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Aqueous solution sampling and the effects of water vapor in glow discharge mass spectrometry

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Aqueous solution sampling and the effects of water vapor in glow discharge mass spectrometry
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Argon ( jstor )
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Ion sources ( jstor )
Ionization ( jstor )
Ions ( jstor )
Plasmas ( jstor )
Signals ( jstor )
Titanium ( jstor )
Water vapor ( jstor )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
Glow discharges ( lcsh )
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Thesis (Ph. D.)--University of Florida, 1992.
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Includes bibliographical references (leaves 221-227)
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Typescript.
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Vita.
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by Philip H. Ratliff.

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AQUEOUS SOLUTION SAMPLING AND THE EFFECTS OF WATER
VAPOR IN GLOW DISCHARGE MASS SPECTROMETRY













By

PHILIP H. RATLIFF


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1992






























This work is dedicated to my parents, whose love
and support helped me get through
my many years of education.













ACKNOWLEDGEMENTS


I wish to express my gratitude to my advisor, Dr. Willard W. Harrison, for

his guidance and encouragement during my graduate career. The qualities that

he has instilled in me as a scientist and a person will not be forgotten. Thanks

to his commitment to the field of glow discharge mass spectrometry and staying

active in research, I have been allowed to attend many national scientific

meetings. This has not only made me a better speaker, but has permitted me to

grow professionally.

I would like to thank the faculty and staff in the University of Florida

Chemistry Department. I would particularly like to thank Evelyn Butler, Susan

Ciccarone, Jeanne Karably, Beverly Lisk, and Wanda Douglas for making the

bureaucracy a little easier to bear. The personnel in the machine shop and

electronics shop are acknowledged for their expertise in constructing many of the

devices required for my research.

I would like to thank my colleagues and friends with whom I have worked

over these years. My colleagues who have gone before me, Christopher

Barshick, Jeffrey Klingler, and Yuan Mei, are especially acknowledged for their

friendship and assistance in the laboratory. The levity they brought to the

research laboratory made every day an adventure. I also would like to thank my

current co-workers, Sue Ohorodnik and Bill Walden, who have had to put up with







me during the writing process when things can get a little hectic. I especially

thank Sue for proofreading this dissertation.

Finally, I would like to express my love and gratitude to my family who have

been very supportive during my graduate career. Although they never pushed me

to pursue this degree, they were with me all the way.

This research has been supported by a grant from the United States

Department of Energy.













TABLE OF CONTENTS


page


ACKNOWLEDGEMENTS .....................................

A BSTRAC T ...............................................


CHAPTERS


1 GLOW DISCHARGE MASS SPECTROMETRY ................ 1

Introduction ..................................... 1
General Description of Gaseous Discharges ......... 2
Fundamental Processes in the Glow Discharge ........... 12
Glow Discharge Sputtering ..................... 12
Glow Discharge Excitation and Ionization ........... 22
Analytical Considerations for GDMS ................... 28
Internal Standards ........................... 29
Relative Sensitivity Factor (RSF) Values ............ 30
Instrum entation .................................. 31
Mass Spectrometer .......................... 31
Neutral Species in the Glow Discharge Plasma ...... 57

2 FUNDAMENTAL STUDIES OF SOLUTION SAMPLES IN
GLOW DISCHARGE MASS SPECTROMETRY ............ 63

Introduction ..................................... 63
Electrothermal Vaporization ......................... 65
Furnace Techniques .......................... 65
Introduction of Micro-Samples into an ICP .......... 69
Solution Samples in the Glow Discharge ................ 70
Introduction ................................ 70
Electrothermal Vaporization/Glow Discharge
Techniques ........................... 71
Ion Source Development ........................... 75
Experiments in a Six-Way Cross ................. 75
Filament on a Probe .......................... 79
Experim ental .................................... 82
Sample Preparation .......................... 82
Sampling Sequence .......................... 82








Data Acquisition ........
Filament Materials ......
Filament-Plasma Interactions ...
Ion Transmission .......
Internal ETV Sample Introduction
Effects of Different Filament
Response to Concentration


Currents.
Changes


Pressure Effects ................
Cathode to Exit Orifice Distance ....


. ... ... ..... 84
. .. ..... .... 84
. 86
. 86
.. ... .. .. ... 92
. ... .. ... ... 92
. .. 96
. .. 98
............ 100


Effects of Sample Positioning on the Filament .....


Memory Effects of the ETV/GD Technique .........
Effects of Contaminant Gas introduction ..........
External ETV Sample introduction ................. ...
Multi-Element Solution Mixtures .....................
Mass Spectral Scanning ......................
Separation of Multi-Element Species with
Filam ent Current ......................
C conclusions ...................................

3 THE EFFECTS OF WATER VAPOR ON GDMS:
STEADY-STATE WATER ADDITION ...................

Introduction ....................................
The Dissociation of Water in Gas Discharges .......
Studies of Water Vapor Effects in Glow Discharges ..
Experim ental ...................................
Results and Discussion ..........................
Effects of Water Vapor on Mass Spectra ..........
Effects of Water on Neutral Species ..............
Effects of Water on Various Plasma Species .......
Effects of Water on Various Cathode
Materials (Matrix Effects) .................
Effects of Water on Plasma Species in
Different Matrices ......................
Removal of Water with a Cryogenic Coil ..........
C conclusions ...................................


.


4 THE EFFECTS OF WATER VAPOR ON GDMS:
PULSED WATER ADDITION ..............


.......... 173


Introduction .................................... 173
Experim ental ................................... 174
Results and Discussion ........................... 180
Transient Behavior of Matrix Species ............. 180
Effects of Pulsed Water on
Other Plasma Species .................. 190
Use of Liquid Nitrogen Cooling ................. 198


103
103
106
107
110
110

112
115


119

119
122
124
130
134
134
139
141

144

151
168
170







Effects of Pulsed Water on the
Atomic Population ..................... 202
Power Supply Operation ...................... 207
C conclusions ................................... 211

5 FINAL REMARKS AND FUTURE DIRECTIONS .............. 214

Final Rem arks ................................. 214
Aqueous Solution Samples ................... 214
Effects of Water Vapor on GDMS ............... 216
Future Directions .................... .. ......... 218
Aqueous Solution Samples .................... 218
Effects of Water Vapor on GDMS ............... 219

REFERENCES ........................................... 221

BIOGRAPHICAL SKETCH ................................... 228













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

AQUEOUS SOLUTION SAMPLING AND THE EFFECTS OF WATER
VAPOR IN GLOW DISCHARGE MASS SPECTROMETRY

By

PHILIP H. RATLIFF

AUGUST, 1992

Chairman: Willard W. Harrison
Major Department: Chemistry

Glow discharge mass spectrometry is an accepted technique for the

analysis of trace elements in solid materials. In this dissertation, the sampling of

small volume aqueous solution samples has been explored. This method uses

electrothermal vaporization of a solution residue for atomization, while a glow

discharge provides the excitation and ionization. The main advantage of this

technique over other glow discharge solution analysis schemes is the increase in

sensitivity for a given sample since the analyte is atomized in a short time. The

effects of the electrothermal filament current on the plasma processes were

studied, since this could influence the discharge processes as well as ion

transport to the mass spectrometer. Variables such as pressure, cathode-to-exit

orifice distance, atomization current, and sample placement on the cathode were

evaluated and the best parameters presented. The method was found to have

relative standard deviations between 15-20 %. Multi-element samples may be







analyzed using either mass spectral scanning or separation of the elements by

their vaporization temperature. The biggest disadvantage of this method is the

irreproducibility, which is attributed to the introduction of contaminant gases with

each sample.

The effects of water vapor on the processes of the glow discharge were

also investigated. This is important since water vapor is always present in the ion

source to some extent. Water vapor exhibits detrimental effects on both

atomization and ionization in the plasma. Mass spectra taken with less than 5%

water vapor resulted in ion signals primarily from H20, H30, ArH, and 02. This is

due to ion molecule reactions and water dissociation occurring in the plasma. A

liquid nitrogen coil was constructed to aid in the removal and control of water

vapor in the ion source. Mass spectra obtained while cooling the source

contained ion signals mainly from the cathode material. Different cathodes were

investigated to observe the varying effects of the water vapor. It was found that

when sputtering reactive metals (such as getters) the water problem may be

minimized. Both steady state and pulsed addition of water were examined to

determine the processes occurring in the plasma.













CHAPTER 1
GLOW DISCHARGE MASS SPECTROMETRY


Introduction

The glow discharge is an old source that has been used over the years

primarily for applications that benefit from conversion of a solid sample into

atoms, ions, and photons that are characteristic of the sample's content. Glow

discharge devices have been constructed in a variety of configurations and have

shown promise in many applications ranging from optical spectroscopy to mass

spectrometry. The earliest reported works examined using the glow discharge for

optical spectroscopy, with one of the first publications appearing in 1916 by

Paschen [1]. Aston [2] used the glow discharge source in the 1940's for its ion

production ability. This source aided Aston's experiments to evaluate and

develop the technique of mass spectrometry, which was then in its infancy.

The majority of the studies reported in this dissertation use mass

spectrometry to sample the ion population produced by a glow discharge source.

Optical methods, such as atomic absorption (AA) and atomic emission (AE), were

also performed in some instances to obtain complementary data on the atomic

population of both ground state and excited species that coexist with ions in the

plasma. The results obtained in these experiments help provide an insight into

the fundamental processes occurring in the glow discharge plasma as well as






2

give some indication to the types of analytical studies that may be conducted and

the considerations that must be made when using this source.

General Description of Gaseous Discharges

Gaseous discharges are obtained upon establishing a flow of electric

current through a gaseous medium [3]. Gas discharges may be classified

according to the current with which they operate. Figure 1 shows a schematic

representation of the characteristics of gaseous discharges with respect to their

voltage and current [4]. There are generally considered to be at least three types

of discharges: 1) the Townsend discharge, 2) the glow discharge, and 3) the arc

discharge. The Townsend discharge operates at very low currents, which give

rise to its nonluminous appearance, since the population of excited species that

provide a discharge with its bright appearance is at very low concentrations. The

Townsend discharge is not self-sustaining and requires external phenomena such

as X-rays or UV light that will produce electrons in the gas to aid in ion

production. A Townsend discharge for analytical mass spectrometry has been

used [5] to ionize the reagent gas rather than high energy electrons emitted from

a hot filament. The central sections of Figure 1 comprise the areas of glow

discharge operation (both normal and abnormal). Abnormal glow discharges are

used in the studies presented here and will be further explained below. The final

section is the area in which an arc discharge operates. The arc discharge has

a very intense luminosity and operates at very high currents, where thermionic

emission from the cathode provides the electrons for this source. This type of
























W V
0

NJ
i


109 10-7


10-3
10


10-1
10


CURRENT (A)





Figure 1 The relationship between voltage and current in a
gaseous discharge [4]. Vb: breakdown voltage, V,: normal cathode
fall potential, Vd: arc voltage.


10+1






4
discharge has found use for analytical spectrographic emission [6] and for the

vapor deposition of thin films [7].

The basic schematic of a glow discharge system is shown in Figure 2. The

main requirements for glow discharge operation are shown in the figure. The

requirements include a low pressure inert gas atmosphere (most often argon), a

cathode (usually the sample to be analyzed), an anode (usually the glow

discharge source housing), and a power supply to provide the high voltage to the

cathode. The dc/pulsed power supply develops a potential difference between

the cathode and the anode, which are both in contact with the low pressure inert

gas. In the dc mode, the power supply provides a constant voltage to the

cathode, while in the pulsed mode the discharge is switched on and off rapidly

with a given frequency. Upon application of the voltage, free electrons that exist

in this low pressure gaseous environment will accelerate toward the anode, which

has a positive bias due to the potential gradient that exists in the chamber. As

these electrons pass through the gas, they collide with and sometimes ionize the

gaseous species. Positive ions are created and accelerate toward the cathode,

which has a negative bias. As the gas ions strike the cathode a variety of species

is liberated, such as neutral atoms and molecules, ionized atoms and molecules,

electrons, and photons. The emitted electrons will accelerate toward the anode

and complete the electrical circuit. With this flow of electrons, the discharge will

maintain a continuous flow of current and be self-sustaining. The pressure regime

that may be used to operate a glow discharge is from about 0.1 10 Torr.






















0
C
0




E



C4



0

.C


1'










0-
.O
(C




0











IU
V.





L O






6
Discharge formation at these pressures requires a potential of 500 2000 V, which

results in a current in the low milliamp range.

The glow discharge is one type of plasma that is used for analytical

studies. A plasma is a partially ionized gas volume that consists of a generally

equal concentration of positively and negatively charged species, as well as a

large number of neutrals. The glow discharge contains a variety of zones that are

evident from Figure 2. These different zones have been characterized with

respect to their light intensity, potential distribution, field strength, space charge,

negative and positive charge regions and the negative current density [8]. The

layers that are present in the glow discharge (from the cathode to the anode) are

the Aston dark space, the cathode layer, the cathode dark space, the Faraday

dark space, the positive column, the anode dark space, and the anode glow. For

all of the regions to be noticeable the anode-cathode distance must be of

sufficient length. In commercial neon lights, the emission that is viewed is from

the positive column, not the negative glow shown here. For the GDMS ion source

the regions from the Faraday dark space out to the anode are missing due to the

close proximity of the anode to the cathode. Also, the Aston dark space that is

near the cathode is not visible because it is surrounded by the cathode glow,

which appears to "coat" the cathode since it is so close to its surface. The

analytically significant processes for GDMS operation occur in the cathode dark

space and the negative glow regions of the discharge, so the discussions here

will only refer to those regions. These regions are named "dark" and "glow"

according to the relative presence or absence of luminosity caused by the






7
radiative relaxation occurring in these regions [9]. The cathode dark space is

dark because there is very little emission from the electron-atom collisions that

occur here, due to the electrons that have been accelerated by the cathode

potential and can no longer sufficiently ionize the discharge gas [10]. In the

negative glow region, however, excitation and ionization will occur. The primary

color of the negative glow is a result of the light emitted upon the radiative

relaxation of the discharge gas. Argon glow discharges are normally a light blue

color, while the emission from neon is red in color.

As previously mentioned, and shown in Figure 1, there are two operational

regimes for a glow discharge: a normal and abnormal cathode fall. In a normal

glow discharge there is a constant increase in current even as the applied voltage

remains steady. The increasing current is compensated for by a subsequent

increase in the area of the cathode that is surrounded by the negative glow

region. Thus the current density, the amount of current for a given area of the

cathode, remains constant [3]. Once the complete cathode area is covered by

the negative glow, any further increase in current requires an increase in the

current density and thus an increase in the discharge potential. This mode is

called an abnormal discharge and is the mode that is almost always used for

analytical glow discharge devices because it is the most stable form. All studies

described in this dissertation used an abnormal glow discharge ion source.

Types of glow discharge devices

The sample to be analyzed is made to be the cathode in the electronic

circuit, while the anode can be constructed of any material. The ion source






8
chamber housing serves as the anode in the mass spectral studies described in

this dissertation. This particular type of glow discharge configuration is called a

coaxial cathode glow discharge. Other types of glow discharges include: 1) the

planar diode, 2) the hollow cathode, 3) the Grimm discharge lamp [11], and 4)

the hollow cathode plume. These configurations are shown in Figure 3.

Coaxial cathode. This cathode is illustrated in Figure 3A and the majority

of the GDMS work that has been done uses this type of cathode. This is due to

the ease of using the ion source chamber of the mass spectrometer as the anode.

Samples for this type of glow discharge are in the form of a 1 2 mm diameter

pin with about five millimeters left exposed to the plasma, or a pressed disk that

is shielded to allow only the top surface of the sample to be exposed to the

plasma. Conducting, bulk samples can be machined into the pin form directly,

while powders (conducting and nonconducting) are pressed into the disk form.

The nonconducting samples must be mixed with a conducting powder before

pressing unless an rf power supply is used. When using rf, care must be

employed to assure that sputtering is localized to the target area of the electrode.

Planar diode. This is the simplest configuration (shown in Figure 3B)

that is used analytically and may also be operated with either dc or rf voltage.

Planar diode glow discharges using an rf power supply were used by Coburn et

al. for mass spectrometry studies of the ion population present in sputter

deposition plasma chambers [12].

Hollow cathode. The hollow cathode lamp, shown in Figure 3C, is

probably the most familiar glow discharge that is used in chemical analysis. It








A




ANODE


CATHODE


S\



,} :-- .. ::.
*.. i*


*'tk : 1 1: i ~


ANODE


1-
+
O-


Figure 3 Various glow discharge configurations.
cathode; B) planar diode; C) hollow cathode lamp; D)
discharge; E) hollow cathode plume.


A) coaxial
Grimm glow


0-


B

+


CATHODE


ANODE








CATHODE
BODY
>


INSULATOR

/ \


SAMPLE
CATHODE


D






WINDOW


ANODE
ANODE


GAS IN/OUT


CATHODE HOLDER


PLASMA
PLUME
r" -rJa


CATHODE -


Figure 3 -- continued.






11
often serves as the light source for atomic absorption or atomic fluorescence

studies, as well as emitting the characteristic lines for emission studies of the

lamp material. For its use as a light source, the only requirement is that the

hollow cathode lamp contains the same material as the atom of interest in the

sample to be analyzed. This source operates at lower voltages and higher

currents than other glow discharge devices due to the constraint of the plasma

within the cathode. This is often referred to as the "hollow cathode effect" [13].

This effect produces a large increase in the intensity of the radiation emitted from

the negative glow, making it attractive as an optical emission source. This results

in a large increase in the excitation and ionization that is observed in the hollow

cathode cavity. This effect has been studied and some theories published

[14,15].

Grimm discharge lamp. The Grimm glow discharge configuration was

first introduced in 1968 [11] and is illustrated in Figure 3D. It is often called an

obstructed discharge due to the fact that the cylindrical anode is positioned within

one cathode dark space distance from the cathode. The sputtered region of the

sample is limited to the region of the cathode enclosed by the anode. This

source has found primary use for atomic emission studies. The anodic sampling

of this source produces ions with a narrow energy distribution and is well suited

to analysis in a quadrupole instrument, which behaves best with ions having

narrow energy bands.

Hollow cathode plume. This source is shown in Figure 3E. This source

was developed by Marcus and Harrison [16] primarily for atomic emission studies.






12
The hollow cathode plume (HCP) results from the restriction of a hollow cathode

discharge to a 1.5 mm orifice in the base of a normal hollow cathode. Samples

must be formed in the shape of a disk, with a central orifice, that may be mounted

in the base of a graphite cylinder and used as the cathode. These disks may be

machined or pressed depending on the nature of the sample. HCP discharges

operate at about 1 Torr pressure and currents in the 50 200 mA range. The

applied potential is typically around 1000 V.

Fundamental Processes in the Glow Discharge

The diagram in Figure 4 shows the structure and some of the processes

occurring in the analytical glow discharge. The various mechanisms, atomization

and excitation/ionization, are shown to occur in very distinct areas of the plasma

and its surroundings. The atomization process, known as sputtering, occurs at

the surface of the sample when argon ions strike the cathode surface. The

excitation and ionization processes are shown in the negative glow region and

occur as the result of collisions in this relatively field free area. There is little or

no potential gradient that exists in the negative glow region since virtually all of

the discharge potential is dropped across the small dark space distance (a few

millimeters, a distance that depends on the operating pressure of the plasma).

Thus, the potential gradient is large within this dark space distance. The following

sections will discuss the fundamental processes in more detail.

Glow Discharge Sputtering

Glow discharge sputtering occurs when the discharge gas ions strike the

surface of the cathode. When an argon ion strikes the surface it will penetrate a







Negative
Glow





Illustration of the fundamental processes in the glow
plasma.


Ar


Dark
Space


Figure 4
discharge






14
few atom distances into the lattice sending "shock waves" out in three dimensions

around the impact point. Actually, it has been reported that the species striking

the surface of the cathode is really a "neutralized ion" [17]. These "neutralized

ions" are formed as the ion gets close to the metal surface and is neutralized by

a field-emitting electron.

If sufficient energy is transferred by the surface collision (in excess of the

target's binding energy), a variety of species will be dislodged from the cathode.

The masses of the collision partners and the individual cross sections (which

depend on the ion velocity and the electronic structure of the partners) will play

a role in the sputtering process [17]. Positive secondary ions are often emitted,

but the strong potential gradient in the cathode dark space will cause these ions

to return to the surface of the cathode and redeposit. These are the species of

interest in the technique of secondary ion mass spectrometry (SIMS), where the

ions formed at the surface of the sample are analyzed. Electrons are also emitted

in GDMS and they are accelerated away from the cathode by the field gradient

and pass into the negative glow region. The emission of electrons from the

cathode surface sustains the discharge and allows it to continue running. Thus,

the glow discharge is a self-sustaining discharge that requires no external

excitation to operate. The third particles that are emitted from the surface are

ground state neutral atoms from the cathode material. These atoms are the

analytically interesting species that are studied with GDMS and make up the main

component of the sputtered particle flux that leaves the cathode surface [18,19].

These species migrate into the negative glow by diffusion, where they may be






15

subsequently atomized and ionized. It is this distinct production of discrete atoms

from a solid medium that gives GDMS its relative freedom from matrix effects.

Essentially, the solid is being converted into a "gaseous solution" of sample atoms

(the analyte) within the argon discharge gas (the solvent).

The efficiency of sputtering (i.e., the number of cathode atoms removed

from the surface per impinging ion) is determined by a parameter called the

sputter yield (Y) [20]:


S9.6 x 104 W (1)
M i' t

where W is the weight loss of the cathode (in g) due to the sputtering process, M

is the atomic weight, i+ is the ion current (in A), and t is the total sputtering time

(in s). The ion current, i+, is related to the total current of the discharge, i, by the

following equation:


i (2)
1+y

where y is the number of secondary electrons released by one ion. Studies of
the experimental factors that influence sputtering have been performed mainly

using focused high energy ion beams similar to those used in SIMS [21]. The

sputter yield is mainly dependent upon the incident ion mass, the angle of

incidence of the incoming ion, the energy of the incoming ion, and the target

material [17,22]. A rigorous theoretical consideration of sputtering and sputter

yields has been published [23]. In the next few sections, some of the factors that

influence sputtering will be discussed.








Incident ion mass

The incident ions that are encountered in glow discharge sputtering are

most frequently those from noble gases. Noble gases are used in GDMS

because their relatively inert nature will minimize any interactions with the cathode

material and they have the ability to provide a good sputter yield since they

behave like hard spheres in the "billiard ball-type" collisions. Bay et al. [24] have

published a theory to explain the dependence of a sample's sputter yield on the

mass of the ions that are bombarding the sample. The maximum sputter yield of

a given target material is proportional to an energy transfer factor, y, which may
be determined as follows:

4M M2
Y. (3)
(MA + M2)2

where M, and M, are the masses of the incident ion and the target atom,

respectively. It can be seen that the maximum energy transfer will occur when M,

= M2, and the highest sputter yield should be obtained. For this reason argon

has been found to be the best choice for sputtering most medium weight atomic

elements [22,25]. Sputter yields have been measured using a variety of noble

gases by Wehner et al. and were shown to have a periodic trend with the best

sputter yields obtained with the noble metals Cu, Ag, and Au [17]. Zinc and

cadmium have still higher yields due to their low heat of sublimation but that

makes these elements undesirable to have in a high vacuum system [17]. Typical

sputter yields obtained using 400 eV Ar for a variety of target materials are






17

shown in Figure 5 [26,27]. As the mass of the incoming ions is increased, greater

sputter yields are obtained for heavier target elements.

Incident ion angle of collision

Oechsner [28] has reported a large quantity of data concerning the

influence that the angle of incidence has on the sputtering yields. At angles of

60 700 from the normal, it was found that sputter yields (in the energy range
from 0.5 2 keV) rise to maximum values which are 1.5 2.4 times higher than

that obtained for normal incidence ions (see Figure 6). The sputter yield showed

increases in this range for metals that normally would produce low sputter yields

[17]. In these ion beam studies, it was determined that the ion current density

decreases with the cosine of the angle a between the beam and the surface
normal. Thus, if there is a sputtering rate increase, it is much less than the yield

increase. The dotted line in Figure 6 shows the reciprocal of the cos a
dependence of Y(a)/Y(0). Above this curve is the region where a higher sputter

removal rate is achieved with an ion beam. However, unless the surface is

rotated or sputtered from different angles, it is most often best to sputter with

incoming ions normal to the sample to alleviate problems that occur with surface

roughening [17].

Incident ion energy

Wehner has performed experiments using a high vacuum beam to

determine that the minimum kinetic energy that is required to dislodge an atom

from its matrix is approximately four times the heat of sublimation of the material

[17,29]. For argon, the threshold for sputtering ranges from 13 eV for aluminum





18










0
S0)
_.-.______. (------0



0)
oo

>O --- -- m





CO E
Z 0 *0
- ----- Q -- c z?


e--~-




o o E


E
0 o
r3-h-o

N 0 w 0 v No 0 w w0

CJ ci c i :- :- -: O O

AG OO IV Cai3A










2.5


Xe

Ar+

SKr+/
1 .5 -0 /






1.0
00 300 60 900
a (with respect to normal)


Figure 6 Sputter yield increase of Cu with bombardment of various
noble gas ions under oblique incidence. The dashed line is the 1/cosa
curve, separating rate increase from rate decrease. [28]






20

to 33 eV for tungsten. Once this threshold is surpassed, the sputter yield will

increase rapidly with increasing ion energy, reach a plateau, and then decrease

as the energy continues to rise. This plateau region generally occurs with about

1 keV ion energy, but will vary slightly with the discharge gas and the sample.

The yields for some elements under different conditions are shown in Table I.

These values will show some scatter because the sample may have different

surface roughness, a preferred crystalline orientation within a nominally

polycrystalline sample, or the possible buildup of impurity layers on the target

during sputtering [17].

As previously mentioned, when an ion hits the sample surface it becomes

imbedded in the matrix and its kinetic energy is absorbed by the atoms in the

vicinity. At lower ion energies the penetration is limited to the surface of the

sample where the atoms are disturbed and ejected from the matrix. However, at

higher energies the penetration depth of the ion increases, which results in

damage to the sample matrix at a distance too far from the surface to affect these

atoms. Thus the sputter yield will decrease at these higher energies [17].

In the glow discharge, the energy of the incoming ion is not easily

determined because of the energy losses caused by the collisions that the

incoming ion encounters as it passes through the discharge dark space.

Attempts to determine this energy in the glow discharge have been addressed

[30,31]. Calculations (based on the pressure and cathode fall potential of the

discharge) have been performed to determine the average energy of the incoming

ions hitting the cathode under typical discharge conditions [25]. An ion energy




















TABLE I. Sputtering Yields in Atoms/Ion for Some Elements
Under Different Argon Ion Bombardment Energies

Element 500 eV 1 keV 10 keV
C 0.12
Al 1
Si 0.5 0.6
Ti 0.5 2.1
Cr 1.2
Fe 1.1 1.3
Ni 1.45 2.2
Cu 2.4 3.6 6.6
Mo 0.8 1.1 2.2

Ag 3.1 3.8 8.8
Au 2.4 3.6 8.4


Source: Reference [17].






22

range of 100 eV 575 eV, corresponding to an applied voltage range from 400 -

2000 V dc, was found to be the typical value for the bombarding ions.

Target material

The dependence of sputter yield versus target material was shown in

Figure 5, where different elements under identical ion bombardment conditions

showed periodic trends in their sputter yield. The sputter yield will increase within

any one period of the periodic table. These differing sputter yields may become

a problem when analyzing alloys or other materials that contain many elements

and is called preferential sputtering [32,33]. It was found that preferential

sputtering will cause the surface composition of the sample to change and be

enriched by the species with the lower sputter yield. The initial consequences of

this process is a difficulty in obtaining an atom population in the gas phase that

is proportional to the concentrations of the species in the bulk sample. However,

the processes inherent to the glow discharge will passively correct for this

difference, because the cathode surface enrichment of the lower sputter yield

species will lead to an eventual equilibrium state [34]. Thus, after the initial

sputtering time required to establish this equilibrium has passed, the gas phase

atom population will be proportional to that of the bulk sample.

Glow Discharge Excitation and Ionization

Aside from being an efficient source of atom generation, the glow

discharge will also excite and/or ionize the sputtered sample atoms through a

variety of energy transfer processes. These processes occur primarily in the

negative glow region, because the negative glow region is an environment rich






23

in collisions that may result in the ionization of the sample atoms. Kinetic and/or

potential energy may be transferred by a collision partner that is sufficient to

excite electrons of the target species to higher energy states [35]. In some cases

the electron may be removed completely. It was determined by Harrison and

Loving that ionization is occurring throughout the volume of the plasma, but only

the ionization that takes place directly adjacent to the exit orifice will be observed

by the mass spectrometer [36]. This is because an ion that is formed in other

regions of the plasma will undergo further collisions on its way through the source

and would never make it to the ion exit orifice.

There are three primary types of collisions that are occurring in the

negative glow region of the discharge. They are collisions involving discharge

electrons, collisions involving excited discharge species, and collisions involving

discharge ions. These collisions will lead to excitation, ionization, and some

recombination of discharge species. Table II lists some of the most important

ionization mechanisms that occur in the negative glow region. The two

mechanisms that are thought to provide a major contribution to the overall

ionization of sputtered species in the plasma are electron ionization (El):


M + e- M' + 2e- (4)

where M is a neutral analyte atom, e- is an electron, and M+ is an analyte ion,

and Penning ionization (PI):


M + Ar* MA + ArO + e-












TABLE II. Possible Ionization Mechanisms in
the Glow Discharge

I) Electron Ionization:

M + e M+ + 2e

II) Penning Ionization:

M + Ar* M+ + Ar + e

III) Associative Ionization (Homonuclear):

Ar* + Ar Ar2+ + e

IV) Associative Ionization (Heteronuclear):

MO + Ar* ArM+ + e

V) Nonsymmetric Charge Transfer:

Ar+ + M -. M + Ar

VI) Symmetric (Resonance) Charge Transfer:

Ar+ast) + Ar (w) Ar ft) + Ar+(s

VII) Dissociative Charge Transfer:

Ar+ + MO M+ + O + Ar


M = neutral analyte atom
M+ = analyte ion
Ar = argon metastable atom
Ar = neutral argon atom
e = electron






25

where Ar* is an argon metastable atom and Ar is a neutral argon atom. These

modes of ionization will be discussed in the following sections.

Electron ionization

Electron ionization (El) in the glow discharge is accomplished through

electrons that are accelerated across the cathode fall region or by electrons

existing in the discharge that possess energy in excess of the ionization potential

of the target atom. This mechanism is an important source of ionization in the

glow discharge [37], and is thought to be the primary ionization path for the

discharge gaseous species in the plasma [38]. If a collision occurs involving an

electron possessing an energy below the ionization potential, but above the

lowest excitation potential of the target atom, electronic excitation of the species

will occur [39]. If the excited species can radiatively relax, then the transition may

be probed by optical spectroscopy. If radiative relaxation is forbidden, the excited

state may exist in the plasma for milliseconds before an alternative deexcitation

process may occur [40]. These species are referred to as metastable species

and they may lead to further excitation and ionization of other species. If an

electron possesses little or no kinetic energy, it is subject to recombination

collisions with discharge ions. The excess energy may result in the creation of

excited neutrals or metastables [41].

The electrons that are present in the glow discharge differ in both their

origin and their energy. There are three general classes of electrons based on

these differences [37]. First, there are the fast electrons with energies exceeding

25 eV. These have been accelerated away from the cathode across the dark






26

space. The second class is the electrons that were created by ionizing collisions

in the negative glow region. These are referred to as secondary electrons and

have an average energy about 7 eV. The final class of electrons is called

"ultimate electrons." These are electrons that have been thermalized (with a mean

energy less then 1 eV [42]) by collisions with other plasma species.

Penning ionization

As mentioned in the previous section, long-lived metastable species may

be formed within the glow discharge plasma and be deexcited by collisional

relaxation processes. When this occurs, the transfer of energy from the

metastable species to the collision partner results in excitation or ionization of the

atom. Two such processes are considered to play a major role in glow discharge

ionization. These are Penning ionization, [43] which was shown earlier in

equation (5), and associative ionization [44], which will be discussed in the next

section.

If the internal energy of the metastable atom is greater than the ionization

energy of the collision partner, then an electron-ion pair is created. As mentioned

previously, this process is believed to be the primary ionization mechanism for

sputtered species in the glow discharge. Table III shows some of the energy

levels of the noble gas atoms that are used for glow discharge operation. Argon

is usually the preferred gas for GDMS because of its metastable internal energies

of 11.5 and 11.7 eV. These energy levels are above the first ionization energies

of most elements, below the second ionization energies of many elements, and

are below the ionization energy of atmospheric contaminants.























TABLE III. Low-lying Metastable Levels
of Rare Gas Atoms


Gas Metastable ionization
Energy, eV Energy, eV
He 19.8, 20.7 24.58
Ne 16.6, 16.7 21.56
Ar 11.5, 11.7 15.76
Kr 9.9, 10.5 14.00
Xe 8.3, 9.4 12.13


Source: Reference [8].








Other ionization mechanisms

Associative ionization. Associative ionization, shown in equation (6),


MO + Ar* ArM' + e- (6)

is the mechanism that is responsible for the production of polyatomic species that

contain discharge gas atoms (i.e., the argides in an argon discharge) [38]. The

mechanism is similar to the Penning process except that the collision partners

remain together and form an ion cluster. The electron that is released during the

reaction will carry off any excess energy.

Charge transfer reactions. The charge transfer reactions listed in

Table II are of two types [37]: symmetric and asymmetric. Symmetric reactions

involve collisions of like partners. These reactions may lead to the formation of

fast neutral atoms as an accelerated gas ion collides with a neutral gas atom [37].

It is believed if this happens in the dark space region, sputter enhancement will

occur because of an increase in the number of species bombarding the surface.

Asymmetric reactions are a potentially important mechanism for the ionization of

sputtered species in some glow discharge configurations. Asymmetric reactions

are believed to be the principal ionization mechanism for sputtered species in an

obstructed discharge [45] and are responsible for the production of excited ions

of sputtered material in hollow cathode vapor lasers [46].

Analytical Considerations for GDMS

Glow discharge mass spectrometry has found wide application for the

analysis of a variety of samples over the past decade, from pure metals and






29
semiconductors to thin films and ceramics. This section will be used to discuss

the considerations that must be made for analytical work with a GDMS system.

These considerations are vital for those applications where an accurate

concentration value is needed to determine impurities in high purity samples [47]

or the abundances of certain critical components, such as in bulk alloys [48] and

semiconductors [49].

Internal Standards

Internal standards in GDMS employ the matrix element as the internal

standard for semi-quantitative analysis. It is assumed that the concentration of

the matrix element of a sample is known. In this method, the sensitivity (the ratio

of the signal intensity to its concentration) observed for the reference element

(e.g., iron in a stainless steel sample) will allow an estimation of the concentration

of the analyte using the following equation:


C X M (7)
Irf
where Cn,, is the unknown concentration of the analyte, I,, is the signal intensity

of the analyte, C,, is the known concentration of the reference element, and I, is

the signal intensity of the reference element. These direct comparisons are valid

in GDMS since the variation in elemental sensitivities for this technique is on the

order of three. In some techniques the sensitivity among the elements varies by

orders of magnitude and the direct comparison shown above would not give even

semi-quantitative results.







Relative Sensitivity Factor (RSF) Values

For precise quantitative analysis, external standards must be used for

GDMS. Two approaches are possible: 1) the use of relative sensitivity factor

(RSF) values obtained from standard materials that are similar in content to that

of the unknown, and 2) using sets of standards to construct a calibration curve

from which the unknown concentration is related to the ion signal intensity.

RSF values were first employed in spark source mass spectrometry (SSMS)

and correct for reproducible variations in the observed sensitivities among the

elements. These variations can arise from physical, chemical, and instrumental

differences. To use RSF values, an element of known concentration must be

present in the sample and the analysis conditions used for the unknown and the

reference must be the same. The RSF is defined as a ratio of the sensitivity of an

analyte element to the sensitivity of the reference element [50]. The concentration

of the analyte and the reference must both be known in the standard, while only

the reference, element concentration needs to be known for the unknown. The

RSF value is calculated as follows:


RSF ( Ca (8)
( / C.. )
where the components of the equation are the same as for Equation (7) shown

for semi-quantitative analysis. The RSF value is then employed as a correction

factor for the intensity ratio calculation shown in equation (7). The unknown

concentration of the analyte is then calculated as follows:









C -x x re (9)

RSF values have been obtained for a variety of sample types and were

determined in this laboratory for stainless steel samples by using an NIST

standard reference material 1264A [51]. RSF values in GDMS have also been

reported in the literature for a variety of elements [52]. For RSF values to be most

accurate, separate RSF values should be calculated for every instrument on which

the analysis is to be performed. The calibration curve method has been reported,

using a Grimm glow discharge, with the use of seven stainless steel standards

[53].

Instrumentation

Mass Spectrometer

The research presented in this dissertation is centered around the use of

a mass spectrometer for fundamental studies of the glow discharge ion source

processes. As mentioned earlier, the glow discharge plasma is an efficient

atomization source containing processes that may excite and ionize many of the

atoms of interest. The mass spectrometer provides an efficient detection system

for the ions produced in this source, as well as providing the isotopic information

for the elements of interest. The mass spectrometer provides spectra that are

simpler and easier to interpret than optical instruments, since each element has

specific mass-to-charge ratios for each of its isotopes. Thus the only interference

problems occur with different elements having an isotope at the same nominal

mass or from polyatomic species that occur at the same nominal mass as an






32

element of interest. The former problem can be avoided by using an isotope that

does not overlap with another element. Almost all elements in the periodic table

have at least one isotope with a nominal mass that is apart from any other

elemental isotope. The problems of polyatomic interference are more difficult to

solve. Studies have been performed previously using a triple quadrupole mass

spectrometer with a glow discharge source to investigate the collision-induced

dissociation of some polyatomic species that are present in the plasma [54]. A

double quadrupole system has also been reported for sampling an rf powered

glow discharge source, with one quadrupole being used as the dissociation

chamber [55]. Both of these methods were successful in using low energy

collisions with argon target atoms to reduce and/or eliminate species such as M2+,

MAr+, MO+, Ar2+, and residual gas species.

The experiments for this dissertation were performed on two different mass

spectrometers. One of the instruments is based on Extranuclear (now called

Extrel) components and is a more research oriented instrument since its source

components are readily accessible and easily modified. This instrument was

constructed at the University of Virginia and has been described in detail

elsewhere [56,57]. This instrument is shown in Figure 7. The second instrument

is a prototype mass spectrometer built by Finnigan MAT in Bremen, Germany

using Balzers components. This instrument has a more commercial design (i.e.,

the components are not as easily accessible or modified) and has previously

been described [35,51]. A cross section of the Finnigan instrument is shown in

Figure 8. This instrumentation section of the dissertation will be used to compare
































X
0
CD
-J
w
LU
C0
w
CU
i0








-U


c
(D
C

E
0





E
0








CL
0
E





.0
U)


-C
2









4- -
a.





%-c

0 0






-CT











Do


U-


34











c
C
0


0


N
o


c
0


C
-(5




0E
E(D





c
0C3




So.
E


-o




c
L0


=5LU
-10



Q Cn
<
FO
co ..1


>-r






35

and contrast these two instrument designs and discuss their unique components

as well as any special considerations that must be made for each. This section

is not meant to be a complete and detailed outline of each individual instrument,

although the primary mass spectrometer components will be discussed, since this

generally applies to either instrument as well as any other quadrupole based

instrument. The Extrel instrument, being more accessible to changes and

modifications, is the instrument where new ideas and new source designs are first

attempted. Once successful, the modified instrumentation or experiments may

be adapted and performed on the Finnigan. The conversion of an experimental

scheme from one instrument to the other gives the researcher an idea of the

difficulties that might be encountered when taking a new idea and trying to use

a commercial instrument to run the experiments. The aspects of instrument

modification and successful adaptation to a more restricted and commercially

designed instrument are always of high concern. This is especially a problem that

may be experienced in an industrial environment where applications usually

require adaptation of an experimental scheme to a commercial instrument.

Ion source

One of the most apparent differences between the two instruments is the

design of the ion source. On the Extrel instrument the ion source is external to

the mass spectrometer chamber and may be isolated by a sliding gate valve that

can be opened and closed between the ion source and the analyzer chamber.

The position of the gate is shown in Figure 7. When the gate is down, a 1.5" hole

is in position between the ion source exit orifice and the skimmer cone of the






36

instrument. With the gate closed the ion source may be vented to atmosphere

while maintaining a high vacuum in the mass spectrometer chamber. The source

is usually vented with argon gas when changing the sample being used for

experimentation. This alleviates some contamination that may occur if air is

allowed into the source. The ion source, which is constructed from a 2 3/4" six-

way cross (MDC Vacuum, Hayward, CA), is larger in volume than the Finnigan

source.

The Finnigan instrument has its ion source mounted on a stainless steel

tube and inserted into a position within the first pumping chamber of the mass

spectrometer. The Finnigan ion source is constructed from a 1" Cajon ultra-torr

tee and its position in the instrument is shown in Figure 8. This source cannot

be vented to atmosphere without venting the entire mass spectrometer. Thus a

sample introduction assembly was constructed [51] to allow the sample to be

changed without venting the instrument. This assembly is shown in Figure 9.

The principal components of this assembly are labeled in the figure. The ion

source is attached to a stainless steel tube that is welded to a vacuum flange and

attached to the instrument through an adjustable bellows. The positioning of the

discharge chamber in three dimensions is controlled by three turnbuckle screws

located 1200 apart around the bellows. This allows the ion exit orifice of the
discharge chamber to be aligned coaxially with the skimmer cone orifice, as well

as allowing the distance between the two to be adjusted. The ball valve in the

assembly allows the mass spectrometer to maintain its vacuum after removing the

direct insertion probe. While the probe is in place, a 1/2" Cajon connector seals









DISCHARGE
CHAMBER


ADJUSTABLE
BELLOWS


SAMPLE
INTRODUCTION Ar IN
ASSEMBLY B
BALL
VALVE


TO
ROUGHING
PUMP



DIRECT
INSERTION
PROBE



Figure 9 Glow discharge mass spectrometer sample
introduction assembly for the Finnigan instrument.






38
around the probe with an O-ring allowing the vacuum to be maintained. A

roughing pump is required to pump air out of the small volume between the

Cajon connector and ball valve when a new sample is inserted. Otherwise, the

short burst of air will cause a high pressure condition in the instrument, causing

the protection circuit to shut off the pumping system to protect the turbomolecular

pumps. During normal operation the roughing pump is isolated from the system.

The design of the direct insertion probes used in these experiments has been

described elsewhere [51]. The probes are identical for both instruments other

than their length. A different probe was constructed for the electrothermal

introduction of solution samples and will be described in Chapter 2.

Einzel lenses and Bessel box

The einzel lenses are constructed from stainless steel tubes and are used

to direct the ions formed in the source into the mass spectrometer. These lenses

are only used for the transport of ions and are only present on the Extrel

instrument in this laboratory. Einzel lenses are required in this system due to the

relatively long distance from the ion exit orifice to the skimmer cone. This

distance cannot be minimized because the Extrel instrument includes the gating

valve to isolate the source from the mass analyzer. These lenses are not shown

in Figure 7 but are contained within the arm of the six-way cross between the ion

exit orifice and the mass spectrometer skimmer cone. The einzel lens system is

shown in the left half of Figure 10. The lenses are powered by a 1000 volt power

supply (Hewlett-Packard Model 6521A) that allows all of the lens voltages to be

independently adjusted from 0 to -1000 V. Einzel lenses are not required in the
















x



UJ
wO
CU)
m




LU
w

















z
rw
UJ


w
N
Z
mU


C
0)




C
x
0







co
I)
a)




,0


c


OW
rD
0
c
0 (D






CE

0
U-CT


'51
00






40

Finnigan instrument since the bellows assembly allows the ion exit orifice of the

source to be placed very close to the skimmer cone.

After the skimmer cone in both instruments lies a Bessel box energy

analyzer, which allows only a small bandwidth of ion energies to pass into the

quadrupole mass filter. The construction of a Bessel box is shown in the right

half of Figure 10, illustrating the complete lens system for the Extrel instrument.

The Bessel box lenses are powered by an Extrel Ionizer Control Model 020-2.

The lensing system for the Finnigan is shown in Figure 11 that illustrates the lens

layout as well as the position of the quadrupole analyzer. These lenses may each

be biased from 0 to +/- 140 V and this value is determined by the control circuit

illustrated in the figure.

The principle behind using an energy analyzer is that the quadrupole

system will operate most efficiently with ion energies from 0 10 eV and the glow

discharge source produces ion energies from 0 20 eV [57,58]. Also any neutral

species and photons that come from the ion source will collide with the center

beam stop that is positioned in the middle of the second lens (housing). The

Bessel box will allow ions within 1 2 eV of the voltage applied to the housing to

be passed into the quadrupole; this may be tuned not only to give maximum ion

signal to the detector but also to allow "tuning" of the mass spectral peaks to give

a Gaussian distribution around the nominal mass number. Ions with a small

energy will be repelled back out of the Bessel box by the voltage on the housing.

Ions with large energies will collide with the housing or the central beam stop

since they are too energetic to be curved through the device. Ions in the correct






41












I[ |a|IE
-6 I


F- c7 ? 100:1
J-,
0. o .

QC
S& & > 1 0 :
75 =



0 0 0 C .2 9 9 a
I 0


2 4r-
0


a + E*

I I oI



A* U





E C5 E ce W -- c us c 9
r S O p 0 ) -
a, -C


U)






42

energy band will be deflected around the central beam stop and then focused

through the exit lenses into the quadrupole mass analyzer.

Quadrupole mass filter

The mass separation system for both instruments is based on a

quadrupole mass filter. This mass separation technique has widespread use and

has been previously described [59]. Mass separation with a quadrupole system

is based on the mass-to-charge ratio (m/z) of the various ions produced from the

sample. The major difference of the quadrupole systems on the two instruments

in this laboratory is the size of the rods that are used. The Extrel system has

stainless steel rods that are 220 mm in length and 19 mm in diameter. This

provides an internal field radius (the distance from the central axis of the rods to

the closest surface of each rod) of 8.22 mm providing a mass range of 1 380

amu. The Finnigan instrument has 200 mm long rods with a diameter of 8 mm,

resulting in a field radius of 3.45 mm that passes specific masses from 1 511

amu. The Finnigan system uses a QMA 150 quadrupole system (Balzers,

Liechtenstein). Thus, both systems give a wide enough mass range to pass all

of the elemental species of interest (approximately 1 250 amu). The basic

operating principles of both quadrupole systems are the same and will be

described below.

A quadrupole system consists of a set of four stainless steel electrodes,

which ideally have a hyperbolic cross section, that are accurately positioned in a

radial array. A block diagram of a quadrupole system is shown in Figure 12.

Since the construction and mounting of hyperbolic rods is more difficult and



















SOURCE


DETECTOR
DETECTOR


Schematic diagram of a quadrupole mass analyzer.


Figure 12






44

expensive, many quadrupole systems employ electrodes that have a circular

cross section. Denison [60] has previously shown that a good approximation to

an ideal hyperbolic field may be obtained if the radius of the electrodes (r) is

related to the quadrupole field radius (ro) by the expression,


r 1.148 ro (10)

The filtering mechanism of the mass analyzer is obtained by the

simultaneous application of dc and ac signals. This will produce an ion trajectory

through the quadrupole that follows a very complicated path with only one

specific mass-to-charge ratio being passed at any one time. The operation of the

quadrupole system will be explained here in a qualitative manner by considering

the physical effects in the X-Z and Y-Z planes separately. The Z axis is defined

along the length of the rods, while the X and Y axes pass through the center of

the opposite rod pairs (see Figure 12). A positive dc potential is applied to the

two rods lying in the X-Z plane, while the rods in the Y-Z plane experience a

negative dc potential [61]. The rf potentials applied to the pairs of rods are

opposite in polarity to one another and are 1800 out of phase.
Here the previously mentioned planes will be considered separately as to

the mass-dependent effects of each pair on the trajectories of some ions. First,

the trajectories in the X-Z plane will be discussed. If an ion is heavy or the

frequency of the rf potential is very high the ion will feel primarily the effect of the

average potential applied to the electrode. In this case heavy ions are mostly

influenced by the positive dc potential and will be focused onto the center axis

of the rods. The times when the dc potential is negative will have a negligible






45
effect on heavy ions. However if the ion is light the varying rf potential may have

a greater affect on its trajectory. If it is sufficiently light it may experience an

acceleration during the negative voltage cycle that will cause it to collide with an

electrode. By this means ions are filtered according to their mass-to-charge ratio.

Thus ions with a mass below a critical m/z value will be filtered out of the beam

and ions above the critical m/z will be transmitted through the device to the

detector. By increasing the rf voltage, heavier ions can be displaced by the rf

field. Thus a high-pass mass filter is formed in the X-Z plane (see Figure 13A).

Second, the effects in the Y-Z plane will be considered. The potential

applied to the electrodes in the Y axis is always equal in magnitude and opposite

in sign to the potential applied to the electrodes in the X axis. Thus, as

mentioned previously, the rf potentials in the X-plane electrodes are 1800 out of
phase with the potentials in the Y-plane electrodes. Also the dc potential applied

to the X-Z plane is positive, while that of the Y-Z plane is negative. As before,

heavy ions will be primarily influenced by the average dc potential. However, in

the Y-Z plane this potential is negative. This means that the heavy ions will be

defocussed away from the central axis of the system and eliminated from the

beam. Conversely, the ions that are lighter will respond to the focussing action

that occurs when the positive portion of the alternating field becomes larger than

the static negative potential. If the frequency and the magnitude of the rf field are

well chosen, the rf potential may be considered to correct the trajectories of light

ions and prevent them from striking the rods along the y axis. Increasing the rf

voltage increases the mass-to-charge ratio that is allowed to pass through the



















MASS


MASS


MASS


Figure 13 Mass filtering in the quadrupole. A) X-Z plane, high-
pass filter; B) Y-Z plane, low-pass filter; C) combination of both
planes, band-pass filter [59].






47
quadrupole. This brief explanation has been discussed in more detail and some

models proposed by Dawson [62]. Thus in the Y-Z plane, the electrodes and

applied potential combine to result in a low-pass mass filter (see Figure 13B).

If an ion is to travel through the quadrupole to the detector on the other

side, the ion must be stable in both the X-Z and Y-Z planes. Thus an ion must

be sufficiently light so as not to be eliminated by the low-pass filter of the Y-Z

plane but not so light that it is eliminated by the high-pass filter in the X-Z plane.

Thus a band-pass filter is created where a narrow mass range of ions are

transported through the quadrupole filter to the detector (see Figure 13C). The

mass corresponding to the center of the mutual stability region is governed by the

magnitude of the applied ac and dc potentials.

The equations of motion in a quadrupole system can be quite complicated.

Therefore, a brief discussion will be presented here and more information may be

obtained from the references cited within this section. The motion of the ions in

the x and y directions depends on the variation of the potential, 0f, with time as

expressed in the following equation [63],


( U- Vcos( ot) (11)

where U is the dc voltage, V is one half of the peak-to-peak rf voltage and (

equals 2jrf with f being the rf frequency. It was determined that for a sinusoidally

operated hyperbolic mass filter, the potential distribution (p) at time (t) is then

expressed as [59],









< [U + Vcos ( ot)] (x2 (12)
2r

where x and y are the distances along the given coordinate axes, r, is the

quadrupole field radius. Equation (12) is then differentiated relative to the x and

y axes to calculate the intensity of the electric field (E) along the X-Z and Y-Z

planes [59]. The results are as follows:


8x r.

and


Ey [U + Vcos (ot ) (14)
by r
These equations can then be mathematically converted to the Mathieu type

equations by first considering the equations of the force that an ion feels in each

plane and substituting Newton's law (F = ma) [62]. The final results of these

calculations are shown in Equations (15) and (16),

d + [U + Vcos( t)] ex 0 (15)
dt2 mr

and


_[U + Vcos ( ot)] 0 (16)
dt2 mri






49

The solutions to these equations are usually unbounded, which corresponds to

the rejection of ions from the mass filter. Stable solutions may be obtained only

for certain values of m, r,, f, U, and V. These solutions will correspond to

conditions that will give a successful transmission of an ion through the

quadrupole. The following substitutions (known as the Mathieu parameters) are

usually made to allow these equations to be written in a single expression [63].

The Mathieu parameters are:

4eU
a -4 (17)


and

2eV
q (18)


These substitutions represent the dc voltage counterpart (a) and the rf voltage

amplitude (q) [59,63]. A typical stability diagram is shown in Figure 14, which is

constructed by plotting dc voltages that yield stable solutions vs. rf voltages that

yield stable solutions. The area below the triangular region represents the values

of dc and rf voltages resulting in a stable ion trajectory for this mass.

To generate a mass spectrum, the alternating voltage, V, is scanned, with

the direct voltage, U, following it with a given ratio along a scan line that is

anchored at the origin in the diagram. Two different scan lines are shown in

Figure 14. The top one will result in more triangular peaks, but will have a lower

intensity since the area of stability traversed is small. The lower scan line will

increase the signal intensity of the ion but will result in broad trapezoidal peaks.














0 0
zl




zq g o
uj- z a
a. Z c

,5 ,o0 2
D D co N






S0 0)



v i d c
T ca
I 0V






1----I----I ---T ||
CdC
I I0



o i&

ZL E
C\I


0 .MeA?




6 0 6






51

Each given mass will have a slightly different stability diagram. Figure 15

(modified from reference [63]) shows the stability diagrams for four different

masses superimposed on the same diagram with actual voltages on the axes.

This diagram may be used to demonstrate the principles of resolution and

intensity and how they interrelate. The mass scan line given is one that may

typically be used in GDMS where unit mass resolution is required. Note that as

the mass scan line passes out of the stable region for mass 28 it just begins to

enter the stable region for mass 29. This will give unit resolution. If the slope of

the line is decreased, then the mass scan line would intercept more of each ion's

stable area (resulting in greater intensity), but for two adjacent ions the mass scan

line would pass through portions of their stability region at the same time. Thus,

the operator must realize that when using a quadrupole system there are trade

offs between good resolution and good signal intensity.

Ion detection

Both instruments have dual detection systems. Each has a Faraday cup

to detect the matrix species and other large ion signals. For the detection of

minor and trace species, the Extrel system utilizes a channeltron continuous

dynode electron multiplier (Galileo Electro-Optics Corporation) and the Finnigan

uses a discrete dynode electron multiplier (Balzers SEV 217, Liechtenstein). Both

of these detectors are set off-axis to the quadrupole axis. These detectors may

be operated in an analog mode that detects the small currents hitting the

detectors or an ion counting mode that will detect discrete events when single

ions strike the detector.













SCO CM


O)
o\ m
\ c(
1 o

0

CM o
CO

L E.




LL E
S\ *E
\\ I E <1




10 0 10 0






I-- -Oo 00
\ E






3oDVIOA Oa






53

Faraday cup. The Faraday cup is the simplest detector employed in

mass spectrometry. It consists of a metal plate (electrode) that is used simply to

collect the ions that strike it. The ions are neutralized on the surface of the

electrode by a transfer of electrons, and the current that results is equal to the

incoming ion current. The Faraday electrode is enclosed, except for a small

entrance aperture that allows the incoming ions to pass through. This shielding

will prevent secondary electrons from escaping before striking the electrode,

which could lead to erroneous readings. The detected currents on the Faraday

electrode are usually small and a high gain signal amplifier is needed. The

Faraday detector requires a well insulated system in order to reduce any leakage

currents. The minimum current that is detectable using this detector is

approximately 10"14 amps [63].

Channel (continuous dynode) electron multiplier. Channel electron

multipliers (CEMs) are widely used as detector devices in many quadrupole mass

spectrometer systems [64]. The characteristics of CEMs that have lead to their

widespread use include their excellent signal-to-noise (with dark counts of less

than 0.5 counts per second), a stable dynode surface that can be exposed to air

without degradation of performance, low power requirements, compact size, and

a narrow gain distribution of output pulses. CEMs are constructed from a

formulation of glass that is heavily doped with lead. The fabrication produces a

glass that exhibits secondary emissive properties and resistive characteristics.

The Channeltron detector typically has a resistance of about 109 ohms. Electrical

contacts are deposited on both ends of the channel to allow electrical contact of






54

the CEM to an external high voltage. The external voltage will replenish the

charge on the channel wall, as well as accelerate low energy secondary electrons

to a level where they will create cascading secondary electrons upon collision

with the wall. Gain on the order of 108 is possible. This detector should be

operated in a vacuum of at least 106 Torr, since higher pressures will lead to

increased background signal and the life of the detector will be shortened.

Ion detection in the CEM is as follows. If a particle has sufficient energy

to cause emission of at least one secondary electron upon striking the interior

surface of the detector then it may be detected. The secondary electron will be

accelerated by the electrostatic field inside the channel until it hits the surface

again, which should result in the release of one or more secondary electrons.

This cascading process may occur 10 20 times in the channel depending on the

design of the multiplier. The gain of a CEM does not depend on the length of the

channel or the diameter of the channel alone. The gain is a function of the ratio

of the channel length to its inner diameter [65] as well as a function of the

potential difference from end to end (i.e., the applied high voltage). Using this

knowledge, channeltron multipliers have been constructed with very small sizes

and in arrays. The detection efficiency of a CEM was found to be a function of

the incoming ion energy, its velocity, and the degree of ionization [66].

Discrete dynode electron multiplier. The secondary electron multiplier

(SEM) operates in a fashion somewhat similar to the CEM. In the SEM, ions

coming from the mass spectrometer are accelerated by an electric field and strike

an electrode (or dynode), which is made of a material with a high coefficient of






55

secondary electron emission. These electrons will in turn produce more

secondary electrons that are accelerated by the field to a second dynode where

they hit and produce other secondaries. This process repeats until a large

number of electrons from the last dynode strike a Faraday plate, which generates

a large signal. The current is amplified as mentioned previously in the discussion

of the Faraday cup. The first dynode of the detector is usually operated at -1000

to -3000 volts that results in an overall gain of 104 108. Thus a current of 10'18

A would be multiplied to about 10-12 A, which is easily measured by the amplifier.

Analog filtering with a bandwidth of a few seconds is usually needed to obtain a

good signal-to-noise ratio [63]. The SEM, as well as the CEM, gives improved

detection limits and an improved response time over the Faraday cup, which

allows the instrument to be scanned faster.

One of the disadvantages with electron multipliers (both CEMs and SEMs)

is that the gain will change as the detector ages. This can be corrected by

measuring gain curves often and adjusting the operating voltage of the detector.

The gain is usually calculated by comparing the voltage detected on an electron

multiplier with that on the Faraday cup using the same ion beam. The SEM and

CEM can both operate in pulse counting modes, where each individual incident

ion produces one pulse. Each single ion that strikes the detector will produce

about 10' electrons, which is a readily detectable pulse. The detection limit of the

pulse counting mode is determined by the background count rate, which is

usually about 1 count per second. Pulse counting circuits are more expensive

than are analog measurement circuits; as a result, the ion currents should be






56
below about 1012 A for this method to be worthwhile. Also, if the current is too

high, pulse pile-up will occur in the detection system and the counting becomes

inaccurate.

Vacuum system

The vacuum systems of the two instruments differ, but the requirements

and care that must be taken to maintain a vacuum system are virtually the same.

The Extrel system uses two diffusion pumps (NRC, Model VHS-4), while the

Finnigan uses two turbomolecular pumps (the first chamber is evacuated with a

330 L/s pump and the analyzer chamber is pumped by a 170 L/s pump). One of

the difficulties that is encountered when merging a glow discharge system is the

pressure difference required to operate the source and the mass analyzer. This

problem is alleviated by the use of differential pumping through small apertures

between three chambers in the instrument. The ion source operates at a pressure

of about 1 Torr while the first chamber maintains a pressure of about 104 Torr.

These chambers are separated by a 0.5 mm ion exit orifice. The analyzer

chamber is maintained at about 106 Torr during operation and is separated from

the first chamber by the small hole in the skimmer cone. The pressure is

measured in the ion source of both instruments with thermocouple gauges

(Teledyne-Hastings, Model DV-4D).

Data collection

Both instruments were operated primarily using a Hewlett-Packard 9816

computer system to control the mass scanning as well as collect the data. A

block diagram of the detection and data acquisition system for the Channeltron






57
detector of the Extrel instrument is shown in Figure 16. The system for the

Finnigan is virtually identical in its operation.

For detection in the analog mode the signal is passed through a

preamplifier and an electrometer. The signal is then directed to an oscilloscope

(Tektronix). The Extrel instrument's oscilloscope (Tektronix Model 5223 with

digitizing time base Model 5B25N) allows real-time mass spectra to be observed

since the quadrupole controller operates in conjunction with the oscilloscope to

allow an external voltage ramp (from the time base output) to control the mass

scan. The analog signal may also be routed through a voltage-to-frequency

converter to measure the ion signals on a multichannel analyzer (MCA) (Tracor-

Northern TN-7200). Finally, the system is connected to the Hewlett-Packard

computer via an IEEE bus system to allow the signal to be converted to a digital

signal that may be stored and manipulated with the computer software.

In counting mode, the signal is fed into an amplifier system that will

eliminate any signal pulses that are below a given threshold discriminatorr

feature). The amplification stage provides a high speed source of pulses (30 ns)

for signal counting devices. The signal can then be directed to the MCA and

collected as counts per channel dwell time. The HP computer may then be used

to read the MCA data for subsequent storage, data manipulation, and printing of

hard copies with the laser printer.

Neutral Species in the Glow Discharge Plasma

The mass spectrometer system will only allow the study of species that are

ionized in the glow discharge ion source. However, it is often advantageous to






























--
cQ
n


E






"C


c


C
0


o
-a

0)


0




-0
0
E




o



Q-
LC
OU0






59

correlate the ion population that is measured with the mass spectrometer to the

neutral and excited atomic population that exists in the ion source. This is

accomplished by interfacing the techniques of mass spectrometry with atomic

absorption (AA) to measure the ground state atomic species, or atomic emission

(AE) to measure the excited neutral species. These experiments were performed

only on the Extrel system since its six-way cross ion source will allow

simultaneous measurements to be made.

Mass spectrometer/atomic absorption system

A block diagram of the MS/AA system is shown in Figure 17. A

commercial neon or argon filled hollow cathode lamp (HCL) was focused down

to a diameter of about 3 mm that was directly adjacent to the ion exit orifice. After

passing through the negative glow, the beam was focused into a 0.5 M Ebert

monochromator (Jarrell-Ash, Waltham, Massachusetts). The intensity of the light

beam was detected by a photomultiplier tube (Hamamatsu R956, Bridgewater,

NJ) coupled with a photometer (Model 110, Pacific Instruments, Concord, CA).

The HCL was powered at a constant current of 6 10 mA and a peak voltage of

300 V by a DC power supply (Hewlett-Packard Model 6525A). The output of the

HCL was modulated at 50 100 Hz by a mechanical chopper (Scitec

Instruments). A reference signal from the chopper was directed to a lock-in

amplifier (Model 5301, EG&G/Princeton Applied Research, Princeton, NJ), as was

the signal from the photometer. The lock-in will discriminate against any

background emission signal that is produced in the negative glow. The output

signal from the lock-in amplifier was then directed into a voltage-to-frequency







































IH l,


OE
oe
C-

12'o






61

converter to allow the signal to be collected on an MCA. The MCA data can then

be read by the HP computer and manipulated or stored. The signal may also be

simultaneously monitored on an oscilloscope.

Mass spectrometer/atomic emission system

A block diagram of the MS/AE system is shown in Figure 18. The emission

experimental setup is basically the same as used for absorption, only much

simplified. With AE, there is no need for an HCL, chopper, or lock-in amplifier.

The emission signal is collected from the negative glow and focused into the

monochromator with one pair of lenses. The detected signal is then fed directly

into the oscilloscope and voltage-to-frequency converter, and subsequently into

the MCA. The collected data is then stored and manipulated by the HP

computer. This system is much simplified since there is no background

subtraction requirement for emission measurements.
























































2 E







LL )
LL. Q(













CHAPTER 2
FUNDAMENTAL STUDIES OF SOLUTION SAMPLES IN
GLOW DISCHARGE MASS SPECTROMETRY


Introduction

As discussed in the first chapter, glow discharge mass spectrometry has

found the majority of its applications in the analysis of solid samples [67,68].

However, this chapter will focus on the introduction of solution samples into a

GDMS system. Solution analysis with a glow discharge is an advantageous

avenue to explore, since many samples already exist in this form. A scheme has

been developed that couples a glow discharge, using its ionization abilities, with

an electrothermal vaporization device, which is very efficient for sample

atomization. This method will reduce any need to manipulate the solution before

glow discharge analysis (i.e., mixing with a solid material), although the solvent

must be removed prior to placing the sample into the mass spectrometer, since

the glow discharge is normally operated at a low pressure (1 Torr). This method

uses electrothermal vaporization of the solution residue to introduce the analytes

into the glow discharge plasma.

The analysis of solution samples is an obviously important facet of

chemistry. Many classic analytical methods were developed from the needs for

solution sample analysis. One of the fastest growing modern techniques for the

analysis of trace elements in solution is the inductively coupled plasma (ICP) [69].






64

The detection of analytes in an ICP may be performed using atomic absorption

(AA), atomic emission (AE), and/or mass spectrometry (MS). ICP-MS uses

differential pumping and skimmer cones with small orifices (similar to that

described in Chapter 1 for GDMS) to allow the mass spectrometer to operate in

the region of 10e to 10" Torr, while the ion source operates at 760 Torr (1

atmosphere). The ICP technique is amenable to the introduction of a large flow

of solution, since the samples are introduced through pneumatic nebulizers that

remove most of the solvents. Solvent particulates that may enter the source can

produce detrimental effects in the plasma, and the properties of the aerosol

relative to the analytical performance of the ICP are of great concern. Differences

will occur in ICP emission signals depending on whether the incoming aerosol is

wet or dry [70]. It was found that the small amount of residual water remaining

after the desolvator did not change the plasma conditions. When the dry aerosol

was mixed with H20/HNO, droplets, there was a reduction in the ICP emission

signal by 43.1 %. Compared to the GDMS source operation, the ICP is able to

handle solution samples more easily, due to its higher source operating pressure,

larger pumping capacity, and ability to handle large fluxes of material. The argon

flow alone into an ICP is in L/min, whereas the glow discharge only can handle

a few mL/min to maintain its operating pressure.

This chapter will begin by briefly reviewing some of the modern analytical

techniques that are used for the analysis of trace metals in small volume solution

samples. The emphasis of this discussion will center on methods using

electrothermal vaporization of the analyte, since the glow discharge solution






65

analysis system was developed using an electrothermal filament for sample

atomization. Finally, the experiments performed during the development of this

method will be discussed.

Electrothermal Vaporization

Electrothermal vaporization (ETV) is an important technique that is used to

provide controlled and fast atomization of a sample deposited onto an

electrothermal device. This device can take many different forms, such as a

filament of graphite furnace. ETV occurs when a sample, is heated to a

temperature above the vaporization point of the elements to be analyzed. As a

result, neutral and ionic elemental species are released from the electrothermal

device. The atomic population produced may be detected and analyzed by a

variety of methods, including AE and AA. The atoms may then be subsequently

excited (either from the ETV energy itself, or by other external methods) to

produce ions that can be detected by ionic emission, ionic absorption, or, as in

the present study, quadrupole mass spectrometry. The following sections will

give some background and examples of some techniques using ETV.

Furnace Techniques

Many solution analysis schemes use a heating device (such as a flame or

furnace) to produce excitation of the species to be analyzed. This section will

describe some techniques that use a graphite tube furnace in conjunction with a

glow discharge hollow cathode plasma to study the atoms and ions from a

solution sample. The methods discussed here rely on the glow discharge

processes to provide the excitation or ionization, so they are termed "nonthermal






66

excitation" methods. The ETV/GDMS method that was developed for the solution

analysis to be described later was based on some of the principles of these

techniques.

Furnace atomic nonthermal excitation spectroscopy

The solution sampling method reported in this dissertation is a variation on

the furnace atomic nonthermal excitation spectroscopy (FANES) technique [71].

In FANES, a thin layer residue is analyzed after solvent evaporation of the solution

sample. The evaporation of the solvent and removal of the analyte from the

graphite furnace surface is accomplished by varying degrees of heat generated

in the furnace by a high current flow. The analyte is then detected by atomic

emission from the graphite tube glow discharge. A block diagram of the FANES

source is shown in Figure 19. The analysis procedure is as follows: 1) pipetting

the sample at atmospheric pressure, 2) drying the sample at atmospheric

pressure, 3) ashing the sample at atmospheric pressure, 4) pumping out the

source, 5) filling the source with argon, 6) ignition of the hollow cathode

discharge, 7) setting the power supply for atomization, 8) detection of the

emission pulse, and 9) cooling the source. Each measurement cycle lasts about

one minute and is under microprocessor control. The discharge used for FANES

is the hollow cathode type that was discussed in Chapter 1. The FANES

technique was first reported by Falk et al., who described a new emission source

with independent atomization and excitation processes [71,72]. The source was

found to have limits of detection comparable to flameless atomic absorption, while

orders of magnitude better than ICP emission. Detection limits of the FANES






67



o
S o z oo-F
X z Or
( F- D m



oC





oA--
w0
C



5)





< C a-

E


Icw
0

-J o
o Om
OO E














z <0 c
w Z C -C







za-


z 'z
m~( -1 O .9
0~C > Om "
O RE o






68
technique, using a 50 /L sample, were in the range of 0.0007 to 16 /g/L
depending upon the element under study. Also, the FANES method was shown

to exhibit a high dynamic range and multi-element capability. This technique has

been compared to other atomic spectroscopic methods in a paper by Falk et al.

[73]. Similar to FANES, the furnace ionic nonthermal excitation spectroscopy

(FINES) technique has been used to measure the emission from ions that are

produced in the source described above. Since this method is also considered

nonthermal, the ions are produced from energy provided by the glow discharge

plasma in the source.

Molecular nonthermal excitation spectroscopy

The technique of molecular nonthermal excitation spectroscopy (MONES)

was developed for the studies of molecules in the FANES source. This method

has been used for the determination of negative ions that combine with metallic

species. For instance, the halides F and CIl have been determined by the

nonthermal excitation of MgF and MgCI molecules [74]. For this determination,

the glow discharge was formed within a graphite tube operated at 2000 oC. The
solution containing the halide is mixed with a solution containing the Mg2+ ions

as an additive, with a concentration higher than the halide to promote reaction to

form the MgX molecule. Upon formation of the MgX in the gas phase, molecular

absorption measurements were made and detection limits of 0.5 ng of fluorine

and 0.24 ng of chloride were obtained. This method shows how reactions may

be carried out and studied in an electrothermal device. MONES experiments

were developed after preliminary FANES studies indicated the formation of free






69

atoms and small molecules in these discharges [74,75,76]. Recently a microwave

induced plasma has been coupled with the MONES method [77]. In these

experiments, nitrogen and oxygen were determined in gaseous and aqueous

samples through the formation and emission of NH and OH radicals.

Introduction of Micro-Samples into an ICP

As pointed out in the introduction, ICP methods have provided a means for

analyzing solution samples that are introduced into the torch in vast quantities

with a large supporting gas flow. The ability of the ICP to desolvate a solution

sample and submit ions to the mass spectrometer makes it a very prominent

technique for bulk liquid analysis. However, if the amount of solution to be

analyzed is in short supply and only a small quantity is available, a conventional

ICP cannot be used.

Recently, interest has grown in measuring microliter samples with the ICP

by interfacing it with an electrothermal source. This was performed by attaching

an ETV device to the ICP torch using 1 2 feet of plastic tubing. The ICP argon

gas is passed through the graphite furnace as the sample is vaporized, producing

an atomic population that is representative of the sample. These atoms are then

swept, by the gas flow, into the ICP torch for subsequent ionization. The ICP may

be operated in single ion monitoring mode to obtain the maximum sensitivity for

a given element or in mass scanning mode to provide multi-element analysis.

The advantages of coupling ETV and ICP-MS include detecting elemental sample

amounts in the low femtogram (fg) region, performing trace analysis on very small

volume samples (ug or pL), and alleviating the matrix and solvent interference






70

that may be encountered with a typical ICP-MS analysis. Sensitivity values from

20 80 counts/fg have been reported, resulting in detection limits in the low fg

range [78]. The measurement precision is in the range of 5 20 %. Calibration

curves have been found to be linear over 2 3 orders of magnitude, although

quantitation usually requires using the standard additions method for analysis.

Memory effects and matrix influences are considered serious constraints for the

ETV-ICP/MS technique and these problems are under investigation.

Solution Samples in the Glow Discharge

Introduction

As previously mentioned, the most common samples analyzed in GDMS

are solid materials in the form of pins machined from bulk solids or disks formed

in a die using powdered samples. The research presented in this chapter is on

the development of a method to introduce small volume aqueous solution

samples into the GDMS ion source to broaden its range of application. Since the

glow discharge source requires operation in a low pressure environment, it is not

amenable to the introduction of solvents in a sample. If a solution sample is

introduced into the discharge plasma without desolvation, it may extinguish the

negative glow. Therefore, strict control is needed to limit the amount of solvent

that is brought into the source. Some methods have been employed over the

years in an attempt to provide GDMS solution analysis and reduce solvent effects

in the vacuum system. The methods employed generally require placing the

sample onto a metal substrate and evaporating the solvent, leaving a thin film of

solution residue (containing the analyte atoms of interest) on the cathode surface.






71

Methods that were performed earlier in this laboratory involved the use of a

copper rf hollow cathode source for solution analysis [79], as well as applying a

solution to a cupped cathode [80]. In these experiments, the applied sample was

evaporated, and then atomized and ionized by the discharge processes. This

allowed for analysis of the solution residue by sputter atomization into the glow

discharge.

Electrothermal Vaporization/Glow Discharge Techniques

When combining electrothermal vaporization and glow discharge ionization,

the strengths of both systems may be used. The electrothermal source provides

a high current that will sufficiently atomize the solution residue in a short time,

while the glow discharge is a very efficient source for excitation and ionization

because of the energetic plasma processes. The glow discharge can be used by

itself to sputter atomize the sample, but this process is limited by the lower

currents allowed in the discharge. Thus, sputter atomization is a slower process

that releases the sample over a longer time, reducing sensitivity. For optimum

sensitivity, the cathode should be carefully chosen to be nonporous. If the

cathode is porous, the solution residue will reside at least partially inside the

cathode and will take even longer to analyze. The analyte signal in this case will

be constant and long lived, but the detection limits will be reduced. An alternative

method has been investigated where the solution was mixed with a conducting

powder, dried, and then pressed into a discharge cathode for subsequent

analysis [81]. The method is thought to be a step forward in the construction of

specific standard cathodes to be used for quantitative glow discharge analysis






72

with relative sensitivity factors, since solid standards containing the elements of

interest are sometimes hard to find.

Electrothermal vaporization/glow discharge atomic emission

Electrothermal vaporization into a glow discharge (GD) emission source

has been demonstrated, using a tungsten wire for atomization of solution samples

[82]. In these experiments, silver and boron solutions were studied using atomic

emission. The discharge was operated at a current of 60 mA and at 1.2 Torr

argon pressure. The maximum filament current of 16 A was used for sample

vaporization. The effect of various solution additives on the emission signal was

also studied and the presence of up to 1000 ppm of aluminum, calcium,

magnesium or sodium had little or no effect when added to a silver solution.

Electrothermal vaporization/alow discharge mass spectrometry

In ETV/GD analysis, the heating of the filament serves as the primary

atomization step. Although some atomization will still occur by glow discharge

sputtering, the amount of sample loss by sputtering will depend on the

temperature of the filament and the time required for the solution residue to be

removed by thermal methods. The glow discharge provides the only apparent

mechanisms for ionization for the analyte under investigation, but the lack of ionic

detection in the absence of a glow discharge is not conclusive evidence that ions

are not being formed directly from the energy provided by the filament. The ions

formed, if any, may be of the wrong energy to pass successfully through the ion

lenses of the mass spectrometer and make it to the detector. One disadvantage

of the ETV method is the need to devise detection schemes that will accurately






73

handle transient signals. The signal peak that is produced from a small volume

solution sample will typically last only a few seconds, thus fast detection systems

are needed.

The schematic diagram shown in Figure 20 shows a simplified overview of

the plasma and its processes for ETV/GD analysis. The analyte to be studied (Zn

in this case) is shown to reside on the surface of the cathode (the tungsten

filament). The plasma sputtering process will liberate both tungsten and zinc

atoms from the surface as the argon ions bombard the filament. This results in

some loss of analyte before initiation of the filament current, but the discharge

"on" time will be minimized in the sampling scheme, as described later. As with

a normal glow discharge, Penning and electron ionization will occur with both the

solution residue and filament material atoms.

The results presented in this chapter are intended to show the usefulness

of this method for introduction of solutions into the glow discharge. Most of the

experimental data are concerned with the fundamental processes that occur in the

glow discharge and demonstrate how different characteristics of the discharge

environment (such as pressure, voltage, and current) may affect the ion signal

and change the analytical usefulness of the method. Also, the presence of a

filament carrying current in the vicinity of the glow discharge may show some

positive or negative effects on the ion transmission and the signal detected by the

mass spectrometer.

This ETV/GD method is of interest as well since it provides a separation of

the atomization and ionization steps in the analysis. Experiments that separate













+
**


e
0


+


/


/.


NEGATIVE
GLOW


*Ar
SZn
0 W


DARK
SPACE


F --- RESIDUE
CATHODE (FILAMENT)


Figure 20 Simplified diagram of the glow discharge processes
occurring in the ETV/GD system.


*


- "-'),VO


* o e-
II






75
atomization and ionization are always of interest since this provides another

dimension with which to evaluate the fundamental processes of the glow

discharge. Other experiments have been performed to show the utility of

separating atomization and ionization steps in the glow discharge analysis. It has

been shown that laser ablation can provide a means of atomizing the sample into

a discharge plasma, which subsequently provides the ionization mechanism [83].

These studies show that the atomization and ionization processes are both

controlled by the laser and discharge parameters. In parallel, the primary

atomization may be thought to occur upon heating the filament, while the primary

ionization is provided by the glow discharge.

Ion Source Development

Experiments in a Six-Way Cross

The initial studies were performed using the Extrel ion source (six-way

cross) that was described in Chapter 1. A diagram of this source, with the ETV

device in place is shown in Figure 21. The source allows easy access to all of

the components inside and application of the solution sample to the filament. A

dual high voltage feedthrough, mounted in a 2.75" rotating flange (MDC High

Vacuum Products Corp.; Hayward, CA), was used to hold the filament and its

supporting assembly. This allowed for ease in filament alignment and

replacement. The filament loops for these experiments were 4 mm in diameter

and 2.75 mm long and were constructed by coiling a rhenium ribbon 3 turns

around a 3.2 mm metal rod. The filament was held in place by two stainless steel

rods and a small copper support wire in such a manner that the center of the coil













oc')


0
*-














4.
0














E
(D




L.

0
0)









o



o
4-




s-x
Li






77
was aligned coaxially with the exit orifice. This was done because the majority of

the solution residue will lie on the inside of the filament loop and ions traveling out

from the center of the coil will most likely be directed through the ends of the coil

and toward the exit orifice. Set screws secured the support rods to the dual

feedthrough as well as mount the filament in a fixed position. Glass shields were

constructed to protect the stainless steel rods from the discharge environment,

allowing only the filament loop and about 10 mm of a copper support wire to be

unprotected. The external cathode was mounted on the end of a probe and

placed in the mass spectrometer through a bellows assembly that allowed for

placement of the cathode in a variety of positions around the filament coil. The

discharge was operated in both constant current and constant voltage mode

using a Heinzinger HNCs 2500-150 ump. power supply, allowing operation up to

a maximum of 2500 V or 150 mA. The studies discussed here were performed

at about 1 mA current and voltages ranging from 400 800 V, with pressures in

the range of 0.45 2.5 Torr. The current loop was powered by a MAT 261

filament power supply (Finnigan MAT; Bremen, Germany), which allows up to 10

A of current to be supplied to the filament for sample atomization. The maximum

current varied from 4 6.6 amps, depending on the filament material and the

analyte to be studied. Table IV shows some materials that were initially studied

in order to find an appropriate filament to provide a "white hot" glow capable of

vaporizing the analyte of interest. The filament power supply was insulated since

it floats at a high potential when the filament was used as the discharge cathode.

A stepper motor assembly was constructed to control accurately the output






















TABLE IV. Properties of Some Possible Filament Materials


Maximum
Material Diameter (mm) Current (A) Observations
Al 0.25 4 glows & melts
Brass 1.2 6.6 -
Chromel 0.35 2.6 red glow
Cu 0.9 6.6 -- -
Ni 1 6.5 ---
Re (ribbon) 0.1 x 0.75 4.2 white glow
Ta 0.015 4.5 white glow
W 0.010 5.5 white glow






79

current of the MAT 261. The current was supplied to the filament by RG-59

coaxial cable and high vacuum feedthroughs.

Filament on a Probe

A problem with this type of source is that there may be slight differences

in the construction of individual filament coils. Also with the rigid support system

there is no way to optimize the distance from the cathode to the exit orifice, which

is an important parameter for detecting the maximum ion signal with GDMS.

Figure 22 shows how the ion signal of tungsten, sputtered from the coiled

electrothermal filament, varies with the cathode-to-exit orifice distance. It can be

seen that the maximum ion signal occurs when the cathode is about 7 mm away

from the ion exit orifice, a typical value for a 1 Torr glow discharge. With these

problems at hand, it was realized that a direct insertion probe (DIP) should be

used in an effort to solve the problem of positioning. A 0.5" outer diameter DIP,

shown in Figure 23, with a 10-turn tungsten filament was constructed to allow the

introduction of a solution residue sample into the ion source without venting and

with the ability to position accurately the filament coil with respect to the ion exit

orifice. The DIP also will allow the use of an auxiliary cathode, if so desired, since

the probe contains four vacuum feedthroughs. Two of the feedthroughs carry the

electrothermal current and discharge voltage for the filament, while one may carry

voltage for an auxiliary discharge cathode. Initial experiments using the DIP show

comparable results to those obtained in the six-way cross, although the ion signal

can be maximized with the DIP due to its flexibility.

















































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Experimental

Sample Preparation

Solutions were prepared by mixing an appropriate amount of a metal

nitrate with distilled water to obtain the desired cation concentration. The

chemicals that were used are as follows: nickel nitrate {Ni(N03)2 6H20}

(Mallinckrodt Inc.; Paris, Kentucky; Analytical Reagent Grade), zinc nitrate

{Zn(N03)2 6H20} (Fisher Scientific; Fair Lawn, NJ; Certified A.C.S.), cupric nitrate

{Cu(NO3),2 21/2H20} (Fisher Scientific; Fair Lawn, NJ; Certified A.C.S.), ferric

nitrate {Fe(N03)3 9H20} (Mallinckrodt Inc.; Paris, Kentucky; Analytical Reagent

Grade), and silver nitrate {AgNO3} (Mallinckrodt Inc.; Paris, Kentucky; Analytical

Reagent Grade). Enough concentrated nitric acid (Fisher Scientific; Fair Lawn,

NJ; Reagent A.C.S. grade) was added to the solutions to provide an acidic

concentration of 1 2 % HNO3. Fundamental studies of the source properties

were carried out with analyte concentrations of approximately 1000 ppm.

Sampling Sequence

The sampling sequence that was devised for the electrothermal

introduction of solution samples went through a variety of stages in its

development. There were a few considerations to make in order to ensure the

most accurate analysis of the samples. First, the solvent on the filament had to

be completely evaporated to minimize any detrimental effects that may be

obtained by excess water being introduced into the mass spectrometer. Second,

the current used for solvent evaporation had to be less than that required to

remove the analyte from the filament. Third, the filament material should be






83
stable for the evaporation step. Some of the filament materials could not

withstand the lower evaporation currents when carried out in an air atmosphere.

This was particularly true for the rhenium ribbon, which would completely oxidize

and break unless the evaporation was performed at a low pressure (= 20 Torr)
or in an inert gas atmosphere. Finally, the discharge operation time before the

filament current is initiated should be long enough to ensure a stable discharge,

but as short as possible to minimize any loss of sample analyte due to sputtering.

The final version of the sampling sequence that was used for sample

introduction on the DIP is as follows: After bringing the source up to atmospheric

pressure, the solution samples (ranging from 10 40 1L) were directly pipetted
onto the filament loop by a microliter pipet [Volac Model # R 880/A, 5 50 UL
adjustable volume; Great Britain]. The solutions were evaporated at about 1.5 A

filament current for 2 min. The current was increased to 2 A, and the evaporation

step continued another 2 min (after this time the coil surface appeared to be dry).

The probe was placed into the instrument and the roughing pump applied for 1

min. The sample was allowed to sit under high vacuum conditions for 2 min after

placing it into the ion source. The argon was then introduce at the working

pressure, usually 1 Torr, and the pressure was allowed to equilibrate for 1 min.

The detector was then turned on for 30 s. The MCA was turned on for 5 s to

measure any background signal that may be present. The discharge was turned

on and allowed to run for 5 s. The filament current supply was turned on to the

appropriate atomization current for the sample. After data collection, the filament

was allowed to continue heating for 2 min to drive off any remaining species and






84

reduce memory effects. The filament assembly was allowed to cool for about 5

min before another cycle was repeated with the next sample.

Data Acquisition

When studying solution samples with the electrothermal source, the data

obtained consisted of a transient signal lasting a few seconds. A schematic

representation of the data acquisition scheme is shown in Figure 24. The

quadrupole was set to pass the mass of interest and the signal from the detector

was monitored using the MCA. The MCA was first turned on to allow detection

of any background signal. The discharge was turned on, at t = 0 s in the figure,

for 5 s to allow stabilization of the discharge voltage. This time was chosen as

the shortest time to allow a pre-sputtered filament coil to attain a stable discharge.

This time was minimized to allow most atomization to take place due to the

electrothermal vaporization. The filament was then turned on, at t = 5 s in the

figure, and quickly rises to the desired current. It may be noted that the spikes

that are produced at this time are due to electrical noise in the system upon

initiation of the filament current. After about a 1 s delay, the ion signal from the

solution residue is detected as a peak.

Filament Materials

The results that were obtained with different filament materials are

discussed in this section. The materials under active investigation were rhenium,

tantalum, and tungsten. These were the only three filaments that resulted in the

"white hot" glow required for vaporization of the samples (see Table IV for the

filaments attempted and some observations). The initial study used a rhenium






















4 8 12 16
Time (s)


STABILIZATION
PERIOD


GLOW DISCHARGE


ON


OFF
ELECTROTHERMAL FILAMENT

Figure 24 Diagram of the relationship between the detected ion
signal with respect to the operation of the glow discharge and
electrothermal filament.


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1500


1200


900


600


300

0


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OFF






86

ribbon as the atomization source. Studies were performed two ways: 1) with the

filament acting as the discharge cathode for ionization, and 2) with ionization

being provided by the discharge supported by an auxiliary cathode (as shown in

Figure 21). Both methods produced comparable ion signals for silver solutions,

but the latter technique was subsequently adopted because of its simpler design.

If the rhenium ribbon is subjected to multiple runs, the glow discharge sputtering

process will "eat" through the ribbon and cause a discontinuity in the current loop.

Tantalum wires were also tried as an ETV filament but they too degraded very

quickly. The majority of the experiments was performed using a tungsten wire.

This wire was the most stable of the three in terms of producing sample ion

signals and the long term ability to withstand the sputtering processes.

Filament-Plasma Interactions

The glow discharge processes and operation may be disturbed by the

operation of the electrothermal device within the ion source region. This section

will describe some considerations that must be made when coupling an

electrothermal device, carrying a high current, with the glow discharge device.

The experiments described in this section were performed to observe the changes

in a typical glow discharge signal when applying a current to the electrothermal

filament.

Ion Transmission

The first concern when using the static coil experimental setup was the

effect that the filament current would have on the transmission of ions from the

auxiliary pin cathode. In these experiments, the discharge cathode was separate






87

from the filament and the cathode pin could be positioned around the filament.

Different positions were investigated for the placement of the discharge pin

cathode. These included placing the pin behind the filament, such that atoms

from the pin must traverse through or around the filament to reach the exit orifice,

placing the pin inside the filament and co-axial to the ion exit orifice, causing the

negative glow region to be confined in the cylindrical space defined by the coil,

and placing the pin beside the filament coil. The ion signal transmission was

found to be greatest with the pin beside the filament coil, so this configuration

was used. This orientation was also allowed with the direct insertion probe.

Effects of different filament currents

Changes may occur in the glow discharge processes when the filament is

carrying a relatively high current (three orders of magnitude more than the

discharge itself). A glow discharge pin cathode was operated in the constant

voltage mode while being positioned beside of the filament. As the filament

current was increased from 0 to about 5 A, the discharge current was monitored.

The results of this experiment are shown in Figure 25. The glow discharge

current is observed to decrease slightly over the range of 0 4 A of filament

current. After this point the glow discharge current shows a dramatic increase.

This may be the result of electron emission from the filament, which may add to

the flow of electrons toward the anode and thus increase the measured current

of the gaseous conductor.

The next experiments were performed to observe how the filament current

would affect the ion signals when the electrothermal filament itself was used as






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89

the discharge cathode, since this much simpler experimental design was used

most often. The effect of filament current on the Re+ signal is shown in Figure 26.

The reported signal is the peak height of the 87Re+ isotope after scanning a

mass range containing the rhenium isotopes 25 times. The initiation of the

filament current results in a fairly constant decline of the ion signal, when

operating with a 1 mA constant current discharge. The discharge voltage

changes from 598 V with no filament current to 740 V with 3 A filament current.

Overall, there is about a 67% decrease in the ion signal from 0 3 A, probably

due to changes in ion transport induced by the current present near the plasma,

possibly as a result of an induced magnetic or electrical field in the area.

These changes were also observed when running the discharge at a

constant voltage of 500 V. Figure 27 shows the ion signals for some gaseous

species (H20 at m/z = 18, COH and N2H at m/z = 29, and ArH at m/z = 41) and

one sputtered species (Re at m/z = 187) with increasing filament current. All of

the ions in this experiment respond in a similar manner and seem to follow the

same trend as the discharge current (as seen in Figure 25). Thus, it appears that

the filament current (and the subsequent changes in the discharge current) has

a major effect on the magnitude of the ion signals obtained from the discharge.

This observed increase at the higher current led to subsequent operation of the

discharge in the constant voltage mode for the analysis of solutions, since high

currents will most likely be used and this will result in a higher ion signal.

However, analyte signals from solutions may respond differently depending on

their atomization temperature.






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AQUEOUS SOLUTION SAMPLING AND THE EFFECTS OF WATER
VAPOR IN GLOW DISCHARGE MASS SPECTROMETRY
By
PHILIP H. RATLIFF
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1992

This work is dedicated to my parents, whose love
and support helped me get through
my many years of education.

ACKNOWLEDGEMENTS
I wish to express my gratitude to my advisor, Dr. Willard W. Harrison, for
his guidance and encouragement during my graduate career. The qualities that
he has instilled in me as a scientist and a person will not be forgotten. Thanks
to his commitment to the field of glow discharge mass spectrometry and staying
active in research, I have been allowed to attend many national scientific
meetings. This has not only made me a better speaker, but has permitted me to
grow professionally.
I would like to thank the faculty and staff in the University of Florida
Chemistry Department. I would particularly like to thank Evelyn Butler, Susan
Ciccarone, Jeanne Karably, Beverly Lisk, and Wanda Douglas for making the
bureaucracy a little easier to bear. The personnel in the machine shop and
electronics shop are acknowledged for their expertise in constructing many of the
devices required for my research.
I would like to thank my colleagues and friends with whom I have worked
over these years. My colleagues who have gone before me, Christopher
Barshick, Jeffrey Klingler, and Yuan Mei, are especially acknowledged for their
friendship and assistance in the laboratory. The levity they brought to the
research laboratory made every day an adventure. I also would like to thank my
current co-workers, Sue Ohorodnik and Bill Walden, who have had to put up with
in

me during the writing process when things can get a little hectic. I especially
thank Sue for proofreading this dissertation.
Finally, I would like to express my love and gratitude to my family who have
been very supportive during my graduate career. Although they never pushed me
to pursue this degree, they were with me all the way.
This research has been supported by a grant from the United States
Department of Energy.
IV

TABLE OF CONTENTS
page
ACKNOWLEDGEMENTS iii
ABSTRACT viii
CHAPTERS
1 GLOW DISCHARGE MASS SPECTROMETRY 1
Introduction 1
General Description of Gaseous Discharges 2
Fundamental Processes In the Glow Discharge 12
Glow Discharge Sputtering 12
Glow Discharge Excitation and Ionization 22
Analytical Considerations for GDMS 28
Internal Standards 29
Relative Sensitivity Factor (RSF) Values 30
Instrumentation 31
Mass Spectrometer 31
Neutral Species in the Glow Discharge Plasma 57
2 FUNDAMENTAL STUDIES OF SOLUTION SAMPLES IN
GLOW DISCHARGE MASS SPECTROMETRY 63
Introduction 63
Electrothermal Vaporization 65
Furnace Techniques 65
Introduction of Micro-Samples into an ICP 69
Solution Samples in the Glow Discharge 70
Introduction 70
Electrothermal Vaporization/Glow Discharge
Techniques 71
Ion Source Development 75
Experiments in a Six-Way Cross 75
Filament on a Probe 79
Experimental 82
Sample Preparation 82
Sampling Sequence 82
v

Data Acquisition 84
Filament Materials 84
Filament-Plasma Interactions 86
Ion Transmission 86
Internal ETV Sample Introduction 92
Effects of Different Filament Currents 92
Response to Concentration Changes 96
Pressure Effects 98
Cathode to Exit Orifice Distance 100
Effects of Sample Positioning on the Filament 103
Memory Effects of the ETV/GD Technique 103
Effects of Contaminant Gas introduction 106
External ETV Sample introduction 107
Multi-Element Solution Mixtures 110
Mass Spectral Scanning 110
Separation of Multi-Element Species with
Filament Current 112
Conclusions 115
3 THE EFFECTS OF WATER VAPOR ON GDMS:
STEADY-STATE WATER ADDITION 119
Introduction 119
The Dissociation of Water in Gas Discharges 122
Studies of Water Vapor Effects in Glow Discharges ..124
Experimental 130
Results and Discussion 134
Effects of Water Vapor on Mass Spectra 134
Effects of Water on Neutral Species 139
Effects of Water on Various Plasma Species 141
Effects of Water on Various Cathode
Materials (Matrix Effects) 144
Effects of Water on Plasma Species in
Different Matrices 151
Removal of Water with a Cryogenic Coil 168
Conclusions 170
4 THE EFFECTS OF WATER VAPOR ON GDMS:
PULSED WATER ADDITION 173
Introduction 173
Experimental 174
Results and Discussion 180
Transient Behavior of Matrix Species 180
Effects of Pulsed Water on
Other Plasma Species 190
Use of Liquid Nitrogen Cooling 198
VI

Effects of Pulsed Water on the
Atomic Population 202
Power Supply Operation 207
Conclusions 211
5 FINAL REMARKS AND FUTURE DIRECTIONS 214
Final Remarks 214
Aqueous Solution Samples 214
Effects of Water Vapor on GDMS 216
Future Directions 218
Aqueous Solution Samples 218
Effects of Water Vapor on GDMS 219
REFERENCES 221
BIOGRAPHICAL SKETCH 228
VII

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
AQUEOUS SOLUTION SAMPLING AND THE EFFECTS OF WATER
VAPOR IN GLOW DISCHARGE MASS SPECTROMETRY
By
PHILIP H. RATLIFF
AUGUST, 1992
Chairman: Willard W. Harrison
Major Department: Chemistry
Glow discharge mass spectrometry is an accepted technique for the
analysis of trace elements in solid materials. In this dissertation, the sampling of
small volume aqueous solution samples has been explored. This method uses
electrothermal vaporization of a solution residue for atomization, while a glow
discharge provides the excitation and ionization. The main advantage of this
technique over other glow discharge solution analysis schemes is the increase in
sensitivity for a given sample since the analyte is atomized in a short time. The
effects of the electrothermal filament current on the plasma processes were
studied, since this could influence the discharge processes as well as ion
transport to the mass spectrometer. Variables such as pressure, cathode-to-exit
orifice distance, atomization current, and sample placement on the cathode were
evaluated and the best parameters presented. The method was found to have
relative standard deviations between 15-20 %. Multi-element samples may be
VIII

analyzed using either mass spectral scanning or separation of the elements by
their vaporization temperature. The biggest disadvantage of this method is the
irreproducibility, which is attributed to the introduction of contaminant gases with
each sample.
The effects of water vapor on the processes of the glow discharge were
also investigated. This is important since water vapor is always present in the ion
source to some extent. Water vapor exhibits detrimental effects on both
atomization and ionization in the plasma. Mass spectra taken with less than 5%
water vapor resulted in ion signals primarily from H20, H30, ArH, and 02. This is
due to ion molecule reactions and water dissociation occurring in the plasma. A
liquid nitrogen coil was constructed to aid in the removal and control of water
vapor in the ion source. Mass spectra obtained while cooling the source
contained ion signals mainly from the cathode material. Different cathodes were
investigated to observe the varying effects of the water vapor. It was found that
when sputtering reactive metals (such as getters) the water problem may be
minimized. Both steady state and pulsed addition of water were examined to
determine the processes occurring in the plasma.
IX

CHAPTER 1
GLOW DISCHARGE MASS SPECTROMETRY
Introduction
The glow discharge is an old source that has been used over the years
primarily for applications that benefit from conversion of a solid sample into
atoms, ions, and photons that are characteristic of the sample’s content. Glow
discharge devices have been constructed in a variety of configurations and have
shown promise in many applications ranging from optical spectroscopy to mass
spectrometry. The earliest reported works examined using the glow discharge for
optical spectroscopy, with one of the first publications appearing in 1916 by
Paschen [1], Aston [2] used the glow discharge source in the 1940’s for its ion
production ability. This source aided Aston’s experiments to evaluate and
develop the technique of mass spectrometry, which was then in its infancy.
The majority of the studies reported in this dissertation use mass
spectrometry to sample the ion population produced by a glow discharge source.
Optical methods, such as atomic absorption (AA) and atomic emission (AE), were
also performed in some instances to obtain complementary data on the atomic
population of both ground state and excited species that coexist with ions in the
plasma. The results obtained in these experiments help provide an insight into
the fundamental processes occurring in the glow discharge plasma as well as
1

2
give some indication to the types of analytical studies that may be conducted and
the considerations that must be made when using this source.
General Description of Gaseous Discharges
Gaseous discharges are obtained upon establishing a flow of electric
current through a gaseous medium [3]. Gas discharges may be classified
according to the current with which they operate. Figure 1 shows a schematic
representation of the characteristics of gaseous discharges with respect to their
voltage and current [4], There are generally considered to be at least three types
of discharges: 1) the Townsend discharge, 2) the glow discharge, and 3) the arc
discharge. The Townsend discharge operates at very low currents, which give
rise to its nonluminous appearance, since the population of excited species that
provide a discharge with its bright appearance is at very low concentrations. The
Townsend discharge is not self-sustaining and requires external phenomena such
as X-rays or UV light that will produce electrons in the gas to aid in ion
production. A Townsend discharge for analytical mass spectrometry has been
used [5] to ionize the reagent gas rather than high energy electrons emitted from
a hot filament. The central sections of Figure 1 comprise the areas of glow
discharge operation (both normal and abnormal). Abnormal glow discharges are
used in the studies presented here and will be further explained below. The final
section is the area in which an arc discharge operates. The arc discharge has
a very intense luminosity and operates at very high currents, where thermionic
emission from the cathode provides the electrons for this source. This type of

VOLTAGE (V)
3
CURRENT (A)
Figure 1 The relationship between voltage and current in a
gaseous discharge [4]. Vb: breakdown voltage, Vn: normal cathode
fall potential, Vd: arc voltage.

4
discharge has found use for analytical spectrographic emission [6] and for the
vapor deposition of thin films [7],
The basic schematic of a glow discharge system is shown in Figure 2. The
main requirements for glow discharge operation are shown in the figure. The
requirements include a low pressure inert gas atmosphere (most often argon), a
cathode (usually the sample to be analyzed), an anode (usually the glow
discharge source housing), and a power supply to provide the high voltage to the
cathode. The dc/pulsed power supply develops a potential difference between
the cathode and the anode, which are both in contact with the low pressure inert
gas. In the dc mode, the power supply provides a constant voltage to the
cathode, while in the pulsed mode the discharge is switched on and off rapidly
with a given frequency. Upon application of the voltage, free electrons that exist
in this low pressure gaseous environment will accelerate toward the anode, which
has a positive bias due to the potential gradient that exists in the chamber. As
these electrons pass through the gas, they collide with and sometimes ionize the
gaseous species. Positive ions are created and accelerate toward the cathode,
which has a negative bias. As the gas ions strike the cathode a variety of species
is liberated, such as neutral atoms and molecules, ionized atoms and molecules,
electrons, and photons. The emitted electrons will accelerate toward the anode
and complete the electrical circuit. With this flow of electrons, the discharge will
maintain a continuous flow of current and be self-sustaining. The pressure regime
that may be used to operate a glow discharge is from about 0.1 - 10 Torr.

DC/PULSED
POWER
SUPPLY
LIMITING
RESISTOR
<$>
O
(+)
FARADAY DARK
SPACE
NEGATIVE
GLOW
CATHODE DARK
SPACE
ANODE
CATHODE
(-)
Figure 2 Schematic of a glow discharge circuit showing the primary regions
of the plasma.
cn

6
Discharge formation at these pressures requires a potential of 500 - 2000 V, which
results in a current in the low milliamp range.
The glow discharge is one type of plasma that is used for analytical
studies. A plasma is a partially ionized gas volume that consists of a generally
equal concentration of positively and negatively charged species, as well as a
large number of neutrals. The glow discharge contains a variety of zones that are
evident from Figure 2. These different zones have been characterized with
respect to their light intensity, potential distribution, field strength, space charge,
negative and positive charge regions and the negative current density [8], The
layers that are present in the glow discharge (from the cathode to the anode) are
the Aston dark space, the cathode layer, the cathode dark space, the Faraday
dark space, the positive column, the anode dark space, and the anode glow. For
all of the regions to be noticeable the anode-cathode distance must be of
sufficient length. In commercial neon lights, the emission that is viewed is from
the positive column, not the negative glow shown here. For the GDMS ion source
the regions from the Faraday dark space out to the anode are missing due to the
close proximity of the anode to the cathode. Also, the Aston dark space that is
near the cathode is not visible because it is surrounded by the cathode glow,
which appears to "coat" the cathode since it is so close to its surface. The
analytically significant processes for GDMS operation occur in the cathode dark
space and the negative glow regions of the discharge, so the discussions here
will only refer to those regions. These regions are named "dark" and "glow"
according to the relative presence or absence of luminosity caused by the

7
radiative relaxation occurring in these regions [9]. The cathode dark space is
dark because there is very little emission from the electron-atom collisions that
occur here, due to the electrons that have been accelerated by the cathode
potential and can no longer sufficiently ionize the discharge gas [10]. In the
negative glow region, however, excitation and ionization will occur. The primary
color of the negative glow is a result of the light emitted upon the radiative
relaxation of the discharge gas. Argon glow discharges are normally a light blue
color, while the emission from neon is red in color.
As previously mentioned, and shown in Figure 1, there are two operational
regimes for a glow discharge: a normal and abnormal cathode fall. In a normal
glow discharge there is a constant increase in current even as the applied voltage
remains steady. The increasing current is compensated for by a subsequent
increase in the area of the cathode that is surrounded by the negative glow
region. Thus the current density, the amount of current for a given area of the
cathode, remains constant [3], Once the complete cathode area is covered by
the negative glow, any further increase in current requires an increase in the
current density and thus an increase in the discharge potential. This mode is
called an abnormal discharge and is the mode that is almost always used for
analytical glow discharge devices because it is the most stable form. All studies
described in this dissertation used an abnormal glow discharge ion source.
Types of glow discharge devices
The sample to be analyzed is made to be the cathode in the electronic
circuit, while the anode can be constructed of any material. The ion source

8
chamber housing serves as the anode in the mass spectral studies described in
this dissertation. This particular type of glow discharge configuration is called a
coaxial cathode glow discharge. Other types of glow discharges include: 1) the
planar diode, 2) the hollow cathode, 3) the Grimm discharge lamp [11], and 4)
the hollow cathode plume. These configurations are shown in Figure 3.
Coaxial cathode. This cathode is illustrated in Figure 3A and the majority
of the GDMS work that has been done uses this type of cathode. This is due to
the ease of using the ion source chamber of the mass spectrometer as the anode.
Samples for this type of glow discharge are in the form of a 1 - 2 mm diameter
pin with about five millimeters left exposed to the plasma, or a pressed disk that
is shielded to allow only the top surface of the sample to be exposed to the
plasma. Conducting, bulk samples can be machined into the pin form directly,
while powders (conducting and nonconducting) are pressed into the disk form.
The nonconducting samples must be mixed with a conducting powder before
pressing unless an rf power supply is used. When using rf, care must be
employed to assure that sputtering is localized to the target area of the electrode.
Planar diode. This is the simplest configuration (shown in Figure 3B)
that is used analytically and may also be operated with either dc or rf voltage.
Planar diode glow discharges using an rf power supply were used by Coburn et
al. for mass spectrometry studies of the ion population present in sputter
deposition plasma chambers [12].
Hollow cathode. The hollow cathode lamp, shown in Figure 3C, is
probably the most familiar glow discharge that is used in chemical analysis. It

9
Figure 3 Various glow discharge configurations. A) coaxial
cathode; B) planar diode; C) hollow cathode lamp; D) Grimm glow
discharge; E) hollow cathode plume.

10
ANODE
CATHODE
BODY
INSULATOR
SAMPLE
CATHODE
D
WINDOW
GAS IN/OUT
E
CATHODE HOLDER
Figure 3 -- continued.

11
often serves as the light source for atomic absorption or atomic fluorescence
studies, as well as emitting the characteristic lines for emission studies of the
lamp material. For its use as a light source, the only requirement is that the
hollow cathode lamp contains the same material as the atom of interest in the
sample to be analyzed. This source operates at lower voltages and higher
currents than other glow discharge devices due to the constraint of the plasma
within the cathode. This is often referred to as the "hollow cathode effect" [13].
This effect produces a large increase in the intensity of the radiation emitted from
the negative glow, making it attractive as an optical emission source. This results
in a large increase in the excitation and ionization that is observed in the hollow
cathode cavity. This effect has been studied and some theories published
[14,15],
Grimm discharge lamp. The Grimm glow discharge configuration was
first introduced in 1968 [11] and is illustrated in Figure 3D. It is often called an
obstructed discharge due to the fact that the cylindrical anode is positioned within
one cathode dark space distance from the cathode. The sputtered region of the
sample is limited to the region of the cathode enclosed by the anode. This
source has found primary use for atomic emission studies. The anodic sampling
of this source produces ions with a narrow energy distribution and is well suited
to analysis in a quadrupole instrument, which behaves best with ions having
narrow energy bands.
Hollow cathode plume. This source is shown in Figure 3E. This source
was developed by Marcus and Harrison [16] primarily for atomic emission studies.

12
The hollow cathode plume (HCP) results from the restriction of a hollow cathode
discharge to a 1.5 mm orifice in the base of a normal hollow cathode. Samples
must be formed in the shape of a disk, with a central orifice, that may be mounted
in the base of a graphite cylinder and used as the cathode. These disks may be
machined or pressed depending on the nature of the sample. HCP discharges
operate at about 1 Torr pressure and currents in the 50 - 200 mA range. The
applied potential is typically around 1000 V.
Fundamental Processes in the Glow Discharge
The diagram in Figure 4 shows the structure and some of the processes
occurring in the analytical glow discharge. The various mechanisms, atomization
and excitation/ionization, are shown to occur in very distinct areas of the plasma
and its surroundings. The atomization process, known as sputtering, occurs at
the surface of the sample when argon ions strike the cathode surface. The
excitation and ionization processes are shown in the negative glow region and
occur as the result of collisions in this relatively field free area. There is little or
no potential gradient that exists in the negative glow region since virtually all of
the discharge potential is dropped across the small dark space distance (a few
millimeters, a distance that depends on the operating pressure of the plasma).
Thus, the potential gradient is large within this dark space distance. The following
sections will discuss the fundamental processes in more detail.
Glow Discharge Sputtering
Glow discharge sputtering occurs when the discharge gas ions strike the
surface of the cathode. When an argon ion strikes the surface it will penetrate a

13
Figure 4 Illustration of the fundamental processes in the glow
discharge plasma.

14
few atom distances into the lattice sending "shock waves" out in three dimensions
around the impact point. Actually, it has been reported that the species striking
the surface of the cathode is really a "neutralized ion" [17]. These "neutralized
ions" are formed as the ion gets close to the metal surface and is neutralized by
a field-emitting electron.
If sufficient energy is transferred by the surface collision (in excess of the
target’s binding energy), a variety of species will be dislodged from the cathode.
The masses of the collision partners and the individual cross sections (which
depend on the ion velocity and the electronic structure of the partners) will play
a role in the sputtering process [17], Positive secondary ions are often emitted,
but the strong potential gradient in the cathode dark space will cause these ions
to return to the surface of the cathode and redeposit. These are the species of
interest in the technique of secondary ion mass spectrometry (SIMS), where the
ions formed at the surface of the sample are analyzed. Electrons are also emitted
in GDMS and they are accelerated away from the cathode by the field gradient
and pass into the negative glow region. The emission of electrons from the
cathode surface sustains the discharge and allows it to continue running. Thus,
the glow discharge is a self-sustaining discharge that requires no external
excitation to operate. The third particles that are emitted from the surface are
ground state neutral atoms from the cathode material. These atoms are the
analytically interesting species that are studied with GDMS and make up the main
component of the sputtered particle flux that leaves the cathode surface [18,19],
These species migrate into the negative glow by diffusion, where they may be

15
subsequently atomized and ionized. It is this distinct production of discrete atoms
from a solid medium that gives GDMS its relative freedom from matrix effects.
Essentially, the solid is being converted into a "gaseous solution" of sample atoms
(the analyte) within the argon discharge gas (the solvent).
The efficiency of sputtering (i.e., the number of cathode atoms removed
from the surface per impinging ion) is determined by a parameter called the
sputter yield (V) [20]:
y 9.6 X 104 W (1)
m ■ r • t
where W is the weight loss of the cathode (in g) due to the sputtering process, M
is the atomic weight, i+ is the ion current (in A), and t is the total sputtering time
(in s). The ion current, /+, is related to the total current of the discharge, /, by the
following equation:
where y is the number of secondary electrons released by one ion. Studies of
the experimental factors that influence sputtering have been performed mainly
using focussed high energy ion beams similar to those used in SIMS [21]. The
sputter yield is mainly dependent upon the incident ion mass, the angle of
incidence of the incoming ion, the energy of the incoming ion, and the target
material [17,22], A rigorous theoretical consideration of sputtering and sputter
yields has been published [23]. In the next few sections, some of the factors that
influence sputtering will be discussed.

16
Incident ion mass
The incident ions that are encountered in glow discharge sputtering are
most frequently those from noble gases. Noble gases are used in GDMS
because their relatively inert nature will minimize any interactions with the cathode
material and they have the ability to provide a good sputter yield since they
behave like hard spheres in the "billiard ball-type" collisions. Bay etal. [24] have
published a theory to explain the dependence of a sample’s sputter yield on the
mass of the ions that are bombarding the sample. The maximum sputter yield of
a given target material is proportional to an energy transfer factor, y, which may
be determined as follows:
4 M, M2
(M, * M#
(3)
where M1 and M2 are the masses of the incident ion and the target atom,
respectively. It can be seen that the maximum energy transfer will occur when M1
= M2, and the highest sputter yield should be obtained. For this reason argon
has been found to be the best choice for sputtering most medium weight atomic
elements [22,25]. Sputter yields have been measured using a variety of noble
gases by Wehner et al. and were shown to have a periodic trend with the best
sputter yields obtained with the noble metals Cu, Ag, and Au [17], Zinc and
cadmium have still higher yields due to their low heat of sublimation but that
makes these elements undesirable to have in a high vacuum system [17]. Typical
sputter yields obtained using 400 eV Ar+ for a variety of target materials are

17
shown in Figure 5 [26,27]. As the mass of the incoming ions is increased, greater
sputter yields are obtained for heavier target elements.
Incident ion angle of collision
Oechsner [28] has reported a large quantity of data concerning the
influence that the angle of incidence has on the sputtering yields. At angles of
60 - 70° from the normal, it was found that sputter yields (in the energy range
from 0.5 - 2 keV) rise to maximum values which are 1.5 - 2.4 times higher than
that obtained for normal incidence ions (see Figure 6). The sputter yield showed
increases in this range for metals that normally would produce low sputter yields
[17]. In these ion beam studies, it was determined that the ion current density
decreases with the cosine of the angle a between the beam and the surface
normal. Thus, if there is a sputtering rate increase, it is much less than the yield
increase. The dotted line in Figure 6 shows the reciprocal of the cos a
dependence of Y{a)/Y{0). Above this curve is the region where a higher sputter
removal rate is achieved with an ion beam. However, unless the surface is
rotated or sputtered from different angles, it is most often best to sputter with
incoming ions normal to the sample to alleviate problems that occur with surface
roughening [17].
Incident ion energy
Wehner has performed experiments using a high vacuum beam to
determine that the minimum kinetic energy that is required to dislodge an atom
from its matrix is approximately four times the heat of sublimation of the material
[17,29]. For argon, the threshold for sputtering ranges from 13 eV for aluminum

ATOMIC NUMBER
Figure 5 Sputter yield of multiple elements under 400 eV argon ion
bombardment [26,27],

Y (a) / Y(0)
19
0° 30° 60° 90°
a (with respect to normal)
Figure 6 Sputter yield increase of Cu with bombardment of various
noble gas ions under oblique incidence. The dashed line is the 1/cosa
curve, separating rate increase from rate decrease. [28]

20
to 33 eV for tungsten. Once this threshold is surpassed, the sputter yield will
increase rapidly with increasing ion energy, reach a plateau, and then decrease
as the energy continues to rise. This plateau region generally occurs with about
1 keV ion energy, but will vary slightly with the discharge gas and the sample.
The yields for some elements under different conditions are shown in Table I.
These values will show some scatter because the sample may have different
surface roughness, a preferred crystalline orientation within a nominally
polycrystalline sample, or the possible buildup of impurity layers on the target
during sputtering [17].
As previously mentioned, when an ion hits the sample surface it becomes
imbedded in the matrix and its kinetic energy is absorbed by the atoms in the
vicinity. At lower ion energies the penetration is limited to the surface of the
sample where the atoms are disturbed and ejected from the matrix. However, at
higher energies the penetration depth of the ion increases, which results in
damage to the sample matrix at a distance too far from the surface to affect these
atoms. Thus the sputter yield will decrease at these higher energies [17].
In the glow discharge, the energy of the incoming ion is not easily
determined because of the energy losses caused by the collisions that the
incoming ion encounters as it passes through the discharge dark space.
Attempts to determine this energy in the glow discharge have been addressed
[30,31], Calculations (based on the pressure and cathode fall potential of the
discharge) have been performed to determine the average energy of the incoming
ions hitting the cathode under typical discharge conditions [25], An ion energy

21
TABLE I. Sputtering Yields in Atoms/Ion for Some Elements
Under Different Argon Ion Bombardment Energies
Element
500 eV
1 keV
10 keV
C
0.12
Al
1
Si
0.5
0.6
Ti
0.5
2.1
Cr
1.2
Fe
1.1
1.3
Ni
1.45
2.2
Cu
2.4
3.6
6.6
Mo
0.8
1.1
2.2
Ag
3.1
3.8
8.8
Au
2.4
3.6
8.4
Source:
Reference [17].

22
range of 100 eV - 575 eV, corresponding to an applied voltage range from 400 -
2000 V dc, was found to be the typical value for the bombarding ions.
Target material
The dependence of sputter yield versus target material was shown in
Figure 5, where different elements under identical ion bombardment conditions
showed periodic trends in their sputter yield. The sputter yield will increase within
any one period of the periodic table. These differing sputter yields may become
a problem when analyzing alloys or other materials that contain many elements
and is called preferential sputtering [32,33], It was found that preferential
sputtering will cause the surface composition of the sample to change and be
enriched by the species with the lower sputter yield. The initial consequences of
this process is a difficulty in obtaining an atom population in the gas phase that
is proportional to the concentrations of the species in the bulk sample. However,
the processes inherent to the glow discharge will passively correct for this
difference, because the cathode surface enrichment of the lower sputter yield
species will lead to an eventual equilibrium state [34], Thus, after the initial
sputtering time required to establish this equilibrium has passed, the gas phase
atom population will be proportional to that of the bulk sample.
Glow Discharge Excitation and Ionization
Aside from being an efficient source of atom generation, the glow
discharge will also excite and/or ionize the sputtered sample atoms through a
variety of energy transfer processes. These processes occur primarily in the
negative glow region, because the negative glow region is an environment rich

23
in collisions that may result in the ionization of the sample atoms. Kinetic and/or
potential energy may be transferred by a collision partner that is sufficient to
excite electrons of the target species to higher energy states [35], In some cases
the electron may be removed completely. It was determined by Harrison and
Loving that ionization is occurring throughout the volume of the plasma, but only
the ionization that takes place directly adjacent to the exit orifice will be observed
by the mass spectrometer [36], This is because an ion that is formed in other
regions of the plasma will undergo further collisions on its way through the source
and would never make it to the ion exit orifice.
There are three primary types of collisions that are occurring in the
negative glow region of the discharge. They are collisions involving discharge
electrons, collisions involving excited discharge species, and collisions involving
discharge ions. These collisions will lead to excitation, ionization, and some
recombination of discharge species. Table II lists some of the most important
ionization mechanisms that occur in the negative glow region. The two
mechanisms that are thought to provide a major contribution to the overall
ionization of sputtered species in the plasma are electron ionization (El):
M° + ©- - M+ + 2e- (4)
where M° is a neutral analyte atom, e is an electron, and M+ is an analyte ion,
and Penning ionization (PI):
M° + Ar' - M' + Ar° + e~
(5)

24
TABLE II. Possible Ionization Mechanisms in
the Glow Discharge
I) Electron Ionization:
M° + e" M+ + 2e
II) Penning Ionization:
M° + Ar* -* M+ + Ar° + e'
III) Associative Ionization (Homonuclear):
Ar* + Ar° -> Ar2+ + e
IV) Associative Ionization (Heteronuclear):
M° + Ar* -»• ArM+ + e
V) Nonsymmetric Charge Transfer:
Ar+ + M° -» M+ + Ar°
VI) Symmetric (Resonance) Charge Transfer:
Ar+ _l A r° _> Ar° 4- Ar+
(fast) ' M (slow) M (fast) ' M (slow)
VII) Dissociative Charge Transfer:
Ar+ + MO -> M+ + O + Ar°
M° = neutral analyte atom
M+ = analyte ion
Ar* = argon metastable atom
Ar° = neutral argon atom
e = electron

25
where Ar is an argon metastable atom and Af is a neutral argon atom. These
modes of Ionization will be discussed in the following sections.
Electron ionization
Electron ionization (El) in the glow discharge is accomplished through
electrons that are accelerated across the cathode fall region or by electrons
existing in the discharge that possess energy in excess of the ionization potential
of the target atom. This mechanism is an important source of ionization in the
glow discharge [37], and is thought to be the primary ionization path for the
discharge gaseous species in the plasma [38]. If a collision occurs involving an
electron possessing an energy below the ionization potential, but above the
lowest excitation potential of the target atom, electronic excitation of the species
will occur [39]. If the excited species can radiatively relax, then the transition may
be probed by optical spectroscopy. If radiative relaxation is forbidden, the excited
state may exist in the plasma for milliseconds before an alternative deexcitation
process may occur [40], These species are referred to as metastable species
and they may lead to further excitation and ionization of other species. If an
electron possesses little or no kinetic energy, it is subject to recombination
collisions with discharge ions. The excess energy may result in the creation of
excited neutrals or metastables [41].
The electrons that are present in the glow discharge differ in both their
origin and their energy. There are three general classes of electrons based on
these differences [37], First, there are the fast electrons with energies exceeding
25 eV. These have been accelerated away from the cathode across the dark

26
space. The second class is the electrons that were created by Ionizing collisions
in the negative glow region. These are referred to as secondary electrons and
have an average energy about 7 eV. The final class of electrons is called
"ultimate electrons." These are electrons that have been thermalized (with a mean
energy less then 1 eV [42]) by collisions with other plasma species.
Penning ionization
As mentioned in the previous section, long-lived metastable species may
be formed within the glow discharge plasma and be deexcited by collisional
relaxation processes. When this occurs, the transfer of energy from the
metastable species to the collision partner results in excitation or ionization of the
atom. Two such processes are considered to play a major role in glow discharge
ionization. These are Penning ionization, [43] which was shown earlier in
equation (5), and associative ionization [44], which will be discussed in the next
section.
If the internal energy of the metastable atom is greater than the ionization
energy of the collision partner, then an electron-ion pair is created. As mentioned
previously, this process is believed to be the primary ionization mechanism for
sputtered species in the glow discharge. Table III shows some of the energy
levels of the noble gas atoms that are used for glow discharge operation. Argon
is usually the preferred gas for GDMS because of its metastable internal energies
of 11.5 and 11.7 eV. These energy levels are above the first ionization energies
of most elements, below the second ionization energies of many elements, and
are below the ionization energy of atmospheric contaminants.

27
TABLE III. Low-lying Metastable Levels
of Rare Gas Atoms
Gas
Metastable
Ionization
Energy, eV
Energy, eV
He
19.8, 20.7
24.58
Ne
16.6, 16.7
21.56
Ar
11.5, 11.7
15.76
Kr
9.9, 10.5
14.00
Xe
8.3, 9.4
12.13
Source:
Reference [8],

28
Other ionization mechanisms
Associative ionization. Associative ionization, shown in equation (6),
M° + Ar* - ArM' + e~ (6)
is the mechanism that is responsible for the production of polyatomic species that
contain discharge gas atoms (i.e., the argides in an argon discharge) [38], The
mechanism is similar to the Penning process except that the collision partners
remain together and form an ion cluster. The electron that is released during the
reaction will carry off any excess energy.
Charge transfer reactions. The charge transfer reactions listed in
Table II are of two types [37]: symmetric and asymmetric. Symmetric reactions
involve collisions of like partners. These reactions may lead to the formation of
fast neutral atoms as an accelerated gas ion collides with a neutral gas atom [37],
It is believed if this happens in the dark space region, sputter enhancement will
occur because of an increase in the number of species bombarding the surface.
Asymmetric reactions are a potentially important mechanism for the ionization of
sputtered species in some glow discharge configurations. Asymmetric reactions
are believed to be the principal ionization mechanism for sputtered species in an
obstructed discharge [45] and are responsible for the production of excited ions
of sputtered material in hollow cathode vapor lasers [46],
Analytical Considerations for GDMS
Glow discharge mass spectrometry has found wide application for the
analysis of a variety of samples over the past decade, from pure metals and

29
semiconductors to thin films and ceramics. This section will be used to discuss
the considerations that must be made for analytical work with a GDMS system.
These considerations are vital for those applications where an accurate
concentration value is needed to determine impurities in high purity samples [47]
or the abundances of certain critical components, such as in bulk alloys [48] and
semiconductors [49].
Internal Standards
Internal standards in GDMS employ the matrix element as the internal
standard for semi-quantitative analysis. It is assumed that the concentration of
the matrix element of a sample is known. In this method, the sensitivity (the ratio
of the signal intensity to its concentration) observed for the reference element
(e.g., iron in a stainless steel sample) will allow an estimation of the concentration
of the analyte using the following equation:
- U X -S* (7)
'ref
where Canal is the unknown concentration of the analyte, lanal is the signal intensity
of the analyte, Cref is the known concentration of the reference element, and lref is
the signal intensity of the reference element. These direct comparisons are valid
in GDMS since the variation in elemental sensitivities for this technique is on the
order of three. In some techniques the sensitivity among the elements varies by
orders of magnitude and the direct comparison shown above would not give even
semi-quantitative results.

30
Relative Sensitivity Factor (RSR Values
For precise quantitative analysis, external standards must be used for
GDMS. Two approaches are possible: 1) the use of relative sensitivity factor
(RSF) values obtained from standard materials that are similar in content to that
of the unknown, and 2) using sets of standards to construct a calibration curve
from which the unknown concentration is related to the ion signal intensity.
RSF values were first employed in spark source mass spectrometry (SSMS)
and correct for reproducible variations in the observed sensitivities among the
elements. These variations can arise from physical, chemical, and instrumental
differences. To use RSF values, an element of known concentration must be
present in the sample and the analysis conditions used for the unknown and the
reference must be the same. The RSF is defined as a ratio of the sensitivity of an
analyte element to the sensitivity of the reference element [50]. The concentration
of the analyte and the reference must both be known in the standard, while only
the reference element concentration needs to be known for the unknown. The
RSF value is calculated as follows:
RSF -
( Ianal I ^ anal )
Ure,l Cmf)
(8)
where the components of the equation are the same as for Equation (7) shown
for semi-quantitative analysis. The RSF value is then employed as a correction
factor for the intensity ratio calculation shown in equation (7). The unknown
concentration of the analyte is then calculated as follows:

31
( 1
'anal
\RSF)
'anal
1 re f
(9)
RSF values have been obtained for a variety of sample types and were
determined in this laboratory for stainless steel samples by using an NIST
standard reference material 1264A [51]. RSF values in GDMS have also been
reported in the literature for a variety of elements [52]. For RSF values to be most
accurate, separate RSF values should be calculated for every instrument on which
the analysis is to be performed. The calibration curve method has been reported,
using a Grimm glow discharge, with the use of seven stainless steel standards
[53],
Instrumentation
Mass Spectrometer
The research presented in this dissertation is centered around the use of
a mass spectrometer for fundamental studies of the glow discharge ion source
processes. As mentioned earlier, the glow discharge plasma is an efficient
atomization source containing processes that may excite and ionize many of the
atoms of interest. The mass spectrometer provides an efficient detection system
for the ions produced in this source, as well as providing the isotopic information
for the elements of interest. The mass spectrometer provides spectra that are
simpler and easier to interpret than optical instruments, since each element has
specific mass-to-charge ratios for each of its isotopes. Thus the only interference
problems occur with different elements having an isotope at the same nominal
mass or from polyatomic species that occur at the same nominal mass as an

32
element of interest. The former problem can be avoided by using an isotope that
does not overlap with another element. Almost all elements in the periodic table
have at least one isotope with a nominal mass that is apart from any other
elemental isotope. The problems of polyatomic interferences are more difficult to
solve. Studies have been performed previously using a triple quadrupole mass
spectrometer with a glow discharge source to investigate the collision-induced
dissociation of some polyatomic species that are present in the plasma [54], A
double quadrupole system has also been reported for sampling an rf powered
glow discharge source, with one quadrupole being used as the dissociation
chamber [55], Both of these methods were successful in using low energy
collisions with argon target atoms to reduce and/or eliminate species such as M2+,
MAr+, MO+, Ar2+, and residual gas species.
The experiments for this dissertation were performed on two different mass
spectrometers. One of the instruments is based on Extranuclear (now called
Extrel) components and is a more research oriented instrument since its source
components are readily accessible and easily modified. This instrument was
constructed at the University of Virginia and has been described in detail
elsewhere [56,57]. This instrument is shown in Figure 7. The second instrument
is a prototype mass spectrometer built by Finnigan MAT in Bremen, Germany
using Balzers components. This instrument has a more commercial design (i.e.,
the components are not as easily accessible or modified) and has previously
been described [35,51], A cross section of the Finnigan instrument is shown in
Figure 8. This instrumentation section of the dissertation will be used to compare

GATE VALVE
$
BESSEL BOX
Figure 7 Glow discharge mass spectrometer, based on Extrel components,
built at the University of Virginia.
00
00

DISCHARGE
ION SOURCE
ION
OPTICS
SECONDARY
ELECTRON
MULTIPLIER
BALL VALVE
ww/
7
nn
c
s
-bn
r~
.< i
a
i
« ”
>
1 n~
>
1 A
ADJUSTABLE
BELLOWS
330 L7S
TURBO
PUMP
Â¥
QUADRUPOLE
MASS FILTER
170 l/s r
TURBO
PUMP
FARADAY
CUP
Figure 8 Glow discharge mass spectrometer, based on Balzers components,
constructed by Finnigan MAT (Bremen, Germany).

35
and contrast these two instrument designs and discuss their unique components
as well as any special considerations that must be made for each. This section
is not meant to be a complete and detailed outline of each individual instrument,
although the primary mass spectrometer components will be discussed, since this
generally applies to either instrument as well as any other quadrupole based
instrument. The Extrel instrument, being more accessible to changes and
modifications, is the instrument where new ideas and new source designs are first
attempted. Once successful, the modified instrumentation or experiments may
be adapted and performed on the Finnigan. The conversion of an experimental
scheme from one instrument to the other gives the researcher an idea of the
difficulties that might be encountered when taking a new idea and trying to use
a commercial instrument to run the experiments. The aspects of instrument
modification and successful adaptation to a more restricted and commercially
designed instrument are always of high concern. This is especially a problem that
may be experienced in an industrial environment where applications usually
require adaptation of an experimental scheme to a commercial instrument.
Ion source
One of the most apparent differences between the two instruments is the
design of the ion source. On the Extrel instrument the ion source is external to
the mass spectrometer chamber and may be isolated by a sliding gate valve that
can be opened and closed between the ion source and the analyzer chamber.
The position of the gate is shown in Figure 7. When the gate is down, a 1.5" hole
is in position between the ion source exit orifice and the skimmer cone of the

36
instrument. With the gate closed the ion source may be vented to atmosphere
while maintaining a high vacuum in the mass spectrometer chamber. The source
is usually vented with argon gas when changing the sample being used for
experimentation. This alleviates some contamination that may occur if air is
allowed into the source. The ion source, which is constructed from a 2 3/4" six¬
way cross (MDC Vacuum, Hayward, CA), is larger in volume than the Finnigan
source.
The Finnigan instrument has its ion source mounted on a stainless steel
tube and inserted into a position within the first pumping chamber of the mass
spectrometer. The Finnigan ion source is constructed from a 1" Cajon ultra-torr
tee and its position in the instrument is shown in Figure 8. This source cannot
be vented to atmosphere without venting the entire mass spectrometer. Thus a
sample introduction assembly was constructed [51] to allow the sample to be
changed without venting the instrument. This assembly is shown in Figure 9.
The principal components of this assembly are labeled in the figure. The ion
source is attached to a stainless steel tube that is welded to a vacuum flange and
attached to the instrument through an adjustable bellows. The positioning of the
discharge chamber in three dimensions is controlled by three turnbuckle screws
located 120° apart around the bellows. This allows the ion exit orifice of the
discharge chamber to be aligned coaxially with the skimmer cone orifice, as well
as allowing the distance between the two to be adjusted. The ball valve in the
assembly allows the mass spectrometer to maintain its vacuum after removing the
direct insertion probe. While the probe is in place, a 1/2" Cajon connector seals

37
Figure 9 Glow discharge mass spectrometer sample
introduction assembly for the Finnigan instrument.

38
around the probe with an O-ring allowing the vacuum to be maintained. A
roughing pump is required to pump air out of the small volume between the
Cajon connector and ball valve when a new sample is inserted. Otherwise, the
short burst of air will cause a high pressure condition in the instrument, causing
the protection circuit to shut off the pumping system to protect the turbomolecular
pumps. During normal operation the roughing pump is isolated from the system.
The design of the direct insertion probes used in these experiments has been
described elsewhere [51]. The probes are identical for both instruments other
than their length. A different probe was constructed for the electrothermal
introduction of solution samples and will be described in Chapter 2.
Einzel lenses and Bessel box
The einzel lenses are constructed from stainless steel tubes and are used
to direct the ions formed in the source into the mass spectrometer. These lenses
are only used for the transport of ions and are only present on the Extrel
instrument in this laboratory. Einzel lenses are required in this system due to the
relatively long distance from the ion exit orifice to the skimmer cone. This
distance cannot be minimized because the Extrel instrument includes the gating
valve to isolate the source from the mass analyzer. These lenses are not shown
in Figure 7 but are contained within the arm of the six-way cross between the ion
exit orifice and the mass spectrometer skimmer cone. The einzel lens system is
shown in the left half of Figure 10. The lenses are powered by a 1000 volt power
supply (Hewlett-Packard Model 6521 A) that allows all of the lens voltages to be
independently adjusted from 0 to -1000 V. Einzel lenses are not required in the

EINZEL LENSES
BESSEL BOX
Figure 10 Schematic diagram of the einzel lenses and Bessel box energy
discriminator for the Extrel instrument.
03
CO

40
Finnigan instrument since the bellows assembly allows the ion exit orifice of the
source to be placed very close to the skimmer cone.
After the skimmer cone in both instruments lies a Bessel box energy
analyzer, which allows only a small bandwidth of ion energies to pass into the
quadrupole mass filter. The construction of a Bessel box is shown in the right
half of Figure 10, illustrating the complete lens system for the Extrel instrument.
The Bessel box lenses are powered by an Extrel Ionizer Control Model 020-2.
The lensing system for the Finnigan is shown in Figure 11 that illustrates the lens
layout as well as the position of the quadrupole analyzer. These lenses may each
be biased from 0 to +/- 140 V and this value is determined by the control circuit
illustrated in the figure.
The principle behind using an energy analyzer is that the quadrupole
system will operate most efficiently with ion energies from 0 -10 eV and the glow
discharge source produces ion energies from 0 - 20 eV [57,58], Also any neutral
species and photons that come from the ion source will collide with the center
beam stop that is positioned in the middle of the second lens (housing). The
Bessel box will allow ions within 1 - 2 eV of the voltage applied to the housing to
be passed into the quadrupole; this may be tuned not only to give maximum ion
signal to the detector but also to allow "tuning" of the mass spectral peaks to give
a Gaussian distribution around the nominal mass number. Ions with a small
energy will be repelled back out of the Bessel box by the voltage on the housing.
Ions with large energies will collide with the housing or the central beam stop
since they are too energetic to be curved through the device. Ions in the correct

Figure 11 Illustration of the ion optics system for the Finnigan instrument.

42
energy band will be deflected around the central beam stop and then focussed
through the exit lenses into the quadrupole mass analyzer.
Quadrupole mass filter
The mass separation system for both instruments is based on a
quadrupole mass filter. This mass separation technique has widespread use and
has been previously described [59]. Mass separation with a quadrupole system
is based on the mass-to-charge ratio (m/z) of the various ions produced from the
sample. The major difference of the quadrupole systems on the two instruments
in this laboratory is the size of the rods that are used. The Extrel system has
stainless steel rods that are 220 mm in length and 19 mm in diameter. This
provides an internal field radius (the distance from the central axis of the rods to
the closest surface of each rod) of 8.22 mm providing a mass range of 1 - 380
amu. The Finnigan instrument has 200 mm long rods with a diameter of 8 mm,
resulting in a field radius of 3.45 mm that passes specific masses from 1-511
amu. The Finnigan system uses a QMA 150 quadrupole system (Balzers,
Liechtenstein). Thus, both systems give a wide enough mass range to pass all
of the elemental species of interest (approximately 1 - 250 amu). The basic
operating principles of both quadrupole systems are the same and will be
described below.
A quadrupole system consists of a set of four stainless steel electrodes,
which ideally have a hyperbolic cross section, that are accurately positioned in a
radial array. A block diagram of a quadrupole system is shown in Figure 12.
Since the construction and mounting of hyperbolic rods is more difficult and

SWEEP
DC/RF
GENERATOR
RATIO
Figure 12 Schematic diagram of a quadrupole mass analyzer.

44
expensive, many quadrupole systems employ electrodes that have a circular
cross section. Denison [60] has previously shown that a good approximation to
an ideal hyperbolic field may be obtained if the radius of the electrodes (r) is
related to the quadrupole field radius (r0) by the expression,
r - 1.148 ra (10)
The filtering mechanism of the mass analyzer is obtained by the
simultaneous application of dc and ac signals. This will produce an ion trajectory
through the quadrupole that follows a very complicated path with only one
specific mass-to-charge ratio being passed at any one time. The operation of the
quadrupole system will be explained here in a qualitative manner by considering
the physical effects in the X-Z and Y-Z planes separately. The Z axis is defined
along the length of the rods, while the X and Y axes pass through the center of
the opposite rod pairs (see Figure 12). A positive dc potential is applied to the
two rods lying in the X-Z plane, while the rods in the Y-Z plane experience a
negative dc potential [61]. The rf potentials applied to the pairs of rods are
opposite in polarity to one another and are 180° out of phase.
Here the previously mentioned planes will be considered separately as to
the mass-dependent effects of each pair on the trajectories of some ions. First,
the trajectories in the X-Z plane will be discussed. If an ion is heavy or the
frequency of the rf potential is very high the ion will feel primarily the effect of the
average potential applied to the electrode. In this case heavy ions are mostly
influenced by the positive dc potential and will be focussed onto the center axis
of the rods. The times when the dc potential is negative will have a negligible

45
effect on heavy ions. However if the ion is light the varying rf potential may have
a greater affect on its trajectory. If it is sufficiently light it may experience an
acceleration during the negative voltage cycle that will cause it to collide with an
electrode. By this means ions are filtered according to their mass-to-charge ratio.
Thus ions with a mass below a critical m/z value will be filtered out of the beam
and ions above the critical m/z will be transmitted through the device to the
detector. By increasing the rf voltage, heavier ions can be displaced by the rf
field. Thus a high-pass mass filter is formed in the X-Z plane (see Figure 13A).
Second, the effects in the Y-Z plane will be considered. The potential
applied to the electrodes in the Y axis is always equal in magnitude and opposite
in sign to the potential applied to the electrodes in the X axis. Thus, as
mentioned previously, the rf potentials in the X-plane electrodes are 180° out of
phase with the potentials in the Y-plane electrodes. Also the dc potential applied
to the X-Z plane is positive, while that of the Y-Z plane is negative. As before,
heavy ions will be primarily influenced by the average dc potential. However, in
the Y-Z plane this potential is negative. This means that the heavy ions will be
defocussed away from the central axis of the system and eliminated from the
beam. Conversely, the ions that are lighter will respond to the focussing action
that occurs when the positive portion of the alternating field becomes larger than
the static negative potential. If the frequency and the magnitude of the rf field are
well chosen, the rf potential may be considered to correct the trajectories of light
ions and prevent them from striking the rods along the y axis. Increasing the rf
voltage increases the mass-to-charge ratio that is allowed to pass through the

46
A
B
C
Figure 13 Mass filtering in the quadrupole. A) X-Z plane, high-
pass filter; B) Y-Z plane, low-pass filter; C) combination of both
planes, band-pass filter [59].

47
quadrupole. This brief explanation has been discussed in more detail and some
models proposed by Dawson [62]. Thus in the Y-Z plane, the electrodes and
applied potential combine to result in a low-pass mass filter (see Figure 13B).
If an ion is to travel through the quadrupole to the detector on the other
side, the ion must be stable in both the X-Z and Y-Z planes. Thus an ion must
be sufficiently light so as not to be eliminated by the low-pass filter of the Y-Z
plane but not so light that it is eliminated by the high-pass filter in the X-Z plane.
Thus a band-pass filter is created where a narrow mass range of ions are
transported through the quadrupole filter to the detector (see Figure 13C). The
mass corresponding to the center of the mutual stability region is governed by the
magnitude of the applied ac and dc potentials.
The equations of motion in a quadrupole system can be quite complicated.
Therefore, a brief discussion will be presented here and more information may be
obtained from the references cited within this section. The motion of the ions in
the x and y directions depends on the variation of the potential, expressed in the following equation [63],
0 - U - Veos ( of) (11)
where U is the dc voltage, V is one half of the peak-to-peak rf voltage and equals 2jif with f being the rf frequency. It was determined that for a sinusoidally
operated hyperbolic mass filter, the potential distribution ( expressed as [59],

4> - [U + Veos ( ü>f)] -í-*-—^
2^
48
(12)
where x and y are the distances along the given coordinate axes, rQ is the
quadrupole field radius. Equation (12) is then differentiated relative to the x and
y axes to calculate the intensity of the electric field (E) along the X-Z and Y-Z
planes [59]. The results are as follows:
Ex - -4* - -IU * Veos ( oí)] 4
ox /â– 
1 o
and
E,- - W* Veos (at)) X
These equations can then be mathematically converted to the Mathieu type
equations by first considering the equations of the force that an ion feels in each
plane and substituting Newton’s law (F = ma) [62]. The final results of these
calculations are shown in Equations (15) and (16),
+ [U+ Veos ( o)f)] - 0 (15)
mr0
(13)
(14)
-0
mr¡
and
- \U * Veos ( oí )J
dr
(16)

49
The solutions to these equations are usually unbounded, which corresponds to
the rejection of ions from the mass filter. Stable solutions may be obtained only
for certain values of m, rot f, U, and V. These solutions will correspond to
conditions that will give a successful transmission of an ion through the
quadrupole. The following substitutions (known as the Mathieu parameters) are
usually made to allow these equations to be written in a single expression [63].
The Mathieu parameters are:
a
4 eU
m a)
• n
(17)
and
Q -
2eV
mu2
(18)
These substitutions represent the dc voltage counterpart (a) and the rf voltage
amplitude (q) [59,63], A typical stability diagram is shown in Figure 14, which is
constructed by plotting dc voltages that yield stable solutions vs. rf voltages that
yield stable solutions. The area below the triangular region represents the values
of dc and rf voltages resulting in a stable ion trajectory for this mass.
To generate a mass spectrum, the alternating voltage, V, is scanned, with
the direct voltage, U, following it with a given ratio along a scan line that is
anchored at the origin in the diagram. Two different scan lines are shown in
Figure 14. The top one will result in more triangular peaks, but will have a lower
intensity since the area of stability traversed is small. The lower scan line will
increase the signal intensity of the ion but will result in broad trapezoidal peaks.

0.3
MASS SCAN LINE
(TRIANGULAR PEAKS)
0.2 -
0.1 -
0
RANGE OF a
GIVING
STABILITY
MASS SCAN LINE
(TRAPEZOIDAL PEAKS)
Figure 14 Mathieu stability diagram for a single mass-to-charge ratio in a
quadrupole mass spectrometer. [57]
cn
o

51
Each given mass will have a slightly different stability diagram. Figure 15
(modified from reference [63]) shows the stability diagrams for four different
masses superimposed on the same diagram with actual voltages on the axes.
This diagram may be used to demonstrate the principles of resolution and
intensity and how they interrelate. The mass scan line given is one that may
typically be used in GDMS where unit mass resolution is required. Note that as
the mass scan line passes out of the stable region for mass 28 it just begins to
enter the stable region for mass 29. This will give unit resolution. If the slope of
the line is decreased, then the mass scan line would intercept more of each ion’s
stable area (resulting in greater intensity), but for two adjacent ions the mass scan
line would pass through portions of their stability region at the same time. Thus,
the operator must realize that when using a quadrupole system there are trade
offs between good resolution and good signal intensity.
Ion detection
Both instruments have dual detection systems. Each has a Faraday cup
to detect the matrix species and other large ion signals. For the detection of
minor and trace species, the Extrel system utilizes a channeltron continuous
dynode electron multiplier (Galileo Electro-Optics Corporation) and the Finnigan
uses a discrete dynode electron multiplier (Balzers SEV 217, Liechtenstein). Both
of these detectors are set off-axis to the quadrupole axis. These detectors may
be operated in an analog mode that detects the small currents hitting the
detectors or an ion counting mode that will detect discrete events when single
ions strike the detector.

DC VOLTAGE
RF VOLTAGE
Figure 15 Mathieu stability diagram for several mass-to-charge ratios in a
quadrupole mass spectrometer (modified from [63]).
01
ro

53
Faraday cup. The Faraday cup is the simplest detector employed in
mass spectrometry. It consists of a metal plate (electrode) that is used simply to
collect the ions that strike it. The ions are neutralized on the surface of the
electrode by a transfer of electrons, and the current that results is equal to the
incoming ion current. The Faraday electrode is enclosed, except for a small
entrance aperture that allows the incoming ions to pass through. This shielding
will prevent secondary electrons from escaping before striking the electrode,
which could lead to erroneous readings. The detected currents on the Faraday
electrode are usually small and a high gain signal amplifier is needed. The
Faraday detector requires a well insulated system in order to reduce any leakage
currents. The minimum current that is detectable using this detector is
approximately 1014 amps [63].
Channel (continuous dvnodel electron multiplier. Channel electron
multipliers (CEMs) are widely used as detector devices in many quadrupole mass
spectrometer systems [64], The characteristics of CEMs that have lead to their
widespread use include their excellent signal-to-noise (with dark counts of less
than 0.5 counts per second), a stable dynode surface that can be exposed to air
without degradation of performance, low power requirements, compact size, and
a narrow gain distribution of output pulses. CEMs are constructed from a
formulation of glass that is heavily doped with lead. The fabrication produces a
glass that exhibits secondary emissive properties and resistive characteristics.
The Channeltron detector typically has a resistance of about 109 ohms. Electrical
contacts are deposited on both ends of the channel to allow electrical contact of

54
the CEM to an external high voltage. The external voltage will replenish the
charge on the channel wall, as well as accelerate low energy secondary electrons
to a level where they will create cascading secondary electrons upon collision
with the wall. Gain on the order of 108 is possible. This detector should be
operated in a vacuum of at least 10‘6 Torr, since higher pressures will lead to
increased background signal and the life of the detector will be shortened.
Ion detection in the CEM is as follows. If a particle has sufficient energy
to cause emission of at least one secondary electron upon striking the interior
surface of the detector then it may be detected. The secondary electron will be
accelerated by the electrostatic field inside the channel until it hits the surface
again, which should result in the release of one or more secondary electrons.
This cascading process may occur 10-20 times in the channel depending on the
design of the multiplier. The gain of a CEM does not depend on the length of the
channel or the diameter of the channel alone. The gain is a function of the ratio
of the channel length to its inner diameter [65] as well as a function of the
potential difference from end to end (i.e., the applied high voltage). Using this
knowledge, channeltron multipliers have been constructed with very small sizes
and in arrays. The detection efficiency of a CEM was found to be a function of
the incoming ion energy, its velocity, and the degree of ionization [66],
Discrete dvnode electron multiplier. The secondary electron multiplier
(SEM) operates in a fashion somewhat similar to the CEM. In the SEM, ions
coming from the mass spectrometer are accelerated by an electric field and strike
an electrode (or dynode), which is made of a material with a high coefficient of

55
secondary electron emission. These electrons will in turn produce more
secondary electrons that are accelerated by the field to a second dynode where
they hit and produce other secondaries. This process repeats until a large
number of electrons from the last dynode strike a Faraday plate, which generates
a large signal. The current is amplified as mentioned previously in the discussion
of the Faraday cup. The first dynode of the detector is usually operated at -1000
to -3000 volts that results in an overall gain of 104 - 108. Thus a current of 10'18
A would be multiplied to about 10'12 A, which is easily measured by the amplifier.
Analog filtering with a bandwidth of a few seconds is usually needed to obtain a
good signal-to-noise ratio [63]. The SEM, as well as the CEM, gives improved
detection limits and an improved response time over the Faraday cup, which
allows the instrument to be scanned faster.
One of the disadvantages with electron multipliers (both OEMs and SEMs)
is that the gain will change as the detector ages. This can be corrected by
measuring gain curves often and adjusting the operating voltage of the detector.
The gain is usually calculated by comparing the voltage detected on an electron
multiplier with that on the Faraday cup using the same ion beam. The SEM and
CEM can both operate in pulse counting modes, where each individual incident
ion produces one pulse. Each single ion that strikes the detector will produce
about 107 electrons, which is a readily detectable pulse. The detection limit of the
pulse counting mode is determined by the background count rate, which is
usually about 1 count per second. Pulse counting circuits are more expensive
than are analog measurement circuits; as a result, the ion currents should be

56
below about 1012 A for this method to be worthwhile. Also, if the current is too
high, pulse pile-up will occur in the detection system and the counting becomes
inaccurate.
Vacuum system
The vacuum systems of the two instruments differ, but the requirements
and care that must be taken to maintain a vacuum system are virtually the same.
The Extrel system uses two diffusion pumps (NRC, Model VHS-4), while the
Finnigan uses two turbomolecular pumps (the first chamber is evacuated with a
330 L/s pump and the analyzer chamber is pumped by a 170 L/s pump). One of
the difficulties that is encountered when merging a glow discharge system is the
pressure difference required to operate the source and the mass analyzer. This
problem is alleviated by the use of differential pumping through small apertures
between three chambers in the instrument. The ion source operates at a pressure
of about 1 Torr while the first chamber maintains a pressure of about 10 4 Torr.
These chambers are separated by a 0.5 mm ion exit orifice. The analyzer
chamber is maintained at about 10‘6 Torr during operation and is separated from
the first chamber by the small hole in the skimmer cone. The pressure is
measured in the ion source of both instruments with thermocouple gauges
(Teledyne-Hastings, Model DV-4D).
Data collection
Both instruments were operated primarily using a Hewlett-Packard 9816
computer system to control the mass scanning as well as collect the data. A
block diagram of the detection and data acquisition system for the Channeltron

57
detector of the Extrel instrument is shown in Figure 16. The system for the
Finnigan is virtually identical in its operation.
For detection in the analog mode the signal is passed through a
preamplifier and an electrometer. The signal is then directed to an oscilloscope
(Tektronix). The Extrel instrument’s oscilloscope (Tektronix Model 5223 with
digitizing time base Model 5B25N) allows real-time mass spectra to be observed
since the quadrupole controller operates in conjunction with the oscilloscope to
allow an external voltage ramp (from the time base output) to control the mass
scan. The analog signal may also be routed through a voltage-to-frequency
converter to measure the ion signals on a multichannel analyzer (MCA) (Tracor-
Northern TN-7200). Finally, the system is connected to the Hewlett-Packard
computer via an IEEE bus system to allow the signal to be converted to a digital
signal that may be stored and manipulated with the computer software.
In counting mode, the signal is fed into an amplifier system that will
eliminate any signal pulses that are below a given threshold (discriminator
feature). The amplification stage provides a high speed source of pulses (30 ns)
for signal counting devices. The signal can then be directed to the MCA and
collected as counts per channel dwell time. The HP computer may then be used
to read the MCA data for subsequent storage, data manipulation, and printing of
hard copies with the laser printer.
Neutral Species in the Glow Discharge Plasma
The mass spectrometer system will only allow the study of species that are
ionized in the glow discharge ion source. However, it is often advantageous to

DETECTOR
TTL
SIGNAL
CONVERTER
Figure 16 Possible modes for ion signal detection with an electron multiplier
detector.

59
correlate the ion population that is measured with the mass spectrometer to the
neutral and excited atomic population that exists in the ion source. This is
accomplished by interfacing the techniques of mass spectrometry with atomic
absorption (AA) to measure the ground state atomic species, or atomic emission
(AE) to measure the excited neutral species. These experiments were performed
only on the Extrel system since its six-way cross ion source will allow
simultaneous measurements to be made.
Mass spectrometer/atomic absorption system
A block diagram of the MS/AA system is shown in Figure 17. A
commercial neon or argon filled hollow cathode lamp (HCL) was focussed down
to a diameter of about 3 mm that was directly adjacent to the ion exit orifice. After
passing through the negative glow, the beam was focussed into a 0.5 M Ebert
monochromator (Jarrell-Ash, Waltham, Massachusetts). The intensity of the light
beam was detected by a photomultiplier tube (Hamamatsu R956, Bridgewater,
NJ) coupled with a photometer (Model 110, Pacific Instruments, Concord, CA).
The HCL was powered at a constant current of 6 - 10 mA and a peak voltage of
300 V by a DC power supply (Hewlett-Packard Model 6525A). The output of the
HCL was modulated at 50 - 100 Hz by a mechanical chopper (Scitec
Instruments). A reference signal from the chopper was directed to a lock-in
amplifier (Model 5301, EG&G/Princeton Applied Research, Princeton, NJ), as was
the signal from the photometer. The lock-in will discriminate against any
background emission signal that is produced in the negative glow. The output
signal from the lock-in amplifier was then directed into a voltage-to-frequency

Figure 17 Block diagram of the glow discharge mass spectrometry/atomic
absorption system.
O)
o

61
converter to allow the signal to be collected on an MCA. The MCA data can then
be read by the HP computer and manipulated or stored. The signal may also be
simultaneously monitored on an oscilloscope.
Mass spectrometer/atomic emission system
A block diagram of the MS/AE system is shown in Figure 18. The emission
experimental setup is basically the same as used for absorption, only much
simplified. With AE, there is no need for an HCL, chopper, or lock-in amplifier.
The emission signal is collected from the negative glow and focussed into the
monochromator with one pair of lenses. The detected signal is then fed directly
into the oscilloscope and voltage-to-frequency converter, and subsequently into
the MCA. The collected data is then stored and manipulated by the HP
computer. This system is much simplified since there is no background
subtraction requirement for emission measurements.

Figure 18 Block diagram of the glow discharge mass spectrometry/atomic
emission system.

CHAPTER 2
FUNDAMENTAL STUDIES OF SOLUTION SAMPLES IN
GLOW DISCHARGE MASS SPECTROMETRY
Introduction
As discussed in the first chapter, glow discharge mass spectrometry has
found the majority of its applications in the analysis of solid samples [67,68],
However, this chapter will focus on the introduction of solution samples into a
GDMS system. Solution analysis with a glow discharge is an advantageous
avenue to explore, since many samples already exist in this form. A scheme has
been developed that couples a glow discharge, using its ionization abilities, with
an electrothermal vaporization device, which is very efficient for sample
atomization. This method will reduce any need to manipulate the solution before
glow discharge analysis (i.e., mixing with a solid material), although the solvent
must be removed prior to placing the sample into the mass spectrometer, since
the glow discharge is normally operated at a low pressure (1 Torr). This method
uses electrothermal vaporization of the solution residue to introduce the analytes
into the glow discharge plasma.
The analysis of solution samples is an obviously important facet of
chemistry. Many classic analytical methods were developed from the needs for
solution sample analysis. One of the fastest growing modern techniques for the
analysis of trace elements in solution is the inductively coupled plasma (ICP) [69],
63

64
The detection of analytes in an ICP may be performed using atomic absorption
(AA), atomic emission (AE), and/or mass spectrometry (MS). ICP-MS uses
differential pumping and skimmer cones with small orifices (similar to that
described in Chapter 1 for GDMS) to allow the mass spectrometer to operate in
the region of 10-6 to 10'8 Torr, while the ion source operates at 760 Torr (1
atmosphere). The ICP technique is amenable to the introduction of a large flow
of solution, since the samples are introduced through pneumatic nebulizers that
remove most of the solvents. Solvent particulates that may enter the source can
produce detrimental effects in the plasma, and the properties of the aerosol
relative to the analytical performance of the ICP are of great concern. Differences
will occur in ICP emission signals depending on whether the incoming aerosol is
wet or dry [70], It was found that the small amount of residual water remaining
after the desolvator did not change the plasma conditions. When the dry aerosol
was mixed with H20/HN03 droplets, there was a reduction in the ICP emission
signal by 43.1 %. Compared to the GDMS source operation, the ICP is able to
handle solution samples more easily, due to its higher source operating pressure,
larger pumping capacity, and ability to handle large fluxes of material. The argon
flow alone into an ICP is in L/min, whereas the glow discharge only can handle
a few mL/min to maintain its operating pressure.
This chapter will begin by briefly reviewing some of the modern analytical
techniques that are used for the analysis of trace metals in small volume solution
samples. The emphasis of this discussion will center on methods using
electrothermal vaporization of the analyte, since the glow discharge solution

65
analysis system was developed using an electrothermal filament for sample
atomization. Finally, the experiments performed during the development of this
method will be discussed.
Electrothermal Vaporization
Electrothermal vaporization (ETV) is an important technique that is used to
provide controlled and fast atomization of a sample deposited onto an
electrothermal device. This device can take many different forms, such as a
filament of graphite furnace. ETV occurs when a sample, is heated to a
temperature above the vaporization point of the elements to be analyzed. As a
result, neutral and ionic elemental species are released from the electrothermal
device. The atomic population produced may be detected and analyzed by a
variety of methods, including AE and AA. The atoms may then be subsequently
excited (either from the ETV energy itself, or by other external methods) to
produce ions that can be detected by ionic emission, ionic absorption, or, as in
the present study, quadrupole mass spectrometry. The following sections will
give some background and examples of some techniques using ETV.
Furnace Techniques
Many solution analysis schemes use a heating device (such as a flame or
furnace) to produce excitation of the species to be analyzed. This section will
describe some techniques that use a graphite tube furnace in conjunction with a
glow discharge hollow cathode plasma to study the atoms and ions from a
solution sample. The methods discussed here rely on the glow discharge
processes to provide the excitation or ionization, so they are termed "nonthermal

66
excitation" methods. The ETV/GDMS method that was developed for the solution
analysis to be described later was based on some of the principles of these
techniques.
Furnace atomic nonthermal excitation spectroscopy
The solution sampling method reported in this dissertation is a variation on
the furnace atomic nonthermal excitation spectroscopy (FANES) technique [71].
In FANES, a thin layer residue is analyzed after solvent evaporation of the solution
sample. The evaporation of the solvent and removal of the analyte from the
graphite furnace surface is accomplished by varying degrees of heat generated
in the furnace by a high current flow. The analyte is then detected by atomic
emission from the graphite tube glow discharge. A block diagram of the FANES
source is shown in Figure 19. The analysis procedure is as follows: 1) pipetting
the sample at atmospheric pressure, 2) drying the sample at atmospheric
pressure, 3) ashing the sample at atmospheric pressure, 4) pumping out the
source, 5) filling the source with argon, 6) ignition of the hollow cathode
discharge, 7) setting the power supply for atomization, 8) detection of the
emission pulse, and 9) cooling the source. Each measurement cycle lasts about
one minute and is under microprocessor control. The discharge used for FANES
is the hollow cathode type that was discussed in Chapter 1. The FANES
technique was first reported by Falk et al., who described a new emission source
with independent atomization and excitation processes [71,72], The source was
found to have limits of detection comparable to flameless atomic absorption, while
orders of magnitude better than ICP emission. Detection limits of the FANES

ANODE
NEGATIVE
GLOW
WATER-COOLED.
CHAMBER
SAMPLE
INJECTION
0^0
CURRENT
SOURCE
GRAPHITE
ELECTRODE
WINDOW
GRAPHITE
FURNACE
(HOLLOW
CATHODE)
Figure 19 Schematic diagram of a furnace nonthermal excitation spectroscopy
(FANES) source.

68
technique, using a 50 /¿L sample, were in the range of 0.0007 to 16 //g/L
depending upon the element under study. Also, the FANES method was shown
to exhibit a high dynamic range and multi-element capability. This technique has
been compared to other atomic spectroscopic methods in a paper by Falk et at.
[73]. Similar to FANES, the furnace ionic nonthermal excitation spectroscopy
(FINES) technique has been used to measure the emission from ions that are
produced in the source described above. Since this method is also considered
nonthermal, the ions are produced from energy provided by the glow discharge
plasma in the source.
Molecular nonthermal excitation spectroscopy
The technique of molecular nonthermal excitation spectroscopy (MONES)
was developed for the studies of molecules in the FANES source. This method
has been used for the determination of negative ions that combine with metallic
species. For instance, the halides F and Cl' have been determined by the
nonthermal excitation of MgF and MgCI molecules [74], For this determination,
the glow discharge was formed within a graphite tube operated at 2000 °C. The
solution containing the halide is mixed with a solution containing the Mg2+ ions
as an additive, with a concentration higher than the halide to promote reaction to
form the MgX molecule. Upon formation of the MgX in the gas phase, molecular
absorption measurements were made and detection limits of 0.5 ng of fluorine
and 0.24 ng of chloride were obtained. This method shows how reactions may
be carried out and studied in an electrothermal device. MONES experiments
were developed after preliminary FANES studies indicated the formation of free

69
atoms and small molecules in these discharges [74,75,76], Recently a microwave
induced plasma has been coupled with the MONES method [77]. In these
experiments, nitrogen and oxygen were determined in gaseous and aqueous
samples through the formation and emission of NH and OH radicals.
Introduction of Micro-Samples into an ICP
As pointed out in the introduction, ICP methods have provided a means for
analyzing solution samples that are introduced into the torch in vast quantities
with a large supporting gas flow. The ability of the ICP to desolvate a solution
sample and submit ions to the mass spectrometer makes it a very prominent
technique for bulk liquid analysis. However, if the amount of solution to be
analyzed is in short supply and only a small quantity is available, a conventional
ICP cannot be used.
Recently, interest has grown in measuring microliter samples with the ICP
by interfacing it with an electrothermal source. This was performed by attaching
an ETV device to the ICP torch using 1 - 2 feet of plastic tubing. The ICP argon
gas is passed through the graphite furnace as the sample is vaporized, producing
an atomic population that is representative of the sample. These atoms are then
swept, by the gas flow, into the ICP torch for subsequent ionization. The ICP may
be operated in single ion monitoring mode to obtain the maximum sensitivity for
a given element or in mass scanning mode to provide multi-element analysis.
The advantages of coupling ETV and ICP-MS include detecting elemental sample
amounts in the lowfemtogram (fg) region, performing trace analysis on very small
volume samples (wg or /uL), and alleviating the matrix and solvent interferences

70
that may be encountered with a typical ICP-MS analysis. Sensitivity values from
20 - 80 counts/fg have been reported, resulting in detection limits in the low fg
range [78]. The measurement precision is in the range of 5 - 20 %. Calibration
curves have been found to be linear over 2 - 3 orders of magnitude, although
quantitation usually requires using the standard additions method for analysis.
Memory effects and matrix influences are considered serious constraints for the
ETV-ICP/MS technique and these problems are under investigation.
Solution Samples in the Glow Discharge
Introduction
As previously mentioned, the most common samples analyzed in GDMS
are solid materials in the form of pins machined from bulk solids or disks formed
in a die using powdered samples. The research presented in this chapter is on
the development of a method to introduce small volume aqueous solution
samples into the GDMS ion source to broaden its range of application. Since the
glow discharge source requires operation in a low pressure environment, it is not
amenable to the introduction of solvents in a sample. If a solution sample is
introduced into the discharge plasma without desolvation, it may extinguish the
negative glow. Therefore, strict control is needed to limit the amount of solvent
that is brought into the source. Some methods have been employed over the
years in an attempt to provide GDMS solution analysis and reduce solvent effects
in the vacuum system. The methods employed generally require placing the
sample onto a metal substrate and evaporating the solvent, leaving a thin film of
solution residue (containing the analyte atoms of interest) on the cathode surface.

71
Methods that were performed earlier in this laboratory involved the use of a
copper rf hollow cathode source for solution analysis [79], as well as applying a
solution to a cupped cathode [80]. In these experiments, the applied sample was
evaporated, and then atomized and ionized by the discharge processes. This
allowed for analysis of the solution residue by sputter atomization into the glow
discharge.
Electrothermal Vaporization/Glow Discharge Techniques
When combining electrothermal vaporization and glow discharge ionization,
the strengths of both systems may be used. The electrothermal source provides
a high current that will sufficiently atomize the solution residue in a short time,
while the glow discharge is a very efficient source for excitation and ionization
because of the energetic plasma processes. The glow discharge can be used by
itself to sputter atomize the sample, but this process is limited by the lower
currents allowed in the discharge. Thus, sputter atomization is a slower process
that releases the sample over a longer time, reducing sensitivity. For optimum
sensitivity, the cathode should be carefully chosen to be nonporous. If the
cathode is porous, the solution residue will reside at least partially inside the
cathode and will take even longer to analyze. The analyte signal in this case will
be constant and long lived, but the detection limits will be reduced. An alternative
method has been investigated where the solution was mixed with a conducting
powder, dried, and then pressed into a discharge cathode for subsequent
analysis [81 ]. The method is thought to be a step forward in the construction of
specific standard cathodes to be used for quantitative glow discharge analysis

72
with relative sensitivity factors, since solid standards containing the elements of
interest are sometimes hard to find.
Electrothermal vaporization/qlow discharge atomic emission
Electrothermal vaporization into a glow discharge (GD) emission source
has been demonstrated, using a tungsten wire for atomization of solution samples
[82]. In these experiments, silver and boron solutions were studied using atomic
emission. The discharge was operated at a current of 60 mA and at 1.2 Torr
argon pressure. The maximum filament current of 16 A was used for sample
vaporization. The effect of various solution additives on the emission signal was
also studied and the presence of up to 1000 ppm of aluminum, calcium,
magnesium or sodium had little or no effect when added to a silver solution.
Electrothermal vaporization/qlow discharge mass spectrometry
In ETV/GD analysis, the heating of the filament serves as the primary
atomization step. Although some atomization will still occur by glow discharge
sputtering, the amount of sample loss by sputtering will depend on the
temperature of the filament and the time required for the solution residue to be
removed by thermal methods. The glow discharge provides the only apparent
mechanisms for ionization for the analyte under investigation, but the lack of ionic
detection in the absence of a glow discharge is not conclusive evidence that ions
are not being formed directly from the energy provided by the filament. The ions
formed, if any, may be of the wrong energy to pass successfully through the ion
lenses of the mass spectrometer and make it to the detector. One disadvantage
of the ETV method is the need to devise detection schemes that will accurately

73
handle transient signals. The signal peak that is produced from a small volume
solution sample will typically last only a few seconds, thus fast detection systems
are needed.
The schematic diagram shown in Figure 20 shows a simplified overview of
the plasma and its processes for ETV/GD analysis. The analyte to be studied (Zn
in this case) is shown to reside on the surface of the cathode (the tungsten
filament). The plasma sputtering process will liberate both tungsten and zinc
atoms from the surface as the argon ions bombard the filament. This results in
some loss of analyte before initiation of the filament current, but the discharge
"on" time will be minimized in the sampling scheme, as described later. As with
a normal glow discharge, Penning and electron ionization will occur with both the
solution residue and filament material atoms.
The results presented in this chapter are intended to show the usefulness
of this method for introduction of solutions into the glow discharge. Most of the
experimental data are concerned with the fundamental processes that occur in the
glow discharge and demonstrate how different characteristics of the discharge
environment (such as pressure, voltage, and current) may affect the ion signal
and change the analytical usefulness of the method. Also, the presence of a
filament carrying current in the vicinity of the glow discharge may show some
positive or negative effects on the ion transmission and the signal detected by the
mass spectrometer.
This ETV/GD method is of interest as well since it provides a separation of
the atomization and ionization steps in the analysis. Experiments that separate

74
Figure 20 Simplified diagram of the glow discharge processes
occurring in the ETV/GD system.

75
atomization and ionization are always of interest since this provides another
dimension with which to evaluate the fundamental processes of the glow
discharge. Other experiments have been performed to show the utility of
separating atomization and ionization steps in the glow discharge analysis. It has
been shown that laser ablation can provide a means of atomizing the sample into
a discharge plasma, which subsequently provides the ionization mechanism [83],
These studies show that the atomization and ionization processes are both
controlled by the laser and discharge parameters. In parallel, the primary
atomization may be thought to occur upon heating the filament, while the primary
ionization is provided by the glow discharge.
Ion Source Development
Experiments in a Six-Wav Cross
The initial studies were performed using the Extrel ion source (six-way
cross) that was described in Chapter 1. A diagram of this source, with the ETV
device in place is shown in Figure 21. The source allows easy access to all of
the components inside and application of the solution sample to the filament. A
dual high voltage feedthrough, mounted in a 2.75" rotating flange (MDC High
Vacuum Products Corp.; Hayward, CA), was used to hold the filament and its
supporting assembly. This allowed for ease in filament alignment and
replacement. The filament loops for these experiments were 4 mm in diameter
and 2.75 mm long and were constructed by coiling a rhenium ribbon 3 turns
around a 3.2 mm metal rod. The filament was held in place by two stainless steel
rods and a small copper support wire in such a manner that the center of the coil

RHENIUM
FILAMENT
FILAMENT
FEEDTHROUGHS
(HV AND/OR CURRENT)
TO
MS
OPTIONAL
DISCHARGE
CATHODE
Figure 21 Diagram of the ETV/GD ion source used for the initial
experimentation.

77
was aligned coaxially with the exit orifice. This was done because the majority of
the solution residue will lie on the inside of the filament loop and ions traveling out
from the center of the coil will most likely be directed through the ends of the coil
and toward the exit orifice. Set screws secured the support rods to the dual
feedthrough as well as mount the filament in a fixed position. Glass shields were
constructed to protect the stainless steel rods from the discharge environment,
allowing only the filament loop and about 10 mm of a copper support wire to be
unprotected. The external cathode was mounted on the end of a probe and
placed in the mass spectrometer through a bellows assembly that allowed for
placement of the cathode in a variety of positions around the filament coil. The
discharge was operated in both constant current and constant voltage mode
using a Heinzinger HNCs 2500-150 ump. power supply, allowing operation up to
a maximum of 2500 V or 150 mA. The studies discussed here were performed
at about 1 mA current and voltages ranging from 400 - 800 V, with pressures in
the range of 0.45 - 2.5 Torr. The current loop was powered by a MAT 261
filament power supply (Finnigan MAT; Bremen, Germany), which allows up to 10
A of current to be supplied to the filament for sample atomization. The maximum
current varied from 4 - 6.6 amps, depending on the filament material and the
analyte to be studied. Table IV shows some materials that were initially studied
in order to find an appropriate filament to provide a "white hot" glow capable of
vaporizing the analyte of interest. The filament power supply was insulated since
it floats at a high potential when the filament was used as the discharge cathode.
A stepper motor assembly was constructed to control accurately the output

78
TABLE IV. Properties of Some Possible Filament Materials
Material
Diameter (mm)
Maximum
Current (A)
Observations
Al
0.25
4
glows & melts
Brass
1.2
6.6
—
Chromel
0.35
2.6
red glow
Cu
0.9
6.6
—
Ni
1
6.5
—
Re (ribbon)
0.1 x 0.75
4.2
white glow
Ta
0.015
4.5
white glow
W
0.010
5.5
white glow

79
current of the MAT 261. The current was supplied to the filament by RG-59
coaxial cable and high vacuum feedthroughs.
Filament on a Probe
A problem with this type of source is that there may be slight differences
in the construction of individual filament coils. Also with the rigid support system
there is no way to optimize the distance from the cathode to the exit orifice, which
is an important parameter for detecting the maximum ion signal with GDMS.
Figure 22 shows how the ion signal of tungsten, sputtered from the coiled
electrothermal filament, varies with the cathode-to-exit orifice distance. It can be
seen that the maximum ion signal occurs when the cathode is about 7 mm away
from the ion exit orifice, a typical value for a 1 Torr glow discharge. With these
problems at hand, it was realized that a direct insertion probe (DIP) should be
used in an effort to solve the problem of positioning. A 0.5" outer diameter DIP,
shown in Figure 23, with a 10-turn tungsten filament was constructed to allow the
introduction of a solution residue sample into the ion source without venting and
with the ability to position accurately the filament coil with respect to the ion exit
orifice. The DIP also will allow the use of an auxiliary cathode, if so desired, since
the probe contains four vacuum feedthroughs. Two of the feedthroughs carry the
electrothermal current and discharge voltage for the filament, while one may carry
voltage for an auxiliary discharge cathode. Initial experiments using the DIP show
comparable results to those obtained in the six-way cross, although the ion signal
can be maximized with the DIP due to its flexibility.

186W Ion Signal (counts x1 06)
Figure 22 Effects of the cathode-to-ion exit orifice distance on the ion signal
obtained from a sputtered tungsten filament coil.
00
o

1/2" STAINLESS STEEL
PROBE BODY CERAMIC
Figure 23 Diagram of the ETV/GD filament cathode mounted on a direct
insertion probe.
00

82
Experimental
Sample Preparation
Selutiens were prepared by mixing an appropriate ameunt of a metal
nitrate with distilled water to obtain the desired cation concentration. The
chemicals that were used are as follows: nickel nitrate {Ni(N03)2- 6H20}
(Mallinckrodt Inc.; Paris, Kentucky; Analytical Reagent Grade), zinc nitrate
{Zn(N03)2-6H20} (Fisher Scientific; Fair Lawn, NJ; Certified A.C.S.), cupric nitrate
{Cu(N03)2- 21/2H20} (Fisher Scientific; Fair Lawn, NJ; Certified A.C.S.), ferric
nitrate {Fe(N03)3-9H20} (Mallinckrodt Inc.; Paris, Kentucky; Analytical Reagent
Grade), and silver nitrate {AgNOJ (Mallinckrodt Inc.; Paris, Kentucky; Analytical
Reagent Grade). Enough concentrated nitric acid (Fisher Scientific; Fair Lawn,
NJ; Reagent A.C.S. grade) was added to the solutions to provide an acidic
concentration of 1 - 2 % HN03. Fundamental studies of the source properties
were carried out with analyte concentrations of approximately 1000 ppm.
Sampling Sequence
The sampling sequence that was devised for the electrothermal
introduction of solution samples went through a variety of stages in its
development. There were a few considerations to make in order to ensure the
most accurate analysis of the samples. First, the solvent on the filament had to
be completely evaporated to minimize any detrimental effects that may be
obtained by excess water being introduced into the mass spectrometer. Second,
the current used for solvent evaporation had to be less than that required to
remove the analyte from the filament. Third, the filament material should be

83
stable for the evaporation step. Some of the filament materials could not
withstand the lower evaporation currents when carried out in an air atmosphere.
This was particularly true for the rhenium ribbon, which would completely oxidize
and break unless the evaporation was performed at a low pressure 20 Torr)
or in an inert gas atmosphere. Finally, the discharge operation time before the
filament current is initiated should be long enough to ensure a stable discharge,
but as short as possible to minimize any loss of sample analyte due to sputtering.
The final version of the sampling sequence that was used for sample
introduction on the DIP is as follows: After bringing the source up to atmospheric
pressure, the solution samples (ranging from 10 - 40 /¿L) were directly pipetted
onto the filament loop by a microliter pipet [Volac Model # R 880/A, 5 - 50 ¡uL
adjustable volume; Great Britain], The solutions were evaporated at about 1.5 A
filament current for 2 min. The current was increased to 2 A, and the evaporation
step continued another 2 min (after this time the coil surface appeared to be dry).
The probe was placed into the instrument and the roughing pump applied for 1
min. The sample was allowed to sit under high vacuum conditions for 2 min after
placing it into the ion source. The argon was then introduce at the working
pressure, usually 1 Torr, and the pressure was allowed to equilibrate for 1 min.
The detector was then turned on for 30 s. The MCA was turned on for 5 s to
measure any background signal that may be present. The discharge was turned
on and allowed to run for 5 s. The filament current supply was turned on to the
appropriate atomization current for the sample. After data collection, the filament
was allowed to continue heating for 2 min to drive off any remaining species and

84
reduce memory effects. The filament assembly was allowed to cool for about 5
min before another cycle was repeated with the next sample.
Data Acquisition
When studying solution samples with the electrothermal source, the data
obtained consisted of a transient signal lasting a few seconds. A schematic
representation of the data acquisition scheme is shown in Figure 24. The
quadrupole was set to pass the mass of interest and the signal from the detector
was monitored using the MCA. The MCA was first turned on to allow detection
of any background signal. The discharge was turned on, at t = 0 s in the figure,
for 5 s to allow stabilization of the discharge voltage. This time was chosen as
the shortest time to allow a pre-sputtered filament coil to attain a stable discharge.
This time was minimized to allow most atomization to take place due to the
electrothermal vaporization. The filament was then turned on, at t = 5 s in the
figure, and quickly rises to the desired current. It may be noted that the spikes
that are produced at this time are due to electrical noise in the system upon
initiation of the filament current. After about a 1 s delay, the ion signal from the
solution residue is detected as a peak.
Filament Materials
The results that were obtained with different filament materials are
discussed in this section. The materials under active investigation were rhenium,
tantalum, and tungsten. These were the only three filaments that resulted in the
"white hot" glow required for vaporization of the samples (see Table IV for the
filaments attempted and some observations). The initial study used a rhenium

85
ON
OFF
STABILIZATION
PERIOD
GLOW DISCHARGE
ON
OFF
ELECTROTHERMAL FILAMENT
Figure 24 Diagram of the relationship between the detected ion
signal with respect to the operation of the glow discharge and
electrothermal filament.

86
ribbon as the atomization source. Studies were performed two ways: 1) with the
filament acting as the discharge cathode for ionization, and 2) with ionization
being provided by the discharge supported by an auxiliary cathode (as shown in
Figure 21). Both methods produced comparable ion signals for silver solutions,
but the latter technique was subsequently adopted because of its simpler design.
If the rhenium ribbon is subjected to multiple runs, the glow discharge sputtering
process will "eat" through the ribbon and cause a discontinuity in the current loop.
Tantalum wires were also tried as an ETV filament but they too degraded very
quickly. The majority of the experiments was performed using a tungsten wire.
This wire was the most stable of the three in terms of producing sample ion
signals and the long term ability to withstand the sputtering processes.
Filament-Plasma Interactions
The glow discharge processes and operation may be disturbed by the
operation of the electrothermal device within the ion source region. This section
will describe some considerations that must be made when coupling an
electrothermal device, carrying a high current, with the glow discharge device.
The experiments described in this section were performed to observe the changes
in a typical glow discharge signal when applying a current to the electrothermal
filament.
Ion Transmission
The first concern when using the static coil experimental setup was the
effect that the filament current would have on the transmission of ions from the
auxiliary pin cathode. In these experiments, the discharge cathode was separate

87
from the filament and the cathode pin could be positioned around the filament.
Different positions were investigated for the placement of the discharge pin
cathode. These included placing the pin behind the filament, such that atoms
from the pin must traverse through or around the filament to reach the exit orifice,
placing the pin inside the filament and co-axial to the ion exit orifice, causing the
negative glow region to be confined in the cylindrical space defined by the coil,
and placing the pin beside the filament coil. The ion signal transmission was
found to be greatest with the pin beside the filament coil, so this configuration
was used. This orientation was also allowed with the direct insertion probe.
Effects of different filament currents
Changes may occur in the glow discharge processes when the filament is
carrying a relatively high current (three orders of magnitude more than the
discharge itself). A glow discharge pin cathode was operated in the constant
voltage mode while being positioned beside of the filament. As the filament
current was increased from 0 to about 5 A, the discharge current was monitored.
The results of this experiment are shown in Figure 25. The glow discharge
current is observed to decrease slightly over the range of 0 - 4 A of filament
current. After this point the glow discharge current shows a dramatic increase.
This may be the result of electron emission from the filament, which may add to
the flow of electrons toward the anode and thus increase the measured current
of the gaseous conductor.
The next experiments were performed to observe how the filament current
would affect the ion signals when the electrothermal filament itself was used as

Glow Discharge Current (mA)
Figure 25 Effects of the electrothermal filament current on the glow discharge
current when the glow discharge is operating at a constant voltage.
00
00

89
the discharge cathode, since this much simpler experimental design was used
most often. The effect of filament current on the Re+ signal is shown in Figure 26.
The reported signal is the peak height of the 187Re+ isotope after scanning a
mass range containing the rhenium isotopes 25 times. The initiation of the
filament current results in a fairly constant decline of the ion signal, when
operating with a 1 mA constant current discharge. The discharge voltage
changes from 598 V with no filament current to 740 V with 3 A filament current.
Overall, there is about a 67% decrease in the ion signal from 0 - 3 A, probably
due to changes in ion transport induced by the current present near the plasma,
possibly as a result of an induced magnetic or electrical field in the area.
These changes were also observed when running the discharge at a
constant voltage of 500 V. Figure 27 shows the ion signals for some gaseous
species (H20 at m/z = 18, COH and N2H at m/z = 29, and ArH at m/z = 41) and
one sputtered species (Re at m/z = 187) with increasing filament current. All of
the ions in this experiment respond in a similar manner and seem to follow the
same trend as the discharge current (as seen in Figure 25). Thus, it appears that
the filament current (and the subsequent changes in the discharge current) has
a major effect on the magnitude of the ion signals obtained from the discharge.
This observed increase at the higher current led to subsequent operation of the
discharge in the constant voltage mode for the analysis of solutions, since high
currents will most likely be used and this will result in a higher ion signal.
However, analyte signals from solutions may respond differently depending on
their atomization temperature.

5
Filament Current (A)
Figure 26 Effects of the electrothermal filament current on the 187Re ion signal
obtained from a rhenium filament when the glow discharge is operating at a
constant current.
co
o

Ion Signal (V)
Filament Current (A)
Figure 27 Effects of the ETV filament current on various ion signals obtained
from a rhenium filament when the glow discharge is operating at a constant
voltage. A) a-18(H20)+; B) •-29(COH)+ and N2H+; C) ■-41(ArH)+, D ♦-187Re+.

92
Internal ETV Sample Introduction
The internal ETV setup that was used for the majority of the fundamental
solution sample studies for this dissertation is shown in Figure 28. This method
is termed "internal" since the vaporization occurs within the discharge plasma, and
the negative glow is supported by high voltage applied directly to the filament.
This method was chosen as the preferred method to use since it is the most
simple with respect to the instrumentation requirements. One of the problems
that occurs with internal analysis is that the discharge plasma will erode the
filament material over time and may lead to inconsistencies in the atomization
temperature obtained for a given filament. This is because the filament current
power supply output is limited by the wire resistance. As a filament is used and
eroded, the power supply maximum output will decrease.
Effects of Different Filament Currents
A parameter that had to be determined for each individual element was the
current required to vaporize sufficiently the analyte. Figure 29 shows the ion
signal that was detected for a 1000 ppm iron sample that was subjected to
various filament currents. Figure 29A shows that when operating with no filament
current the ion signal is very low (about 30 counts per MCA channel), but is long
lived. Over the observation period shown (about 200 s) the ion signal is constant.
Figure 29B shows the ion signal obtained when the filament is operated at 1 A
(turned on at t = 5 s). The ion signal starts out high and falls to a plateau value
that is higher than observed for operation with no filament current. The initial
signal peak is probably due to the initial sputtering of the surface residue and

TO MS
A
93
NEGATIVE
GLOW
\ION SOURCE
(ANODE)
FILAMENT
(CATHODE)
Figure 28 Diagram of the internal ETV ion source.

Ion Signal (counts) Ion Signal (counts)
94
1000-.
800-
600-
400-
A
200
iirrm H if^ü iijjr
40
i ■“I"
80
120
160
200
Figure 29 Ion signal detected from a 20//L sample of 1000 ppm
iron that was collected while using various electrothermal filament
currents. A) 0 A; B) 1 A; C) 2 A; D) 3 A.

Ion Signal (counts X10 3) lon Si9nal (counts)
95
Figure 29 - continued

96
equilibrium of the discharge plasma. This increase in plateau signal may be due
to the slight heating of the filament material. Figure 29C shows the ion signal that
is obtained when the filament current is operated at 2 A. Again, there is an initial
decrease in ion signal as the filament begins to sputter, but when the filament
current is initiated a signal peak is produced. This peak is narrow and lasts
approximately 5 s, but produces a higher ion signal. The initiation of the filament
current causes a temporary increase in the ion signal as the filament is heated,
but the signal then falls to a plateau value that is higher than observed with 0 or
1 A filament current. The filament current was finally operated at 3 A (see Figure
29D). The initiation of a 3 A filament results in a large ion signal peak that is at
a much higher intensity than any signal previously observed. The maximum ion
signal obtained was about 400,000 counts. Since the ion signal obtained in these
experiments was reported as the area under the peak, it can be seen how the
sensitivity of the ETV/GD technique is increased by using a filament current that
quickly removes the analyte, rather than sputtering the thin film residue to remove
the analyte more slowly.
Response to Concentration Changes
A problem with using this method for quantitation is that the absolute
counts detected for a given sample may differ as a result of changing the filament
as needed. However, the ion signal should be consistent when compared using
a known concentration and the same filament. A series of Ag+ solutions was
prepared having concentrations of approximately 1, 10, 100, 250, 500 and 1000
ppm. The results obtained using one filament are shown in Figure 30. The

07Ag+ Ion Signal (counts x 1 03)
Figure 30 Effects of the silver concentration on the 107Ag ion signal.
CD
"si

98
concentration range shown here is quite small, only three orders of magnitude,
but the samples used in the experiments described here are within this range.
The plot is linear in the lower concentration range, with some deviations being
encountered at the higher sample concentrations. The error bars in the figure are
calculated as the relative standard deviation for a series of three separate runs.
The inconsistencies that were obtained from run-to-run will be discussed further
below.
Pressure Effects
The glow discharge will operate in a stable manner in the pressure range
from a few tenths of a Torr up to five Torr. The purpose of this experiment was
to locate the pressure resulting in the largest ion signal from a given amount of
analyte. Figure 31 shows two plots that were obtained using an iron pin (in a
typical GDMS configuration) while measuring the ion signal obtained at a variety
of pressures. The upper curve was obtained after tuning and optimizing the
system at a pressure of 1 Torr, while the bottom curve was tuned at the low end
of the experimental range (0.45 Torr). Although the intensities of the ion signals
obtained under these different tunings are different, the general trend shows that
for a normal sputtered species the maximum ion signal is obtained at 1.2 - 1.3
Torr argon. This trend is comparable to previous studies of glow discharge
pressure effects. As the pressure is increased or decreased from its maximum
a sharp decline is seen in the ion signal. As pressure decreases, the argon
metastable population falls off causing an ion signal decrease. This is because
Penning ionization, via argon metastables, is the predominant mechanism for

56Fe+ Signal Intensity (V)
Pressure (Torr Ar)
Figure 31 Effects of ion source pressure on the “Fe ion signal intensity from
a stainless steel cathode. The instrument was optimized at: A) 1 Torr argon and
B) 0.45 Torr argon.
CO
CO

100
ionization in the glow discharge. As pressure increases, a point is reached where
the destructive collision rates of the argon metastables will exceed their creation
rate, also causing a decrease in ion signal [84].
The ion signals from a silver sample being vaporized from the filament
surface were also collected at a variety of source pressures. Figure 32 shows the
plot of the ion signal from a 100 ppm Ag+ solution. The ion signal reported was
the total counts reaching the detector in a period of 15 s totally encompassing the
transient ion signal peak. The maximum operating pressure was determined to
lie between 1.0 -1.2 Torr argon. This is in agreement with that obtained using a
normal pin cathode.
Cathode to Exit Orifice Distance
As previously mentioned, one of the primary motivations for designing the
filament source on the probe is the added flexibility of positioning the filament
within the ion source. Figure 22 was presented in an earlier section and shows
the typical ion signal response for a sputtered species from a solid cathode as the
cathode-to-ion exit orifice distance is changed. On the other hand, Figure 33
shows the ion signals from an electrothermally vaporized species (in this case
zinc), as well as from the filament (tungsten) as the cathode-to-exit orifice distance
was changed. Dramatic differences in the optimum sampling distance are
observed between the pin and filament sources. With the presence of a 4 A
filament current and a glow discharge operating at 10 mA the optimum sampling
distance has decreased from about 7 mm to approximately 3.5 mm. This may be
a result of the changes that are occurring within the plasma. The large flux of

07Ag+ Ion Signal (counts x 1 03)
Pressure (Torr Ar)
Figure 32 Effects of ion source pressure on the 107Ag ion signal intensity from
a 2QmL-100 ppm sample.

Relative Ion Signal
from a sputtered species (■- 186W+) and an electrothermal species (•- 64Zn+) while
using a 10 mA constant current discharge and a filament current of 4 A.
102

103
electrons from the filament, or possible differences in the energy of the ions
formed with this source may contribute to this difference. Consequently, further
experiments were performed at this distance to obtain the highest ion signal.
Effects of Sample Positioning on the Filament
One parameter that may cause problems in the ETV/GD technique is where
the sample is placed on the filament. Using a 20 ¡uL sample size, the sample
could be placed in about four distinct locations on the filament. Figure 34 shows
the ion signal obtained in these four positions, where position 1 is the end of the
coil farthest from the ion exit orifice, and position 4 is the end closest to the exit
orifice. There is a generally linear increase in the ion signal as the position of the
sample is changed from one end to the other. Thus, care must be taken in
placing the sample in a reproducible position on the coil. If the maximum ion
signal is desired, then the sample needs to be placed on the end closest to the
ion exit orifice.
Memory Effects of the ETV/GD Technique
Another consideration for ETV analysis is the background signal that will
be obtained when the solvent alone is placed on the filament and analyzed by the
mass spectrometer. Figure 35A shows the signal obtained for a 20 ¡uL sample of
a 1000 ppm silver solution. In the next run, a blank sample (containing only the
acidic distilled water) was loaded onto the filament coil and the ion signal
obtained is shown in Figure 35B. The total ion signal for the silver sample is
about 300 times larger that for the blank.

700
600
500
400
300
200
100
0
2 3
Sample Position
34 Effects of the positioning of the sample application on the ion signal,
n 4 is closest to the ion exit orifice.
104

Ag Ion Signal (counts) Ag Ion Signal (counts x 10
105
CO
Figure 35 Silver ion signals with the ETV/GD system to determine
the presence of memory effects. A) 2QaL sample of 1000 ppm silver
solution; B) 2Qul_ of a blank solution.

106
Effects of Contaminant Gas Introduction
The inconsistency of this method can be attributed mostly to the
introduction of contaminant gaseous species (air and water vapor) brought into
the ion source with each sample. This may be seen by two different experiments.
In the first experiment, samples were run in succession and the ion signal is
observed to decrease with each sample (probably due to the cumulative effects
of constantly introducing more contaminant species). For a 20//I sample of 1000
ppm Ni+ the Tollowing ion signals were obtained for four consecutive runs: 6.16
x 106, 5.61 x 106, 3.20 x 106, and 3.60 x 106. After these four runs, the probe was
allowed to remain in the high vacuum of the mass spectrometer for 90 min. The
next solution sample produced 6.30 x 106 counts, which is close to the signal
obtained for the first run. Subsequent runs again showed a continual decrease
in ion signal counts. Similar trends were observed for other elements analyzed
by this ETV/GD technique. This is probably due to detrimental effects on the
ionization processes in the negative glow, since previous experiments that were
performed with ETV/GD atomic emission showed fairly consistent data [82]. Also,
the contamination effects may be studied by evaporating the solvent in an argon
atmosphere versus an air atmosphere. When the ion source on the Extrel
instrument was used, an argon atmosphere was employed. For a 20 juL sample
of 100 ppm zinc, ion signals of 3.68 x 105, 3.66 x 105, 3.47 x 105, and 3.21 x 105
counts were obtained. If air were drawn into the cross during solvent evaporation,
the following ion signals were obtained: 1.48 x 105, 1.15 x 105, 1.41 x 105, and
1.81 x 105 counts. Thus, it can be seen that this method is very susceptible to

107
atmospheric conditions in which the evaporation step and sample loading are
performed. Chapters 3 and 4 of this dissertation will discuss the effects that water
vapor has on the processes of the glow discharge, since this is probably the most
detrimental contaminant for these aqueous solution studies.
External ETV Sample Introduction
The external ETV setup is shown in Figure 36. The glow discharge is
supported by a pin cathode and the atomization filament is external to the
negative glow. The atoms are required to diffuse to the negative glow for
ionization when using this method. This setup was developed to see if the
changes in the filament due to sputtering were a main cause of the
inconsistencies in quantitative analysis. Figure 37 shows a comparison of an
analysis of 20 samples of 100 ppm Zn obtained with both the internal and the
external ETV method. Three things should be pointed out: 1) The sensitivity of
the internal ETV is much better. This is most likely due to losses as the atoms
diffuse to the negative glow in the external setup. 2) The small ion signal that is
apparent during the pre-sputter period is not existent in the external method,
since the filament is no longer subjected to sputtering. 3) The relative standard
deviations are very similar for both methods over the first four runs of a new
filament as shown, although both methods will deteriorate over subsequent runs.
The external mode, however, will not produce as large a standard deviation (10 -
15%) as the internal mode (20 - 25%). This is probably due to the more stable
filament surface conditions that are anticipated with the external method since the
filament is not being sputtered.

C~&
ELECTROTHERMAL
PROBE
DISCHARGE
PROBE
TO MS
t
Figure 36 Diagram of the external ETV ion source.
108

109
Time (s)
A
B
Figure 37 Comparison of the ion signals obtained for four
consecutive runs when using the internal and the external ETV
systems. A) Internal ETV; B) External ETV.

110
Multi-Element Solution Mixtures
Multi-element samples present challenges for ETV/GD analysis, since the
analyte signal is only present for a few seconds. This makes it difficult to observe
more than one elemental ion signal with each sample. There are, however,
solutions to this problem. An alternative is to run a sample for each element
individually, but the results may be inconsistent if there are changes in the
filament or discharge conditions between experiments. Two methods for
detection of multi-elements in the same sample are the use of a lower atomization
current to remove the atoms in a slower manner so that the ion signal will be
present for a longer time, and taking advantage of different heats of atomic
formation for the elements studied.
Mass Spectral Scanning
In this experiment, the filament current was lower than required for analyte
atomization, thus providing a longer ion signal duration. This method is suitable
for analysis of elements with similar atomization temperatures that cannot be
resolved with different filament currents. This allows for analysis of multi-elements
by mass spectral scanning. By doing this, isotopic ratios may be observed as
well as any interferences that may be present.
An example using this method is the analysis of an iron-manganese
mixture, since both analytes are atomized at a similar temperature. Figure 38
shows the mass spectra (a compilation of 10 scans) for the Fe/Mn sample,
accumulated one minute after discharge initiation, with 0, 1, 2 and 3 A. The
manganese ion is the dominant species in these spectra and shows a maximum

350 i
280-
x
£ 210
o
To 140
c
O)
0)
70-
0
-
A
55
+
Mn
56 +
/•
:J
lIjk
50
60 50
B
t—i—r
ArOH
60 50
m/z
ArHoO
\
60 50
Figure 38 Mass spectra obtained from an iron/manganese mixture using
different currents. A) 0 A; B) 1 A; C) 2 A; D) 3 A.

112
when 2 A of filament current are used. The decrease in ion signal at 3 A is most
likely because the bulk of the analyte was electrothermally vaporized before all 10
scans of the acquisition was completed, resulting in an overall lower ion signal
intensity.
This method would benefit from a computer controlled instrument that can
quickly scan the mass range multiple times while the ion signal is coming off the
filament. The computer system used in this laboratory is far too slow to provide
this capability. Thus, multi-elements that are analyzed must be in a very small
mass range (about 10 amu) to provide any multiple scanning. To perform these
experiments most accurately, a computer system such as those used in liquid
chromatography/MS applications is needed. This would allow multi-element
analysis with the addition of internal standards, which could lead to more accurate
analytical results.
Separation of Multi-Element Species with Filament Current
Since the limitation of mass spectral scanning exists, the multi-element
species were attempted by removing the analytes in sequence according to their
vaporization temperature. This was done by periodically increasing the filament
current, which enabled the sequential removal of the analytes from the element
with the lowest vaporization temperature to that with the highest. Table V shows
some of the thermal properties of the ions under study and the filament currents
used to vaporize them. The data for rhenium are given for comparison. The
solution analyzed in this experiment was a binary mixture of 1000 ppm zinc and
nickel. Figure 39A shows the data obtained for the zinc analyte with the

113
TABLE V. Heats of Formation and Filament Currents
used for the Elements under Study
Element
AHf (kJ/mole)
Filament Current
Used (A)
Fe
415.5
2.5 - 3.4
Mn
283.3
2.5 - 3.4
Zn
130.4
2.0
Ni
430.1
3.0
Ag
284.9
3.0
Re
774
—
AHf is the heat of formation of gaseous atoms from elements in their standard
states.
Source: Reference [85],

Ion Signal (counts x 103 ) Ion Signal (counts x 103
114
Figure 39 Detection of zinc and nickel from a binary mixture as
the filament current is changed. At t = 5 s, the current is turned on
at 2 A, while at t = 50 s the current was increased to 3 A. A) MZn
ion signal; B) MNi ion signal.

115
quadrupole set to pass m/z 64. At t = 5 s, the filament was turned on to 2 A and
the zinc sample was removed from the filament. At t = 50 s, the current was
increased to 3 A. An additional small zinc signal was obtained. Figure 39B
shows the same analysis sequence with the quadrupole set to pass the 58
isotope of nickel. The majority of the nickel signal was obtained only after
increasing the filament to 3 A. In Figure 40, both species were analyzed using
only one sample application. With the quadrupole set at m/z = 64, the filament
was turned on (2 A) at t = 5 s. At t = 40 s, the quadrupole was changed to pass
m/z 58 and at t = 50 s the current was increased to 3 A to remove the nickel. By
using this method of carefully controlling the filament current, multi-element
samples may be separated and analyzed with the ETV/GD method.
Conclusions
This chapter has shown that electrothermal vaporization may be used to
analyze solution samples in a glow discharge mass spectrometer quickly. The
fundamental characteristics of this technique were shown to be similar in most
respects to those of typical GDMS systems. The sampling sequence is an
important part of the method development, since the dryness of the sample is
critical and the timing of the steps needs to be accurate. The method described
above probably could be improved if the entire sequence of events was under
computer control. The experiments discussed above were timed with a stopwatch
for accuracy within about 1 s. The amount of sputtering loss before initiation of
the electrothermal filament should be minimized, but the sputtering time should
be sufficient to provide a stable discharge. This was obtained in a minimum time

Figure 40 Separation of zinc and nickel from a binary mixture using one
sample application. At t = 5 s, the MZn atoms are removed with 2 A current and
at t = 50 s, the MNi atoms are removed with 3 A current.
80
116

117
of 5 s, which does not result in the loss of much ion signal (less then a few
percent of the total). The ion transmission from both an auxiliary cathode and the
filament itself was dependent on the parameters used. Maximum ion signals were
obtained with a constant voltage discharge and the maximum filament current, so
these parameters were used for the majority of the data. The method showed
fairly good linearity in the limited concentration range studied for these
experiments. The optimum operating pressure was found to be about 1.2 Torr
argon, which is consistent with that used for a typical glow discharge application.
The surprising difference that was observed with this source is the cathode-to-exit
orifice distance that produces the maximum ion signal. For the electrothermal
source, the maximum signal was observed with the filament 3.5 mm from the ion
exit orifice. This difference may be a result of the electron production from the
filament, which is absent in a typical glow discharge, or the atoms in the plasma
may be at a different energy level than those of the typical discharge. The
positioning of the sample on the filament is a critical parameter as well, with the
maximum ion signal obtained when the sample is applied at the end of the
filament nearest the ion exit orifice. This method shows no significant memory
effects when a blank sample is used, which is important since many analyses of
this type suffer greatly from memory effects. Only binary element samples were
demonstrated in the data presented here, since the elements must be in a very
narrow mass range for analysis. Improvements in quantitation would be expected
if internal standards were used during the analysis. Binary samples were
separated by ramping the filament current, but this method requires elements that

118
are far apart in their atomization currents unless the power supply is computer
controlled. Internal and external ETV introduction schemes were used and
compared. The internal method is the preferred method, due to its simplicity and
better sensitivity. However, internal ETV introduction suffers from higher relative
standard deviations and the erosion of the filament material. The inconsistencies
of this technique have been contributed to the continual introduction of
contaminant air and water vapor with each subsequent sample. The effects of
water vapor contamination will be addressed in the following chapters.

CHAPTER 3
THE EFFECTS OF WATER VAPOR ON GDMS:
STEADY-STATE WATER ADDITION
Introduction
The next two chapters of this dissertation focus on the effects of water
vapor (which is always present to some extent in a glow discharge ion source) on
the production of the atoms and ions that are used for analytical studies. These
examinations were undertaken since the research presented in Chapter 2 was
determined to be affected by the water and other contaminants that were
introduced into the ion source with each subsequent sample. Also these
deterring effects of water vapor content on the analytical results obtained with
glow discharge are of interest. Water vapor content in the plasma might vary
from run to run depending on the environmental conditions external to the mass
spectrometer to which the sample was exposed. These varying experimental
conditions might subsequently affect the reproducibility of analytical results.
The ion signal intensities that are detected by the mass spectrometer may
be influenced through a variety of means, including the presence of impurities
(such as water vapor) in the glow discharge ion source. It has been determined
that it is very important to use ultra high purity sputtering gases to improve the
long-term stability and accuracy of glow discharge analytical results [20], When
water vapor is present, the resultant mass spectra will contain a variety of ion
119

120
signals that are produced from water vapor or combinations of water vapor
species with the analyte of interest. Water vapor may be introduced by different
means, including leaks in the vacuum system, water occluded within the sample
itself (e.g., a powdered material that must be compressed to form the discharge
cathode), the discharge gas, or water outgassing from the surfaces of the ion
source components or gas lines. This chapter reports on the effects of
introducing a constant concentration of water vapor on the fundamental glow
discharge processes (sputtering and ionization) and the subsequent changes in
a variety of ion signal (e.g., analyte ion, Ar+, H20+, and H30+) intensities that have
been observed using a mass spectrometer.
In this chapter, the effects of water vapor were studied by adding controlled
amounts of water into the mass spectrometer in a steady-state fashion. This
method of addition was chosen for the preliminary studies, so that the water
vapor effects may be initially characterized at a constant concentration. The
effects encountered with a pulsed introduction of a small volume of water vapor
and the resulting time resolved behavior of the ion signals will be discussed in
Chapter 4. Understanding the time resolved behavior of ion signals while pulsing
water vapor first required a thorough comprehension of the processes occurring
with a steady concentration.
Once a constant concentration of water vapor was established in the ion
source, the overall effects of the water vapor presence were noted. After
observing the resultant effects, the ion signals were monitored to observe their
changes while the water was subsequently removed and the plasma returned to

121
its "normal" state. This provides insight into the mechanisms occurring in the
plasma when the water is initially added and the processes resulting in the
plasma environment recovering to its original state. Three methods of water
removal were investigated: 1) the water was pumped from the ion source by the
vacuum system, 2) the water was reacted out of the ion source by using a sample
cathode that actively reacts with water and removes it to the chamber walls (i.e.,
getter elements) and 3) the water was cryogenically removed using a liquid
nitrogen cooled coil.
The effects of water on the plasma processes are important considerations
when using glow discharge ion sources. The water contamination may affect the
analytical results obtained with glow discharge methods by two major pathways:
1) changing the ionization mechanisms in the plasma, and 2) changing the
sputter-atomization processes of the glow discharge, which results in a decreased
atomic population. As mentioned in Chapter 1, ionization processes in the glow
discharge rely mainly on the collisions of analyte atoms with long-lived argon
metastable species (Penning ionization) [35]. Thus, any suppressing effects that
water vapor has on argon metastables would result in a reduction of ion signal
as measured by the mass spectrometer. Water vapor and other organic
molecules in the ion source have been shown to quench the argon metastable
population by an energy transfer from the metastable atom to the water or other
organic molecule [86], As the argon metastable population is decreased, the ion
signal will become lower due to the loss of this ionization pathway.

122
The Dissociation of Water in Gas Discharges
A discussion of the dissociation of water vapor is appropriate here, since
many of these dissociation products may play an active role in the reactions
occurring in the glow discharge. Water vapor was initially studied in gas
discharge tubes as a means of examining the free hydroxyl radical OH [87].
Initially, the final products of the dissociation of water vapor in a gas discharge
at ordinary temperatures were found to consist of hydrogen, oxygen, and water
only. It was later reported that the neutral OH radical was present in these
systems as well [88], and hydrogen atoms as well as OH would be involved in the
reactions of dissociated water vapor. It was also observed that oxygen may be
formed in a discharge tube by the dissociation of OH [89], while Kaufman and Del
Greco [90] found that the relative proportions of OH and O in the discharge
should depend on the rate of the reaction shown in Equation (19):
O + OH - 02 + H (19)
This reaction has no appreciable activation energy and some 02 was expected
to be found in the discharge tube [91]. Bonhoeffer and Person suggested an
overall mechanism for the reactions occurring in dissociated water vapor systems
[92]. These reactions are shown in Table VI. From these results, we may expect
to see reactions in the glow discharge resulting in ions from any or all of the
reactants and products occurring in this mechanism. Some rate constants and
reactions involving dissociated water species, such as OH and 02H radicals as
well as atomic species such as hydrogen and oxygen, have been reviewed in the

123
TABLE VI. Overall Reaction Mechanism of Dissociated
Water Vapor in a Gas Discharge
H20 -*• H + OH
H + H -»■ H2
OH* -* OH
OH + OH + M - H20 + O + M
0 + O -* 02
where M is a surface or third body.
Source: Reference [92],

124
literature [93]. That publication and the references therein give insight to the
many reactions that may occur in a system where water vapor is present.
The products and processes of the ionization in water vapor by electron
impact have been studied by some researchers as well [94,95,96]. The positive
ions under study included H20+, H+, OH+, H30+, 0+, H2+, and 02+. Further
studies were carried out on some of these ions [97,98,99] and the reactions that
were proposed to lead the principal positive ions are shown in Table VII. These
reactions as well as others will be expected to occur in the present glow
discharge system, since the primary ionization mechanism for gaseous impurities
relies on electron impact.
This section has presented an overview of some early work that studied the
dissociation of water vapor in discharge and mass spectrometry systems.
However, these systems failed to consider the reactions that are occurring
between the water vapor species and the normal glow discharge species (i.e., the
discharge gas species and the sputtered species). The following section will
present an introduction to studies that have been reported more recently in glow
discharge systems similar to that used in the experiments presented in this
dissertation.
Studies of Water Vapor Effects in Glow Discharges
The focus of this section is on the interactions of water vapor species with
the discharge gas as well as the cathode material. Of primary concern is the
effect water vapor in the discharge chamber will have on the fundamental
processes in the plasma, such as atomization (sputtering) and ionization.

125
TABLE VII. Principal Positive Ions Formed
in Water Vapor by Electron Ionization
Ion
Probable Process
h3o+
H20+ + H20 - H30+ + OH
h2o+
H20 - H20+
OH+
H20 - H + OH+
0+
h2o ^ h2 + 0+
H20 ^ 2H + 0+
+
CM
O
02 - 02+
H+
h2o ^ oh + h+
+
CM
X
h2o ^ 0 + h2+
Source: Reference [98,99],

126
The effects of water in the glow discharge will be better understood by
studying the reactions that occur in the plasma when water molecules are
present. A variety of possible reactions involving argon and water was proposed
by Undinger in a study using a steady-state hollow cathode discharge with 0.15%
water in a balance of argon [100]. The pertinent reactions that were reported are
as follows:
Ar + e~ - Ar + 2e~
(20)
Ar + e~ - Ar2* + 3e"
(21)
Ar* + Ar - Ar2 + e~
(22)
Ar* + HzO - ArH* + OH
(23)
ArH* + H20 - H30* + Ar
(24)
H20* + H20 - H30* + OH
(25)
Ar* + H20 - H20+ + Ar
(26)
H30* + e~ - H20 + H
(27)
where Ar* indicates excited Ar.
Reactions listed in Equations (20) - (22) are related to the formation of the
various argon ions that exist in the plasma, while the more direct interactions of
water vapor are shown in Equations (23) - (27). It is evident from these reactions
that an increase in the water concentration of the plasma will shift the reactions
depicted by Equations (23) - (26) to the right side and will result in a discharge
plasma enriched in H20 and H30 ions. From Equations (23) and (26), it is also
evident that the argon ions, which are mainly responsible for the sputtering

127
processes, are being consumed by the added water. Reaction (25) was shown
to proceed rapidly and for this reason the intensity of H30+ is often greater than
that of H20+ when traces of H20 are present in the plasma [101]. It was
determined that H20+ is formed mainly by the ion-molecule reaction shown in
Equation (26) [100], As a result of these observations, one could expect reduced
sputtering of the analyte species and a mass spectrum that contains two
significant ion signals at mass-to-charge ratios 18 and 19. Also, other products
from the decomposition of water that were outlined above (e.g., O or OH) or
water-analyte cluster species may be present in the ion source, depending on the
plasma conditions and the cathode sample.
A decrease in sample atomization is another major concern as a result of
the presence of water vapor in the glow discharge source. Water molecules may
dissociate in the plasma leaving a large amount of hydrogen ions or other
fragment ions that could decrease the analyte sputtering rate by replacing some
of the argon ions in the source. A decrease will occur if the discharge current is
being partially supported by these fragment ions (e.g., H+) in the discharge, rather
than by argon ions alone. It has been shown that the greater fraction of the
current in a gaseous conductor is carried by the species with the highest mobility
[3], and the mobility of the positive ions vary with mass (with lower mass ions
exhibiting higher mobility) [102], Stern and Caswell found that the hydrogen ions
in a sputtering system have between 20 and 40 times the mobility of the argon
ions depending on the discharge conditions [103]. When the hydrogen ions carry
a large fraction of the current, they will produce a reduced sputter rate due to

128
their low mass [104]. Sputter yields with a variety of species have been
investigated and the sputter efficiency was shown to be related to the mass of the
sputtering ion, with low mass ions resulting in lower sputter yields [104],
Therefore, any small fragment species in the plasma could carry a large amount
of the discharge current, but not effectively sputter the analyte.
Gough et al. [105] showed by atomic absorption that sputtering of the
sample in the presence of water is not as efficient as sputtering with dry argon
and will decrease the concentration of sputtered neutrals in the plasma. It was
noted that the absorption of ground state Cr atoms decreased and the precision
of the measurements worsened when changing from an argon gas supply with
3 ppm water to one with 17 ppm. Similar effects have also been demonstrated
while introducing methane gas into the plasma and monitoring the sputtered atom
population with atomic absorption [106], It was postulated that as the methane
concentration is increased in the discharge gas, more of the discharge current will
be carried by the highly mobile hydrogen ions, methane ions, and other fragment
ions coming from the methane molecule.
The effects of water vapor on glow discharge sputter-atomization was also
investigated by Larkins in an argon discharge gas containing various amounts of
water (up to 140 ppm) using atomic absorption measurements [107], A decrease
in absorbance signal was observed and the magnitude of these water induced
effects varied depending on the sample being sputtered and the discharge
current. At a level of 140 ppm water vapor, the reduced absorbance values varied
from 12% for nickel in steel to about 77% for chromium in aluminum. These

129
effects were observed to be more prominent when sputtering was carried out
using lower discharge currents. The decrease in the atomic population was
shown to be the result of lower sample erosion rates as well as processes in the
plasma such as gas-phase reactions between water molecules and fragments with
free atoms.
With these detrimental effects being considered, experimental methods
need to be developed that will avoid, or at least reduce, these problems. Initial
steps that may be taken to eliminate water contamination are using ultra pure
argon discharge gas and avoiding vacuum leaks in the mass spectrometer,
although these preventive steps alone are not sufficient. Other methods that have
been shown to decrease the residual water vapor concentration are the use of a
cryogenic device to remove physically the water vapor and the use of gettering
materials, which will chemically eliminate oxygen-bearing species and other
impurities when sputtered into the discharge chamber [108]. These methods will
be elaborated upon further in a later section of this chapter.
Few in-depth studies have been performed to observe the effects of water
vapor using mass spectral methods. Some initial studies were performed in this
laboratory by Loving and Harrison to demonstrate the changes occurring in an
iron glow discharge as the water content of the plasma was changed [36]. The
study that is described in this dissertation was conducted also to observe the
effects of small amounts of water vapor contamination on the ion signals detected
with a quadrupole mass spectrometer. These fundamental studies give insight
into water reactions that exist in the plasma and the possible changes in ion

130
signals as a result of changing the water concentration. The immediate effects
of water vapor contamination will be demonstrated with mass spectra obtained
first under normal conditions and then with the steady state addition of a small
amount of water vapor. The resulting effects on the neutral analyte species
population as well as the argon metastables were studied by atomic absorption.
The changes that occur upon removing water vapor were studied by first
maintaining a steady-state water vapor concentration in the ion source and then
removing the water vapor. The changing ion signals were then monitored with
time as the water is removed. Experiments were also conducted to explore the
effects encountered when using three different metal matrices (titanium, iron, and
copper) that have varying reactivity toward oxygen, which should indicate the
influence that water has on these elements.
Experimental
These experiments were performed on the quadrupole mass spectrometers
that were described in Chapter 1. The modifications made to the mass
spectrometers for the water experiments will be described in detail below. The
cathode materials used in the experiments were Ti [Aldrich Chemical Co.;
Milwaukee, Wl; #34,885-6, 99.97% pure], stainless steel [National Institute of
Standards and Technology; Gaithersburg, MD; SRM 1265, >99.9% Fe], and Cu
[Johnson Matthey Chem. Ltd.; Royston, England; 99.999% pure]. The titanium
and copper were purchased as 2 mm wires and were cut to an appropriate length
to fit the direct insertion probe. The iron sample was machined into 2 mm pins
from a bulk disk, the length of the pin being determined by the thickness of the

131
standard disk. A five millimeter section of the sample was left exposed to the
negative glow for sputtering. The gas used in these experiments was 99.999%
pure argon [Liquid Air Corporation; San Francisco, CA] and the discharge
pressure was maintained at approximately 1 Torr.
Studying the effects of water vapor first required a means of introducing the
water in a reproducible and steady fashion. An introduction valve assembly,
shown in Figure 41, was constructed to allow the water vapor to bleed into the ion
source in a constant and controllable rate. The assembly is made up of a shut
off valve [Whitey; part# SS-3NBS4], a needle valve [Whitey; Highland Heights,
OH; part# SS-21RS4], and a water reservoir, connected in series to the mass
spectrometer. The water reservoir is made up of a 0.75" diameter glass bulb at
the end of the assembly. It is attached to the assembly by a 0.25" O-ring
connector [Cajon; Macedonia, OH; part# SS-4-UT-1-2]. The entire assembly is
then attached to the ion source by a 0.25" Cajon connector welded into the center
of a 2.75" flange (MDC Vacuum Products; Hayward, CA).
The water vapor concentration was determined using a dewpoint analyzer
[Shaw Moisture Meters; Bradford, England; Model SDA] to the discharge ion
source. Since the moisture analyzer requires a flow of the gas to be measured
across the sensor wire, the sensor was placed in line between the glow discharge
ion source and a vacuum pump to provide the necessary flow of the discharge
chamber gases. The argon gas flow into the ion source was adjusted to maintain
the source operating pressure of 1 Torr. Figure 42 shows a block diagram of this
experimental setup. The sensor is constructed from a small wire composed of an

<
i=TP
rJ-
I
i,
TO ION
SOURCE^-
SHUT-OFF
VALVE
NEEDLE
VALVE
n
WATER
RESERVOIR
Figure 41 Schematic diagram of steady-state water introduction valve
assembly.
132

Figure 42 Block diagram of the experimental set-up to measure the water
vapor pressure in the GDMS ion source.
133

134
aluminum core, a hygroscopic aluminum oxide dielectric layer (a few microns in
thickness), and a thin covering of porous gold film. The pores of the film and
oxide layer are very small and are specific to the size of the water vapor molecule
[109]. The oxide layer will quickly (~ 30 s) reach equilibrium with the water vapor
pressure, which is proportional to the dewpoint temperature. The meter readout
is the dewpoint of the flowing gas in degrees Celsius, which can be subsequently
converted to the water vapor pressure in mm Hg (Torr) [109].
The cryogenic coil studies were performed in the Extrel ion source, which
is constructed from a standard six way cross with 2.75" flanges, as described in
Chapter 1. The source design is depicted in Figure 43, which shows the spatial
relationship of the cryogenic coil to the sample pin and the negative glow. This
coil was constructed in the University of Florida Chemistry Department machine
shop from 0.125" stainless steel tubing welded to a double sided vacuum flange
[MDC Vacuum Products; Hayward, CA; part # 140009] to accommodate the
introduction of both the coil and the direct insertion sample probe simultaneously.
An insulated liquid nitrogen container is attached to the top of the flange, and the
flow through the coil relies on the liquid being drawn through by gravity. A
mechanical vacuum pump may be attached to the coil exit port to maintain the
flow of liquid nitrogen, if required.
Results and Discussion
Effects of Water Vapor on Mass Spectra
The initial indications of the effects that water vapor has on a glow
discharge mass spectrum can be observed by collecting a spectrum while

LIQUID
NITROGEN
IN
*
1/4" SS COIL
PIN SAMPLE
LIQUID
NITROGEN
OUT
NEGATIVE GLOW
Figure 43 Schematic diagram of the Extrel ion source with the cryogenic coil.
135

136
bleeding a small amount of water vapor into the Ion source. Figure 44 shows two
mass spectra that reveal the differences before and after water addition. In Figure
44A, the mass spectrum of a titanium sample is shown after sputtering for about
30 minutes (discharge conditions: 1000 V, 3.8 mA), which is a sufficient sputtering
time to clean the pin surface of any oxide layer and obtain a steady state analyte
ion signal. The observed spectrum demonstrates the reduced magnitude of the
interfering species present in the ion source. With a titanium cathode, the
oxygen-containing gaseous interferants are efficiently removed by the gettering
action of the titanium. The predominant species that appear in the spectrum are
H3+, ArH+, and 48Ti+. H3+ is often present in glow discharge spectra and is
thought to form by the following reactions [110,111]:
H2 + e- - H2 + 2e-
(28)
h2 + h2 - h; + H
(29)
If the ion source is kept clean, the intensity of H3+ will normally continue to
decrease over the sputtering period. The H2 content in the ion source is normally
low, but may arise from a variety of sources. The most prominent source of
hydrogen in a discharge system is from the dissociation of water vapor [104].
Water vapor can evolve from the sample during sputtering, since even ultra pure
metals can contain gaseous impurities [112]. Water vapor also may be
introduced into the source with room air that is brought in with each sample and
will result in an increase of hydrogen [102]. Govier and McCracken observed the
correlation between water vapor pressure and hydrogen pressure in a sputtering

Ion Signal (counts x 10°) Ion Signal (counts x 10
137
m/z
Figure 44 Mass spectra of a titanium pin. A) Typical spectrum
taken after 30 min of sputtering. B) Spectrum taken with 0.05 Torr
of water being introduced.

138
system [113]. Upon discharge initiation the concentration of each was relatively
high but decreased with the sample sputtering time. Another possible source of
hydrogen that has been considered is from back diffusion from the pumping
system [114], and the inability of some pumps (especially turbomolecular) to
remove a low mass species such as hydrogen [115].
In sharp contrast, the spectrum shown in Figure 44B was obtained about
5 min after opening the water introduction valve that allowed approximately 0.05
Torr of water vapor into the ion source (discharge conditions: 1000 V, 2.0 mA;
note the decrease in current with the added water content while the voltage was
held constant). The glow discharge current has been observed in these studies
to be inversely proportional to the amount of water vapor that is present in the ion
source as the voltage is kept constant. Previous studies performed with water
addition in which the current was held constant resulted in an increased voltage
[108], agreeing with the present voltage-current behavior. The dramatic
differences in the ions produced with and without water contamination are
obvious from these figures. As discussed earlier, adding water to the system will
shift equations (23) - (26) toward the right, giving a mixture of H20 and H30 ions
being formed in the ion source. The predominant ions with water addition are
those of the water species as well as a small contribution from molecular oxygen
at m/z 32. This species arises from the electron impact ionization of 02 that is
present upon dissociation of water vapor, as discussed earlier. On this scale, no
ion signal is observed for the analyte elements and the intensities of the other
peaks have also been reduced. This reveals the significant quenching effect that

139
the water vapor has on the ion analyte signals detected from the glow discharge
source.
Effects of Water on Neutral Species
Because ion signals do not supply information about sputtering directly, it
is important to study the effects of water vapor on some neutral species in the
plasma. The atomic absorption measurements performed in these experiments
indicate how the neutral population in the ion source changes during the addition
of water vapor, since absorbance values are directly proportional to the
concentration of the ground state species. Also, the absorbance of the argon
metastable may be monitored to give an indication of its population in the ion
source. Argon metastables are important since they provide the path for Penning
ionization, which is one of the predominant ionization mechanisms for GDMS.
Table VIII lists the effects of the addition of water on absorbance measurements
taken on the argon metastables (A = 811.5 nm) as well as the titanium atoms (A
= 365.3 nm). These data show the reduction in the population of the metastables
due to the quenching effects of the water as well as the decrease in the sputtered
titanium atoms. These effects will occur as some of the argon ions are being
replaced by water fragments and the sputtering becomes less efficient as
previously discussed. Both reductions will contribute to the decrease in the
analyte ion signals that were observed in Figure 44B. These measurements
indicate that any discharge species that rely on sputtering and subsequent
Penning ionization will be reduced upon the addition of water into the system.

140
Table VIII. Changes in the Atomic Neutral Population
of Titanium and Argon Metastable Species in the Plasma
Discharge
Conditions
Ti
Absorbance
Ar*
Absorbance
1 Torr Ar
0.146
0.070
1 Torr Ar +
0.05 Torr H20
0.012
0.016
% decrease upon
adding H20
92
78

141
Effects of Water on Various Plasma Species
The following experiments were performed to investigate the changes that
occur in the plasma species of the glow discharge as water is removed from the
ion source. This was accomplished by first obtaining a steady-state water
concentration in the ion source by opening the water introduction assembly shut¬
off valve, which produces conditions similar to those used when Figure 44B was
obtained. After reaching a steady-state condition, the water valve was then turned
off and the water vapor allowed to pump (or react, depending on the cathode
material being used) out of the system. Figure 45 shows the behavior of the
water vapor pressure during these experiments, as measured by the moisture
meter. The initial value of the water vapor pressure was approximately 0.025 Torr,
which is 2.5% of the total discharge pressure. The behavior of the water
concentration shown in this figure was reproducible and did not vary significantly
during any of the following experiments, even with the use of different cathodes.
As will be seen later, all of the major changes in the ion signals under
investigation were observed in a period after the water vapor concentration had
leveled off as indicated by the measuring device.
Figure 46 shows the time-dependent behavior of four species of interest
from a titanium discharge (H20+, H30+, Ar+, and Ti+). This set of data was
obtained during the same time frame as that used for Figure 45. The H20+ signal
remains mostly constant, while the H30+ signal steadily decreases up to a
"transition point" where both ion signals quickly fall to zero intensity. This trend
would be expected in relation to the water reactions that were described by

H20 Vapor Pressure (mm Hg)
Figure 45 Plot of the changing water vapor pressure over the time frame of the
experiment. The water introduction is turned of at t = 2.8 min.
142

Ion Signal (V)
Figure 46 Changes in ion signals from a titanium pin as the water vapor is
removed from the ion source. •-H20+; ■-H30+; *-Ar+; ♦-Ti+.
143

144
Equations (23) - (26). Conversely, as the water is removed, the Ar+ and Ti+ slowly
begin to increase. At t = 25 min, these ion signals quickly rise to a plateau
intensity that would be comparable to the normally expected values obtained
using a dry ion source. This "transition point" appears to be a time when the
plasma changes from an oxidizing plasma that supports oxygen-containing and
dissociated water vapor species to a more reducing atmosphere that is dominated
by the elemental species. The results shown in Figure 46 would appear to be a
result of changes in both the atomization and ionization processes in the plasma
as the water is removed from the ion source. These types of "redox" transitions
and "titration-like" curves have been demonstrated by Roth when monitoring the
changes in sputtering yields of titanium using 11 keV argon ions in a residual gas
containing various amounts of oxygen, hydrogen, and nitrogen contaminants
[116]. It was shown that as the partial pressures of any of these three
contaminant gases were increased, a transition will be observed in the sputter
yield. The sputter yield was shown to go rapidly from a level observed with argon
alone to a plateau level that is significantly lower. This transition was observed
to occur over a small change in partial pressure. Thus, the sputter yield profile,
with changing concentrations of contaminant species, revealed a similar "transition
point" as observed in Figure 46.
Effects of Water on Various Cathode Materials (Matrix Effects)
Another study of interest involved the observation of the differing effects
that water has on various cathode materials. This study is important to determine
whether different elements exhibit different behavior with respect to the water

145
content in the ion source. This could make analytical determinations of some
elements more difficult if a particular element is found to be more greatly affected
by the water content of the plasma. Thus, the analysis of alloy materials may
prove difficult if the elements making up the alloy have vastly different interactions
with the water vapor.
The materials studied for comparison were Ti, Fe, and Cu. These three
elements exhibit differing oxide bond strengths, listed in decreasing order: Ti-O,
159.3±1.5 kcal/mol; Fe-O, 97.7±3 kcal/mol; Cu-O, 65.2 kcal/mol [85].
The oxide bond strength will contribute to the species observed in a glow
discharge mass spectrum and is of interest in the current studies since the
introduction of water vapor into the plasma should produce an oxidizing
environment [108], The correlation between the oxide bond strength and the
observed species in a glow discharge has been previously studied. It has been
observed that the ratio of the population of dimer species to their single atoms will
increase with the dimer binding energy [117]. Coburn and coworkers [118]
focussed on the species produced from oxide targets and found that with
increasing M-0 bond energy that the ratio MO+/(M+ + MO+) observed with a
quadrupole mass spectrometer also increases. The M-0 bond strength will be
a contributing factor in the present studies. With the relatively high concentration
of water in the plasma, M-0 species will be formed both on the sample surface
and in the gas phase. With the introduction of water the cathode will form an
oxide layer on its surface and in the initial sputtering period the loss in M+ signal
may be the result of the formation of MO+ species. This effect will be more

146
prominent with reactive metals. However, metal-oxide bond strength is not the
only influencing factor as there are many ion molecule reactions occurring in the
plasma to change its characteristics. These considerations will be discussed in
detail later.
Copper is of interest since it has been observed experimentally in this
laboratory to produce continually noticeable amounts of water ion signals even
after long sputtering times. This might be expected since copper atoms will not
react as readily with oxygen species to remove them from the plasma. Thus, with
Cu it is imperative to use a high purity discharge gas to obtain a mass spectrum
with low levels of water contaminants. On the other hand, a titanium cathode will
result in a fairly clean spectrum even without using ultra high purity gas due to its
strong oxide bond strength that allows it to react with these species and remove
them from the plasma. However, the time required to achieve this condition may
take a long sputtering period. This will depend on the extent of the oxide
contamination on the surface of the cathode. If the cathode is being sputtered
in a clean environment that suddenly encounters a small increase in water
content, the recovery would occur quickly since the oxide layer is not allowed to
form extensively on the surface of the cathode. On the other hand, if the cathode
is exposed to extensive oxidation before beginning to sputter, then the time
required to produce a "clean" spectrum would take longer.
Figure 47 shows the matrix ion signals that were obtained as the water was
removed from the source while sputtering these three samples in separate runs.
Again, the water concentration decreases over the time frame in a manner similar

Ion Signal (V)
7.50
6.00
4.50
3.00
1.50
0.00
60
Time (min)
Figure 47 Changes in ion signals of various matrices as the water vapor is
removed from the ion source. •-Fe+; ■-Ti+; a-Cu+.
147

148
to that depicted in Figure 45. Figure 47 will be discussed in three different stages
with respect to time as follows: 1) the initial stage before and just after the water
valve is closed off, 2) the intermediate stage where ion signal transitions are
occurring, and 3) the final stage after the transitions where the ion signals again
reach a steady state intensity and appear to be of a magnitude similar to that
obtained prior to the water addition.
The initial stage exhibits ion signals that are at or very close to zero
intensity. The only observable signal at the beginning of the data collection is
from copper. During this period, the surface of the individual cathodes should be
essentially oxidized due to the constant presence of the water vapor. The copper
exhibits a small ion signal in this initial stage probably because its surface is not
as readily oxidized due to its lower oxygen reactivity. After the water is turned off
(at t = 2.8 min), the copper signal begins to increase more quickly than the other
two (iron and titanium), suggesting that the relatively thinner oxide layer is being
removed more readily and the copper atoms in the sample are beginning to
sputter sufficiently enough to produce more of an ion signal. Also, after the water
is turned off the iron pin just begins to produce an ion signal. At a time of about
17 min, the copper signal has reached its plateau value, while the ion signal
intensities of the titanium and iron remain very small. This again reflects the lower
oxide bond strength of the copper and its relative ease for being sputter removed.
Thus in this first stage of surface cleaning, the species with the lower oxygen
reactivity will begin to sputter sooner due to the inability to retain an oxide layer
on the surface. It should be noted that over the period of 0 - 20 min the

149
appearance time and magnitude of the ion signals are what would be predicted
when considering surface oxidation and its removal. The lower the oxidation of
the surface the quicker sputtering will begin to remove efficiently the sample
atoms. This will result in a faster appearance and a higher ion signal during this
period.
In the intermediate stage, the "titration-like" curve is again observed with the
titanium sample and occurs at the same time observed in Figure 46. The iron
signal shows a similar behavior, but its "transition point" occurs later (at t = 35
min). This demonstrates the ability that iron and titanium cathodes have to
produce a "clean" discharge environment after some amount of sputtering.
However, it takes longer to achieve these conditions when using an iron cathode
since its ability to remove the water contaminants chemically is less efficient than
titanium. This delayed occurrence of the transition point agrees with the lower
oxide bond strength of iron. During this intermediate time where titanium and iron
are quickly increasing, the copper ion signal is still maintaining its plateau value
and shows no similar "transition point." Once the water source is turned off and
the surface sputtering has begun, gas-phase reactions seem to play a more
significant role as to which species will reach its "transition point" first. It is the
species with the highest oxide bond strength that will react with the water species
more efficiently and contain them for subsequent removal to the chamber walls.
Therefore, even though the iron ions were detected first, they were not able to
clean up the ion source by gas-phase reactions as quickly as the titanium, which
removes impurities by its gettering action. Notice that in this middle stage the

150
copper ion signal shows no improvement since its ability to react with the water
species and oxygen is limited by its lower reactivity.
The water ion signals for these experiments are not shown on this graph.
The water ion signals for titanium and iron produce a similar trend as those
illustrated in Figure 46, and fall to zero or low intensity at their corresponding
"transition points.'1 The water ion signals with the copper cathode show a
decreasing trend similar to the water concentration, but never pass through a
transition point or fall to zero intensity since copper is poor in removing the water
species. For comparison, during the final stage of this experiment a titanium or
iron mass spectrum taken at 45 minutes would show a clean mass spectrum with
no H20 or H30 ion signals. This is due to their relatively high reactivity that has
allowed the water species to be reacted out of the chamber. A copper spectrum
taken at a similar time would still have a small contribution from the water species.
Thus, in the final stages, the different reactivities of these elements are apparent
by the overall signal that was achieved over the course of this experiment. The
behavior of water species ions and other ions of interest will be considered further
in the next section.
As demonstrated above, water removal may be accomplished by sputtering
elements that will react with the water and remove it from the ion source. This
form of water removal will depend on the material that is being sputtered in the
plasma. Water removal due to this gettering ability can change the mass
spectrum that is obtained and the amount of time required to produce a "clean"
mass spectrum. Gettering elements have been observed to produce varying

151
effects on the removal of oxygen-containing species and other impurities from the
plasma environment with tantalum and titanium being shown to be quite effective
in producing a lanthanum atomic signal from a lanthanum oxide sample [119].
Therefore, the cathode material may be chosen appropriately to maximize the
amount of water removed by the sputtering processes, if a particular glow
discharge experiment allows for this flexibility.
Effects of Water on Plasma Species in Different Matrices
In this section, the behavior of other plasma species will be discussed while
observing the ion signals in the same time frame as that used in the experiments
of the previous two sections. This section will address and discuss the ion signal
behavior of various plasma species ion signals in the three matrices described
above. The ions that are of interest include: 1) H3+, 2) 0+, 3) OH+, 4) H20+,
5) H30+, 6) Ar2+, 7) N2H+ and COH+, 8) Ar+, 9) ArH+, and 10) Ar2+. The following
discussion will be separated into groups of ions that exhibit similar behavior in
their plasma reactions.
Hydrogen, Oxygen, and Hydroxide. Monitoring the hydrogen ions (at
m/z = 3), the oxygen ions (at m/z = 16), and the hydroxide ions (at m/z = 17)
in the plasma should give an indication of the water dissociation products that are
present in the glow discharge. These three species are the main products of
water dissociation, as discussed earlier. As shown in Table VI and Table VII,
there is a good chance that H2 is present in the water vapor equilibrium, which
would lead to the corresponding ion signal observed at m/z = 3 by the reactions
shown in Equations (28) and (29). Also, any free oxygen atoms that are formed

152
could be ionized to form the 0+ at m/z = 16. The hydroxide species was
discussed earlier to be present as an ion under electron ionization conditions and
as a neutral radical. This species has been studied with respect to its ionization
in a mass spectrometer and its ionization energy was found to be about 13.18 eV
[120].
The ion signals of these three species are shown in Figures 48, 49, and 50
with the three cathodes discussed above. All three ion signals show a similar
behavior, which would indicate that they are involved together in an equilibrium
process. These signals show a fairly constant ion signal over the entire run with
the copper cathode, which has lower reactivity. With the titanium cathode, these
species show little signal before the transition point and have a short lived spike
at this point. With iron, the signals have a sharp increase at the transition point
and then show a gradual decrease over the remainder of the experiment. The
sharp increases at the transition points may correspond to the water species
equilibrium shifting to the more dissociated state in the plasma or where these
species may be better ionized as the water content is reduced. Earlier research
of water vapor in gas discharges has presented similar results, where the water
ion is no longer stable in the discharge and the ions of its dissociation products
become the predominant species. According to Laidler [121], the appearance of
OH+ is due to the formation of an H20+ state that is unstable with respect to H
and OH+. When this condition is present the overall process is represented as
[121]:

Ion Signal (V)
Figure 48 Changes in the H3+ ion signal in various matrices as the water vapor
is removed from the ion source. »-Fe matrix; B-Ti matrix; *-Cu matrix.
153

Ion Signal (V)
Figure 49 Changes in the 0+ ion signal in various matrices as the water vapor
is removed from the ion source. *-Fe matrix; B-TÍ matrix; *-Cu matrix.
154

Ion Signal (V)
Figure 50 Changes in the OH+ ion signal in various matrices as the water
vapor is removed from the ion source. «-Fe matrix; B-Ti matrix; a-Cu matrix.
155

156
e- + H20 - H20+ + 2e~ - H + CW+ + 2©' (30)
Laidler also proposed a similar mechanism for the appearance of 0+ when this
unstable state of H20+ with respect to H2 and 0+ is encountered [121]:
e- + H20 - H20' + 2e~ - H2 + 0+ + 2 A similarly unstable state of the water ion also may be encountered that leads to
H2+ and O by a reaction similar to those shown above. This ion is a precursor to
the H3 ion that is observed in the glow discharge source. All of the reactions
discussed here are considered in more detail in the paper by Laidler, and
potential energy curves are presented to demonstrate that these reactions may
proceed energetically.
Therefore, when this condition occurs in the discharge it is likely that the
intensity of the H20+ signal will sharply decrease while ion signals of hydrogen,
oxygen and hydroxide will become predominant. This is in agreement with the
results discussed above and shown in Figures 48, 49, and 50. Consequently, the
titanium will react in the gas phase with these contaminant species, which may
result in the quick loss of these ion signals after their appearance. Since the iron
pin does not exhibit as large of a gettering capability to clean up impurities as
titanium, these ion signals will slowly decrease as the contaminant species are
removed by the vacuum system or to the chamber walls. Note that all of these
ion signals have a relatively low magnitude (less than 0.6 V) in the mass
spectrometer.

157
Water and protonated water. As mentioned before, the ion signals at
m/z 18 and 19 will give a direct indication of the water content in the plasma,
since the total water in the plasma is an equilibrium between H20+ and H30+.
The ion signal H20+ was proposed to result both from electron ionization (see
Table VII) and by charge exchange with an argon ion as shown in Equation (26).
On the other hand, H30+ comes from the reactions in Equations (24) and (25).
Mann et al. found the intensity of the H30+ signal was proportional to the square
of the water vapor pressure [98]. Figure 51 shows a plot of the H30+ signal
obtained with the copper cathode versus the square of the water vapor pressure
observed by the moisture meter. This is in agreement with the findings of Mann
and co-workers. The data for the copper cathode are shown here since it is
considered to react the least with the water vapor in the discharge. The results
are in agreement with those observed by Mann et al., since this plot is linear
throughout most of the measured range. The H30 was considered to form
through Equation (25) as well as the following by Mann et al.:
H20* + H - HsO+ (32)
Of these reactions, they concluded that Equation (32) is more likely when
considering only energetic grounds. However, the limiting factor for that equation
is that the hydrogen atom concentration may be too low for it to be significant.
Formation of H30+ also may occur via the collision of two metastable water
molecules as shown below [36]:
H20* + H20* - H30♦ + OH-
(33)

H30+ Ion Signal (V)
Square of Vapor Pressure (x1 O'5)
Figure 51 Correlation between the H30+ ion signal in a glow discharge plasma
and the square of the vapor pressure.
158

159
The ion signal response for H20+ (see Figure 52) and H30+ (see Figure 53)
in the discharges of the three cathodes discussed above have been investigated.
The ion signals from both of these species show similar trends. With the copper
cathode, both ion signals gradually fall over the period from 0-12 min, where
they eventually come to a fairly constant plateau value, following the H20 vapor
pressure trend. For the iron and titanium cathodes the H20+ signal is fairly stable
until the transition point is reached. At this time, the ion signals quickly decrease.
The H30+ signal shows a gradual decrease up to the transition point where they
fall to zero intensity. This is the expected trend, and was shown in Figure 46.
Protonated nitrogen and carbon monoxide. These species are monitored
because when the discharge is contaminated by air and moisture they are almost
always present in a large quantity. Thus, these species will give an indication of
the overall cleanliness of the plasma. Also, with the usually high amount of
hydrogen in a "dirty" system, the ion signals from these species are of larger
magnitude than m/z = 28 for N2+ and CO+. The ion signals obtained for these
species with each cathode are shown in Figure 54. In all of the runs, the ion
signals of these species are zero before about 5 min. This may be because they
are not efficiently ionized in the presence of the large amount of water vapor that
is in the plasma. After this period, the ion signal initially increases with each
cathode. For copper, the signal maximizes and then begins to decrease as the
contaminants are slowly removed. With titanium, the signal begins to increase,
but rapidly falls to zero at the transition point, which is probably a result of the
gettering action of the titanium. Wth iron, the signal increases at the transition

Ion Signal (V)
Figure 52 Changes in the H20+ ion signal in various matrices as the water
vapor is removed from the ion source. »-Fe matrix; B-Ti matrix; *-Cu matrix.
160

Ion Signal (V)
Figure 53 Changes in the H30+ ion signal in various matrices as the water
vapor is removed from the ion source. »-Fe matrix; B-Ti matrix; *-Cu matrix.

Ion Signal (V)
Figure 54 Changes in the N2H+ and COH+ ion signals in various matrices as
the water vapor is removed from the ion source. »-Fe matrix; B-Ti matrix; a-Cu
matrix.
162

163
point and then gradually falls. This increase at the transition point also may
reflect a change in ionization at this time. This is similar to that observed with the
hydrogen ions in Figure 48 and shows the lack of iron’s ability to remove
contaminants with gas-phase reactions.
Argon species. The argon content in the plasma is probably an
equilibrium between the Ar+, Ar2+, Ar2+, and the ArH+ species. The Ar+, Ar2*, and
Ar2+ are usually formed through the reactions shown in Equations (20) - (22). The
reactions to produce ArH+ have been studied [122] and the most likely reactions
include Equation (23) as well as:
Ar + H2 - ArH+ + H (34)
Ar* + H - ArH+ + Each of these reactions has been shown to proceed rapidly and from Equation
(35) it seems that ArH+ may be formed by a process requiring much less energy
that required to ionize argon. Thus, when traces of hydrogenous impurities are
present in the plasma the intensity of ArH+ is typically greater than Ar+ [36].
However, Equation (34) will only predominate when there are appreciable
amounts of H2 in the plasma. As shown in Tables VI and VII, H2 may result from
water dissociation.
These four ion signals are shown in Figures 55, 56, 57, and 58,
respectively. Each of these ion signals follows the same general trend, indicating
their coupled involvement in the plasma equilibrium. In copper each of these
species shows a gradual increase followed by a plateau as the plasma is cleaned

Ion Signal (V)
10.00
Time (min)
Figure 55 Changes in the Ar+ ion signal in various matrices as the water vapor
is removed from the ion source. *-Fe matrix; B-Ti matrix; *-Cu matrix.
60
164

Ion Signal (V)
0.75
24 36
Time (min)
Figure 56 Changes in the Ar2+ ion signal in various matrices as the water vapor
is removed from the ion source. »-Fe matrix; B-Ti matrix; a-Cu matrix.
60
165

Ion Signal (V)
Figure 57 Changes in the Ar2+ ion signal in various matrices as the water vapor
is removed from the ion source. *-Fe matrix; B-Ti matrix; *-Cu matrix.
166

Ion Signal (V)
10.00
Figure 58 Changes in the ArH+ ion signal in various matrices as the water
vapor is removed from the ion source. «-Fe matrix; ■-!! matrix; ±-Cu matrix.
167

168
up, which is similar to observations made for other species above. For the
titanium, the signals show a sharp increase at the transition point and then a
constant plateau value. A spike is observed with Ar+, Ar2+, and ArH+ and may
result from the equilibrium with the water dissociation products, which shows a
corresponding spike. This would occur since the water content in the plasma will
affect the argon ion concentration as shown in Equations (23) and (26). These
rapid gas-phase changes are probably enhanced by the gettering effects of the
titanium. With the iron cathode, the ion signals show a shifting increase to a
plateau value at the transition point as well.
Removal of Water with a Cryogenic Coil
As mentioned previously, a coil was constructed to house liquid nitrogen
and serve as a water sink inside the ion source plasma environment. This will
allow for the rapid removal of water vapor and any other species that freeze at a
temperature higher than that of the liquid nitrogen (boiling point: -195.8 °C). The
immediate benefits of using liquid nitrogen cooling can be observed in Figure 59.
Figure 59A was obtained with 0.05 Torr of water being bled into the ion source
in a steady state fashion as was true in earlier experiments. The sample used to
demonstrate this effect was an iron pin and the spectrum obtained with the
addition of water is very similar to that shown in Figure 44B for water added to a
titanium sample. The dominating species are the expected ion signals from H20+
and H30+, as predicted by Equations (23) - (26), as well as a contribution from
02+ at mass-to-charge 32. The liquid nitrogen was then added to the reservoir
and allowed to flow through the coil for about ten minutes while continuing the

Ion Signal (counts x 10°*) Ion Signal (counts x 10
169
Figure 59 Mass spectra of an iron pin. A) Spectrum taken with
0.05 Torr of water being introduced. B) Spectrum taken with 0.05
Torr water introduction and liquid nitrogen cooling.

170
introduction of water vapor into the chamber. The spectrum in Figure 59B was
then taken and the results clearly may be seen. The cryogenic coil has effected
the removal of water vapor and shifted the equilibrium of Equations (23) - (26) to
the left. As a result, more argon ions are available for sputtering, producing a
mass spectrum that is dominated by the sputtered species.
One disadvantage of this method may occur if the analysis takes a long
time (more than 30 min or so). After liquid nitrogen is added to the coil, the water
will begin to freeze out and deposit onto the coil surface. At some later time,
depending on the plasma conditions, the coil will become covered with water
crystals and the water will reach an equilibrium between the gas phase in the cell
and the solid phase on the coil. At this point, the pressure in the discharge
chamber will begin to rise slowly. When this condition is reached the analysis
must be suspended until the coil reaches ambient temperature and the water
vapor is pumped out of the ion source. The time required to reach this point will
depend on the water content in the plasma, the size of the coil, and the ability to
flow fresh liquid nitrogen through the device continuously. A different liquid
nitrogen introduction system is under current development in this laboratory to
alleviate the problems of a stopped-flow condition by applying a pressurized
supply of liquid nitrogen on the inlet end of the coil.
Conclusions
This chapter has demonstrated the effects that water vapor will have on the
plasma processes of the glow discharge as well as some methods to aid in its
control and removal. As was demonstrated earlier, water vapor effects in the

171
plasma can occur to varying degrees depending upon the cathode material that
is used, the purity of the discharge gas, and the integrity of the vacuum system.
The reduction in ion signal observed with the mass spectrometer is most likely
due to a combination of inefficient sputtering, oxidation of the sample surface,
loss of analyte atoms through gas-phase reactions, and quenching of argon
metastable atoms that are responsible for ionization of the analyte atom. Many
surface and gas-phase reactions are occurring in the ion source as the water
vapor content is lowered in the plasma. It was found that during the initial period,
after the steady state introduction of water vapor was suspended, the greatest
factors were attributed mostly to the surface interactions of the cathode. Elements
with a greater reactivity toward the water in the ion source will form a more
tenacious oxide layer and will result in a longer period of time before an ion signal
appears for a sputtered species. Once sputtering is initiated and the surface
begins to be cleaned, then gas-phase reactions seem to dominate. In this realm,
the elements that are more active toward water vapor species will react with them
and remove them from the system, resulting in a cleaner spectrum. A variety of
gas-phase reactions are occurring in the system and these were discussed.
Water dissociation products and hydrogenated species (i.e., N2H and COH) show
similar effects and are considered to be in an interactive equilibrium with one
another. The same is true for the various argon species in the plasma that exhibit
very similar behavior toward the water vapor content in the plasma. The use of
a liquid nitrogen coil was demonstrated to alleviate the effects of added water

172
vapor by freezing out the contaminants and will result in a mass spectrum that is
dominated by the elemental ion signals.
The effects studied in these experiments would become important in the
analysis of an alloy or other sample that contains a variety of elements. Since ion
signals of different elements are affected to different degrees in the presence of
water, calibration data for quantitative analysis (e.g., RSF values) that are obtained
at unknowingly different water concentrations in the plasma can no longer be
reliably used. Therefore, it is especially crucial that the water content in a glow
discharge ion source be controlled to a constant and minimal level for quantitative
analysis.

CHAPTER 4
THE EFFECTS OF WATER VAPOR ON GDMS:
PULSED WATER ADDITION
Introduction
This chapter continues to explore the reactions and interactions of water
vapor in the glow discharge plasma. As discussed in Chapter 3, water vapor that
is present in the glow discharge ion source will have a variety of effects on the
processes that are occurring in the plasma. These effects were determined to be
detrimental to both the analytical studies of the sputtered species and the
fundamental processes occurring in the plasma. This is because water vapor in
the plasma will decrease the amount of sample sputtering as well as quench
argon metastable atoms that are responsible for Penning ionization. The
experiments presented in Chapter 3 were performed by introducing a relatively
large quantity of water to the glow discharge plasma. In those experiments, the
water vapor was introduced in a steady state fashion, affecting not only the
processes occurring in the gas phase, but the interactions on the sample surface
as well. When a constant amount of water vapor was bled into the ion source,
the time required to begin detecting matrix ions was correlated to the metal-oxide
bond strength of the cathode material. This is because the relatively large
amount of water vapor introduced into the source with this method will cause
surface oxidation of the cathode as well as interactions in the gas phase.
173

174
The difference in the experiments presented in this chapter is that the water
vapor was pulsed into the discharge in short bursts. This allowed a small and
controllable amount of water vapor to be introduced into the glow discharge
plasma. These experiments were designed to provide further insight into the
transient changes that occur in the plasma upon an immediate and short lived
change in the water vapor content. Also, since the amount of water vapor that
is introduced into the plasma is small, the recovery time for system re¬
equilibration is shorter than in the previous experiments. This method of water
introduction also should minimize the cathode surface oxidation that was
discussed in Chapter 3. The interactions of the pulsed water vapor should affect
gas-phase processes the most with this experimental scheme. It has been noted
that when sputtering a reactive target material (such as a getter, which exhibits
efficient gas-phase reactions toward oxygen-containing molecules), water vapor
and oxygen species are often not observed, even when deliberately introduced
[102].
Experimental
The only difference in the experimental scheme for these studies that has
not been discussed previously is the pulsed solenoid injection valve and its
controlling circuits. The block diagram for the pulsed introduction of water vapor
into the ion source is shown in Figure 60. This water injection valve is mounted
onto a 2.75" flange and may be connected to either mass spectrometer. A
diagram of the solenoid valve mounted to the ion source of the Extrel instrument
(six-way cross) is shown in Figure 61. A pulse generator (Hewlett-Packard Model

Figure 60 Block diagram of the pulsed solenoid valve water introduction
system.
175

176
GAS GAS
IN OUT
TO
MS
Figure 61 Schematic diagram of the pulsed solenoid injection
valve on the Extrel ion source.

177
3325A) was used to trigger the control box, which in turn initiated the injection
valve power supply to open the solenoid valve. The solenoid injection valve used
in these studies was purchased commercially (General Valve Corporation, Series
9; Fairfield, NJ). The valve was powered by the control box, which contained a
+28 V dc power supply. This control box was built in the University of Florida
electronics shop for this specific purpose. The pulsed solenoid could be
operated in a single-shot fashion, but this method did not allow the operator to
control the open time precisely, since the open time of the valve corresponded
to the amount of time the single-shot button was depressed. When the solenoid
was being governed by the control box, it had specific operating ranges. The
pulsing frequency of the solenoid could be varied between 0 and 120 Hz, while
the open time could be varied from 0.16 ms up to its maximum (a few seconds).
The frequency of pulsing was controlled by the pulse generator and the open time
of the solenoid was varied by a potentiometer in the control box circuit. The
rising edge of the pulse generator square wave triggered the solenoid to open,
in addition to triggering the MCA, which was used to collect all of the data (i.e.,
AA, AE, and MS signals) for these experiments. The MCA was used exclusively
since only one mass could be monitored at a time to observe the individual time-
resolved characteristics.
The water reservoir (a 0.75" glass bulb, as described in Chapter 3) was
attached to the injection valve by a Cajon connector; water vapor residing above
the liquid was pulsed into the ion source. In order to make the amount of water
being injected with each pulse consistent, the space above the liquid water was

178
evacuated, either with an attached roughing pump or through the injection valve
into the ion source. The former method was most often used in order to keep the
ion source contamination to a minimum. Without this process, the first few
injection pulses will contain a large quantity of air with the water vapor. This
process was repeated after the system had remained idle for an extended period
of time, in case there were any air leaks into the water reservoir. The system was
considered to be providing a consistent amount of water vapor when the ion
signal profile changes were the same from pulse-to-pulse, within experimental
error.
Before using the pulsed injection valve, studies were performed by
introducing a short burst of water vapor simply by opening the shut-off valve,
shown in Figure 41, for a short period of time. However, this method of water
vapor introduction was found to be irreproducible. Figure 62A shows the titanium
ion signal measured when using this method. It is clear that this method is not
sufficient for measuring reproducible changes in the plasma as water vapor is
introduced. Not only is the magnitude of the signal loss different for each pulse,
but these varying amounts of water vapor introduction lead to different recovery
times and signal profile behavior. The solenoid system was designed to pulse
small and controllable amounts of water vapor into the mass spectrometer ion
source. Figure 62B shows the reproducibility of the pulsed solenoid introduction
valve method as compared to manually opening the needle valve assembly. The
signal fluctuations upon introducing the water in a 2 min cycle are shown to be
very reproducible with the characteristics of each pulsed signal being the same.

Ti Ion Signal (counts x 10d) Ti Ion Signal (counts x 10
179
Figure 62 Comparison of pulsed water vapor introduction
methods. A) Opening shut off valve by hand; B) Use of the
solenoid valve.

180
The open times that were used in the experiments described in this chapter
generally varied between 5 and 100 ms. The time that was used usually
depended on the observed effects for a given cathode. Shorter open times were
used for an element such as copper, which does not readily remove water vapor
by gas-phase reactions, and longer times were used for reactive elements such
as titanium.
Results and Discussion
Transient Behavior of Matrix Species
This section will discuss the results from the studies of pulsed water vapor
introduction while monitoring the ion signal response of a variety of matrices.
Some of the differences that are observed were anticipated because of the results
reported in Chapter 3. The titanium was expected to suffer the least adverse
effects from the pulsed introduction of water vapor due to its gettering ability. The
reactions of titanium with water in the gas phase should facilitate its fast recovery
to an equilibrium condition and minimize any possible surface contamination. On
the other hand, some cathodes that might suffer from water oxidation on their
surfaces due to their minimal gettering ability, will be affected the most.
Information on the ion-molecule reactions involving metallic element ions
has been scarce until recently. A large volume of work has been published in the
past decade that shows the periodic trends of the reactivities of the first row
transition metals. Although none of this work has involved the reactions of metal
ions with water, some comparisons may be made to the trends observed using
reactions with other species. Many different aspects were found to control the

181
efficiency of the reactions of the first row transition metals, but the general trend
that was observed was metal ions on the left side of the period were more
reactive than the metal ions on the right. This may be due to a variety of factors,
such as the reaction cross section for metal ions (which generally decreases
across the period) [123,124] and the electronic state of the metal ion
[125,126,127], In most cases, the reactions shown in these references (M+ with
H2, D2, 02, and N20) were more efficient with elements on the left side of the
fourth period and decreased across the period. This is in agreement with
observations made in the glow discharge using titanium, iron, and copper ions
in reactions with water vapor.
Titanium. Since titanium is a gettering material, it can exhibit gas-phase
reactions that will speed the cleaning of the discharge environment. However, the
surface of the cathode can become contaminated with water and oxide species
if the amount of water in the plasma is enough to overcome the gettering abilities
of the element. In the experiments presented here, the amount of water that is
introduced is quite small in comparison to those discussed in Chapter 3.
Therefore, it would be expected that the effects of pulsed injection would be
minimal when using the titanium cathode, because the pulse of incoming water
may be consumed in the gas-phase reactions before it is allowed to affect the
surface greatly.
Figure 63A shows the titanium ion signal response to a 30 ms pulse of
water vapor into the GDMS ion source. The signal recovery is very quick for this
cathode material, in comparison to the other cathodes that will be discussed

Ion Signal (counts x 10 ) Ion Signal (counts x 10
182
20 n
co
A
o i 1 1 i i i i i . i
0 60 120 180 240 300
d Time (s)
B
oh
60
I
180
Time (s)
T 1
240 300
Figure 63 Ion signal profiles obtained with a 30 ms pulse of water
vapor. A) Titanium ion signal; B) Titanium oxide ion signal.

183
below. The slope of the recovery is sharp, similar to that shown in Chapter 3.
Since the ion signal is only suppressed for a short time, it is believed that the
water pulse is short enough not to affect the surface but only affect gas-phase
reactions. For a gettering material such as titanium, these gas-phase reactions
will include the formation of metal-oxide species. This is shown in Figure 63B,
where the formation of the titanium oxide ion signal corresponds to the decrease
in the titanium ion signal. When longer open times or faster repetition rates are
used, the ion signal profiles will not reach their plateau value, indicating that the
system is not recovering between pulses. At this point, the amount of water
introduced into the system has overcome the ability of the titanium to getter-clean
the discharge environment and the surface interactions are becoming more
predominant.
Copper. As mentioned in Chapter 3, copper is a species that will often
have water peaks in its mass spectrum unless ultra high purity sputtering gases
are used. This is due to the lack of gettering reactions by copper to help clean
up the discharge environment. Also, since the reactivity of copper is considered
lower than the other elements used in these studies, its recovery to a "clean"
environment will take longer.
Figure 64 shows both the copper ion signal and the copper emission signal
(A = 324.75 nm) upon adding a 5 ms pulse of water vapor to the mass
spectrometer ion source. The shape of the copper recovery should be noted and
compared to the results shown in the last chapter. In both cases the recovery is
fairly slow compared to a species like titanium. Note that the slope of the copper

Figure 64 Signal profiles obtained from a copper pin when a 5 ms pulse of
water vapor was introduced. A) Copper emission; B) Copper ion signal.
184

185
signal as it recovers from the water vapor introduction is not as great as for
titanium, even with the reduced open time for the pulsed solenoid valve. These
results are in agreement with the previous results discussed in Chapter 3.
Iron. Iron was studied also and should exhibit a behavior somewhere
between that of copper and titanium since it lies in the middle of the first row
transition metals. Figure 65 shows the behavior of the iron signal from a stainless
steel pin with the introduction of a 30 ms pulse of water vapor. The number of
pulses in this figure is increased to demonstrate the reproducibility of the pulsing
valve. The water valve was pulsed 10 times, as shown in this figure, and the
integrated ion signal counts over the two minute period was calculated. For this
particular data the signal intensity was 1.0 x 107 ± 1.65 %. This is a typical
standard deviation over ten pulses for the cathode materials under study. The
behavior of the iron signal profiles also shows a quick recovery, as did the
titanium. However, this recovery time is not quite as rapid as for titanium. This
is difficult to see in this figure since there are ten pulses shown. With the iron
cathode there is a sharp transition back to the plateau value as was illustrated in
Chapter 3.
The reproducibility of the results of some of the ion signal profiles that were
studied while sputtering a stainless steel pin are illustrated in Table IX. The
reproducibility was demonstrated to be best for the matrix ion, its oxide ion, and
the argon species. Gaseous contaminant species showed a larger deviation with
the species at m/z 29 (N2H+ and COH+) being the worst.

Ion Signal Intensity (counts x 10
CO
Figure 65 Iron ion signal profiles obtained from a stainless steel pin when a 30 ms
pulse of water vapor was introduced.
186

187
Table IX. Reproducibility of the Pulsed Solenoid Valve on the
Detected Ion Signal Intensities with a Stainless Steel Cathode.
Ion
Signal Intensity (counts)
%RSD
56Fe+
1.0 x 107
1.65
H3+
8.9 x 106
3.00
OH+
8.5 x 105
9.23
H20+
6.0 x 106
7.44
h3o+
3.3 x 106
8.34
Ar2+
3.3 x 106
3.85
COH+, N2H+
4.1 x 105
25.31
ArH+
4.1 x 106
2.82
FeO+
2.1 x 105
3.11

188
Zinc. Zinc is another species that is of interest in glow discharge studies.
Since it is a relatively soft material with a low melting point, it requires a lower
discharge voltage than some of the other materials. This makes it more
susceptible to gaseous contaminants since these species are more dominant
when the glow discharge is operated at a reduced voltage. At these lower
voltages, the "cleaning" processes of the discharge are not as efficient as they
might be if the cathode were operated at a higher voltage. It has been previously
shown, with a stainless steel pin, that as the discharge voltage is increased, the
ion signal intensities of water vapor and other contaminant gaseous species will
decrease [128]. For comparison, the zinc cathode was operated at 1000 V and
3.2 mA, while the stainless steel cathode could be operated at 1250 V and 5.0
mA. These detrimental factors are illustrated in Table X, which lists the
reproducibility of some of the ion signals monitored while using the zinc cathode.
The standard deviations when using the zinc pin are generally higher than for the
stainless steel pin. Even the signal from the zinc cathode itself shows a quite
large %RSD. The zinc oxide signal has a large %RSD as well, but this is
somewhat attributable to the conditions that must be used to monitor this species.
The highest zinc oxide signal will occur at m/z 80, which is also where the argon
dimer is detected. For this reason the zinc oxide signal at m/z 82 was used. This
was formed with the isotope “Zn and oxygen. Since this isotope is less abundant
(about 50 % of the MZn isotope), the detector must be operated at a higher gain
and subsequently more noise may be introduced in the detected signal.

189
Table X. Reproducibility of the Pulsed Solenoid Valve on the
Detected Ion Signal Intensities with a Zinc Cathode.
Ion
Signal Intensity (counts)
%RSD
64Zn+
3.9 x 106
8.77
H3+
9.6 x 106
2.17
h2o+
5.8 x 105
9.32
h3o+
1.2 x 106
22.01
Ar2+
constant decrease
—
ArH+
4.1 x 106
1.73
(66ZnO)+
1.1 x 105
8.12
Note that 66ZnO+ was used, since 64ZnO+ will be interfered by Ar2+.

190
Effects of Pulsed Water on Other Plasma Species
The experiments that are discussed in this section will demonstrate the
effects of the pulsed water on other species that are formed or reside in the glow
discharge plasma. The species that were investigated are similar to those
reported in Chapter 3 (i.e., water species, argon species, and M-0 species).
Water dissociation species. As reported in Chapter 3, the various species
formed upon the dissociation of water vapor were observed to increase at the ion
signal transition point, while the water vapor species showed a corresponding
decrease. In the experiments shown here the plasma environment will not be
subjected to the relatively large amount of water vapor that was present with the
steady state introduction. It is therefore expected that upon the pulsed
introduction of the water vapor into the plasma, the water dissociation species will
show an increase as the water is dissociated by the discharge and will then be
removed from the plasma environment by reactions or pumping. Figure 66
shows the two prominent water dissociation species that were monitored in these
experiments. Figure 66A shows the ion signal behavior of the H3+ signal when
a 30 ms pulse of water was introduced. The cathode being used was a stainless
steel pin. The behavior of these species was the same for all the cathodes, only
the residence times of the individual ions in the plasma were different. These
species were detected longest for copper and shortest for titanium, since the
removal may be expected to be faster with a reactive metal versus a more inert
metal. Figure 66B shows the ion signal response for OH+. The signal profile and
characteristics are very similar to that shown for H3+. This indicates that upon the

Ion Signal Intensity (counts x 10 ) Ion Signal Intensity (counts x 10
191
Figure 66 Signal profiles obtained with a 30 ms pulse of water
vapor into a stainless steel discharge. A) H3+ ion signal; B) OH+ ion
signal.

192
pulsed introduction of water, these species begin to immediately dissociate in the
discharge.
Water species. The water ion signals in the glow discharge plasma (i.e.,
H20+ and H30+) are expected to behave similarly to that described in the previous
section, since the same ion molecule reactions are occurring in the plasma.
Upon the pulsed addition of the water vapor, the water ion signals should
increase with the shifting of the discharge equilibria. The water signal should rise
sharply, and since there will be minimal interactions with the cathode surface, the
gas-phase reaction processes (i.e., metal-water vapor reactions and water
dissociation reactions) should determine the rate at which water vapor is
removed. Thus, as previously mentioned, the reaction efficiency would be
expected to be greater with titanium than with copper.
Figures 67, 68, and 69 illustrate the relative ion signals from the H20+ and
H30+ species, as the water is pulsed into the system with titanium, iron, and
copper cathodes, respectively. For the titanium and iron profiles the water
injection valve was opened for 30 ms, while for the copper data the valve was
opened only for 5 ms. It should be noted that the main difference between these
three figures is the time required to remove the water species from the plasma.
It was observed that the titanium was the best cathode for removing the water
from the ion source, followed closely by iron. Even with the reduced solenoid
open time for the copper cathode, the water species are present in the plasma
for a longer period of time (approximately 45 s, versus 5 -10 s for the other two
cathodes).

Relative Ion Signal Relative Ion Signal
1
193
A
0.5
0 H t r-
0 60
—t* i i 1 r 1 1
120 180 240 300
Time (s)
Figure 67 Water ion signal profiles obtained with a 30 ms pulse of
water vapor into a titanium discharge. A) H20+ ion signal; B) H30+ ion
signal.

Relative Ion Signal Relative Ion Signal
1
194
A
0.5-
60
Time (s)
300
Time (s)
Figure 68 Water ion signal profiles obtained with a 30 ms pulse
of water vapor into a stainless steel discharge. A) H20+ ion signal;
B) HaO+ ion signal.

Relative Ion Signal Relative Ion Signal
195
Time (s)
Figure 69 Water ion signal profiles obtained with a 5 ms pulse of
water vapor into a copper discharge. A) H20+ ion signal; B) H30+
ion signal.

196
Argon species. The species that were monitored to determine the argon
behavior in the plasma were ArH+ and Ar2+. It was determined from the previous
chapter that these two species, along with Ar+ and Ar2+, show a similar behavior,
so only ArH+ and Ar2+ are illustrated. Also, these species were the largest ion
signals obtained for argon containing ions. ArH+ is larger than Ar+ in this system
due to the plasma conditions that are being used (with the water and subsequent
hydrogen contamination, as discussed in Chapter 3). Figure 70A shows the ion
signal behavior of ArH+ as a 30 ms pulse of water was introduced into the
discharge having a stainless steel pin cathode. Figure 70B shows the ion signal
behavior for Ar2* under the same conditions. The ion signals show a decrease
in intensity upon the addition of the water vapor. This is expected in relation to
the plasma reactions that have been previously discussed in Chapter 3. The
extent of the detrimental effects that were observed for different cathodes
correlated with the amount of time it takes to remove the water vapor from the
plasma. As with the other results presented above, copper has the most
detrimental effects, while titanium can keep the plasma environment clean.
Metal-oxide species. With the introduction of water vapor into the ion
source, the plasma environment will become more oxidizing and this can be
demonstrated with the observation of the formation of oxide species in the
plasma. Figure 63 shows the corresponding production of a titanium oxide ion
signal with the reduction in the titanium ion signal. This is direct evidence of the
formation of an oxide species with the introduction of water. The loss in titanium
may be attributed to losses in sputtering, ionization, and an overall loss of some

Ion Signal Intensity (counts x 10 ) Ion Signal Intensity (counts x 10
14 -\
197
0 1 1 1 1 1 i 1 1 1 i
0 4 8 12 16 20
Time (min)
6i
^VyyyyT^i
rysp
B
0 T 1 1 1 1 1 1 1 1 1 1
0 4 8 12 16 20
Time (min)
Figure 70 Argon species ion signal profiles obtained with a 30
ms pulse of water vapor into a stainless steel discharge. A) ArH+
ion signal; B) Ar2+ ion signal.

198
titanium that is reacting with the water and its dissociation species in the plasma.
Figures 63A and 63B should not be compared directly, since the gain on the
detector was increased to obtain the titanium oxide ion signal. Therefore, it
should be noted that the signal intensity of the oxide species compared to the
metal is orders of magnitude smaller. All matrices were found to form a metal-
oxide species in the discharge upon the injection of water vapor. The amount of
oxide formation that occurred in the plasma can be correlated with the M-0 bond
strength. For the species under investigation, the M-0 ion signal intensity
followed the decreasing trend: TiO+ > FeO+ > CuO+.
Use of Liquid Nitrogen Cooling
The liquid nitrogen cooling coil was also used in some of these
experiments. This method was used to see if the water vapor that was injected
in the gas phase will deposit on the coil before interfering with the plasma
interactions. Figure 71A shows the water ion signal profile for a new copper pin,
at a time when the plasma has not yet become "clean." Under these plasma
conditions, the ion signal profile for the water species does not reach a zero
intensity level between pulses. After this profile was obtained, liquid nitrogen was
added to the cryogenic coil and the ion signal profile for H20 was observed, as
shown in Figure 71B. Even with liquid nitrogen cooling, the copper discharge will
still have a larger water ion signal than that obtained with either iron or titanium
cathodes.
Figure 72 shows the effects of liquid nitrogen on the copper ion signal
profile upon the introduction of a 5 ms pulse of water vapor when the liquid

o
60
240
120 180
Time (s)
Figure 71 Water (H20+) ion signal profiles obtained from a copper pin when
a 5 ms pulse of water vapor was introduced. A) without liquid nitrogen cooling;
B) with liquid nitrogen cooling.

Cu Ion Signal (counts x 10
CO
44-
33-
22-
11 -
120 180
Time (s)
300
Figure 72 Copper ion signal profile obtained when a 5 ms pulse of water vapor
was introduced and liquid nitrogen cooling was used.
200

201
nitrogen coil is in the ion source. Figure 64B shows a typical signal profile that
was obtained of the sputtered copper ions under normal conditions. The data
presented in Figure 72 were obtained a few minutes after adding the liquid
nitrogen to the cooling coil. The differences in the recovery time of the ion signal
back to its original equilibrium state can be easily seen. The behavior of the
copper ion signal profile with liquid nitrogen is very similar to the behavior of the
titanium ion signal shown in Figure 63A. Although the recovery time is not quite
as fast, the sharp transition back to an equilibrium state is similar. From this
experiment it may be concluded that the liquid nitrogen coil will remove much of
the water vapor from the gas phase and the inert metal ion signal will not be
affected as much.
As mentioned in Chapter 3, the time that the liquid nitrogen coil may be
used effectively is limited. It was observed that after pulsing the solenoid for a
period of about two hours, the area of the coil facing the injector would become
covered with ice crystals. When this condition occurs, the coil has to be allowed
to warm up and the system must pump away the excess water. However, with
the limited amount of water vapor that is allowed into the source under these
experimental conditions, the useful time of the cryogenic coil is enhanced
compared to the experiments described in Chapter 3. Under typical experimental
conditions, the useful time for liquid nitrogen cooling should be even longer since
there is no external water vapor being added to the source. In these experiments
the water vapor will come from sample introduction, outgassing from the chamber
walls, and the sample itself.

202
Effects of Pulsed Water on the Atomic Population
As discussed earlier in Chapter 3, it is advantageous to monitor the atomic
population in these systems to observe the effects that the addition of water vapor
will have on the neutral and excited atomic populations in the plasma. The
experimental setup used was discussed in Chapter 1 and all of these experiments
were performed on the Extrel instrument.
Atomic absorption and atomic emission measurements were conducted on
the two most prevalent atoms of interest in the glow discharge plasma: the
sputtered (cathode) atoms and the argon metastable atoms. The measurement
of the absorption or emission of the sputtered species will give an indication of
how the water vapor is affecting the sputtering processes in the glow discharge.
As mentioned earlier, it is expected that the addition of water vapor will reduce the
sputtering since some of the water dissociation products (i.e., H+), which have a
low mass and a high mobility, will contribute a large amount to the discharge
current but will do little to aid in sputtering. This will be seen experimentally with
a decrease in the absorbance or emission signal obtained from the species upon
the introduction of water vapor. The absorbance or emission of the argon
metastable atoms will give a direct indication of how the sputtered species
ionization is being affected in the plasma. A decrease in the argon metastable
absorbance or emission value will indicate that either the metastables are being
quenched or their formation is reduced. This will result in a decrease in Penning
ionization, which is primarily responsible for the ionization of sputtered species in
the plasma.

203
Analyte atoms
The AA and AE measurements on the analyte atoms in the plasma give an
indication of the effect that the water vapor introduction has on the sputtering
process. As mentioned previously, the sputtering will be reduced as the water
content in the plasma is increased. Since either AA or AE measurements will give
an indication of the atomic population of the sputtered species in the plasma, it
is not typically crucial to these studies which measurement is chosen. However,
if a cathode that is being measured has a high sputter yield (such as the copper),
it is advantageous to use emission measurements. This is because the
absorbance for copper atoms in the plasma is very high. Thus, when small
pulses of water are introduced into the system, the absorbance measurements will
not change greatly. This would also be true if the sputtered atoms are very
efficient absorbers, where even a small amount of atoms will result in a large
absorbance value.
Figure 73 shows the absorbance and emission signal profiles obtained for
the titanium when a 30 ms pulse of water was allowed into the ion source. As
expected, these signals are reduced when water vapor is pulsed into the plasma,
indicating that the sputtering may be reduced or the atoms may be tied up in the
oxide form during this time. The profile shown in Figure 73A shows a short lived
loss in absorbance signal. The signal reaches its plateau level again after about
15 s. However, the titanium emission signal (see Figure 73B) shows a decrease
in signal that does not regain its plateau value for about 30 seconds. The
formation of excited atoms lags behind the ground state species in reaching their

Emission Signal (counts x 10 ) Absorbance
204
A
Figure 73 Signal profiles obtained with a 30 ms pulse of water
vapor into a titanium discharge. A) Ti absorbance; B) Ti emission.

205
equilibrium value. This may be a result of collisions of the excited species with
water vapor in the ion source that does not allow the excited species population
to regain its equilibrium value until the water vapor is removed.
Argon metastables
The signal behavior of the argon metastable atomic population is shown
in Figure 74. It is important to monitor the argon metastable population since this
determines the extent of the Penning ionization in the glow discharge plasma.
The behavior of the argon metastable profile is very similar to that shown for the
titanium atoms. Upon the introduction of the water vapor into the ion source, the
argon metastable population is reduced due to the efficient transfer of energy
from the argon metastable to the water molecule [86]. Figure 74A shows the
argon metastable absorbance data that give an indication of the neutral argon
metastable species population in the plasma. This measurement is the
absorbance occurring at A = 811.5 nm, which is the transition 3P2 -* 3D3. Figure
74B shows emission at the same atomic line. This is a measurement of the
emission intensity of transitions terminating at a metastable level, which may be
used to estimate the population of argon metastable species. Emission
measurements at this line may produce errors in the absolute number densities
for the metastable population, because all transitions are not accounted for (e.g.,
direct electron excitation and possible electron-ion recombination) [129].
However, generalities on plasma effects may be made using this argon emission
line. Both graphs show a similar profile and time of recovery. However, the

Emission Signal (counts x 10 ) Absorbance
206
Figure 74 Signal profiles obtained with a 30 ms pulse of water
vapor into a titanium discharge. A) Ar* absorbance; B) Ar* emission.

207
argon metastable emission is quenched to a greater extent that the argon
metastable absorbance.
The results presented in these sections illustrate that the effects of water
vapor introduction into the glow discharge are two-fold. Since the titanium
emission is reduced, there must be a loss of sputtering in the plasma. Also, since
the argon metastable population is reduced, the ionization in the plasma is also
decreased.
Power Supply Operation
The behavior of the species under investigation was further explored to be
sure that the signal changes being observed were real and not just an anomaly
of the power being supplied to the cathode. The majority of the studies were
performed with the power supply being operated in the constant voltage mode.
Upon each pulse, a short-lived decrease in the discharge current was also
observed. This decrease in current could possibly be a factor in the decrease of
ion signal that was detected. Recall, that when water was added to the ion
source as described in Chapter 3, there was a correlating decrease in the
discharge current and the discharge current was found to be inversely
proportional to the water concentration in the plasma. However, in those
experiments it was noted that the ion signals did not follow the current
characteristics exactly. For example, many abrupt changes were observed to
occur when the ion signals went through their "transition points." At this particular
point in the experiments, there was not a corresponding quick change in the

208
discharge current. Rather, the current slowly returned to its typical value as the
water was removed.
The purpose of these experiments is to determine if these changes in the
discharge current during constant voltage operation has a more dominant effect
here, since the changes in current and ion signal both occur in a short time
frame. While running the discharge in both constant current and constant voltage
mode, the ion and neutral atomic signal characteristics were observed as before
using MS, AA, and AE.
Constant voltage
When running the discharge with a constant voltage power supply there
will be a constant energy of the bombarding species on the cathode surface, but
as the current changes the number of ions striking the cathode will change.
Figure 75 shows the current profile that was obtained while running the glow
discharge in the constant voltage mode. From these observations, it may be
thought that the decrease in ion signal is a result of the change in discharge
current.
Constant current
When operating the discharge in the constant current mode, the number
of ions impinging onto the cathode surface will remain the same. However, the
energy of the ions arriving at the surface will change as the voltage changes.
Figure 76 shows the response of the discharge voltage as the water is pulsed into
the system. From this observation, one might expect an increase in the ion
signals previously shown.

o
CD
E?
CÜ
O
cn
Q
0 60 120 180 240 300
Time (s)
Figure 75 Discharge current profile obtained from a titanium pin operating at
a constant voltage of 1250 V when a 30 ms pulse of water vapor was introduced.
209

Figure 76 Discharge voltage profile obtained from a titanium pin operating at
a constant current of 4.0 mA when a 30 ms pulse of water vapor was introduced.
210

211
The ion signal profiles obtained under these different operating conditions
show a very similar behavior. Figure 77 shows the titanium ion signal obtained
while operating the discharge in both the constant voltage and constant current
mode. Upon initial observation of these profiles, it was determined that the power
supply operation was not affecting the ion signal profiles that were being
detected. The major difference between these two operating parameters is that
in constant current mode, the recovery time appears to be slightly faster. This is
most likely due to the increase in voltage that occurs when the water is pulsed
into the system. A higher voltage will increase the energy of the plasma and will
allow the plasma environment to "clean up" faster, as mentioned previously.
Therefore, the changes that are being observed upon the pulsed injection of water
vapor into the ion source are not attributable to the differences in the power
supply operation, even though there are changes in the voltage and current as
the other parameter is held constant. Similar profiles were obtained with the
sputtered species and argon metastable atoms in the plasma. The effects of both
operation modes were similar, only the recovery time was slightly enhanced when
running in the constant current mode.
Conclusions
This chapter has described additional experiments that have been
performed to explore the effects of water vapor on the glow discharge plasma
processes. The results that were obtained are in agreement with those observed
in Chapter 3. The water introduction method described in this chapter allowed
the determination of the processes occurring primarily in the gas phase of the

Ti Ion Signal (counts x 10°) Ti Ion Signal (counts x 10
212
Figure 77 Titanium ion signal profiles obtained with a 30 ms
pulse of water vapor into a titanium discharge. A) Constant voltage
operation; B) Constant current operation.

213
glow discharge, since the surface-water interactions are kept to a minimum with
the short pulse times used. As expected, the titanium is the least affected since
it exhibits efficient gas-phase reactions that will keep the water effects to a
minimum. The water dissociation species in the plasma showed an increase
upon the introduction of the water vapor, since the discharge environment will
promote this process. These species then decreased in intensity as they were
pumped and reacted out of the system. The pulsed water showed a detrimental
effect on the argon species in the plasma as well, very similar to that shown in
Chapter 3. The water species ion signals were shown to increase upon the
pulsed introduction of water vapor and the recovery time for the system to reach
equilibrium was different for each cathode. The titanium cathode removed the
water and its effects the fastest, while the copper cathode took the longest. The
effects on sputtering were monitored by observing the changes in the atomic
absorption and atomic emission of the analyte species. All of the cathodes under
study showed a 40 - 60 % decrease in the analyte atomic population upon the
introduction of the water vapor. Also the effects on Penning ionization were
observed by the decrease in the argon metastable atomic population. These
experiments show that even a small introduction of water vapor into a glow
discharge environment can have effects that last from a few seconds to about one
minute.

CHAPTER 5
FINAL REMARKS AND FUTURE DIRECTIONS
The experiments performed for this dissertation were a progression from
the introduction of aqueous solutions into the GDMS ion source to studies of
water vapor effects. Since the aqueous solution method was limited by the
introduction of contaminants with each sample, it was of interest to study the
water vapor effects on the fundamental processes and analytical utilities of the
glow discharge. Water vapor problems are difficult to avoid since the analysis
must be performed quickly after initiating the glow discharge to avoid sample
loss. With a normal pin cathode, it often takes as much as 10 -15 min to obtain
a stable discharge. Therefore, if the cathode is brought in and out of the ion
source in the same manner described earlier for the solution analysis and data
were collected in the first minute of sputtering, similar levels of standard
deviations might be obtained. Thus, the inherent operation of the glow discharge
limits this type of sample introduction, where data collection starting immediately
after initiating the discharge is required.
Final Remarks
Aqueous Solution Samples
Electrothermal vaporization has been used to analyze solution samples
quickly in a GDMS system and its characteristics were shown to be similar to
those of other GDMS systems. The sampling sequence is an important part of
214

215
the method development, since sample dryness and sampling sequence timing
need to be accurate. The amount of sputtering loss before initiation of the
electrothermal filament should be minimized, but the initial sputtering time should
be sufficient to provide a stable discharge. This was obtained in a minimum time
of 5 s, which does not result in a large loss of ion signal (a few tenths of a
percent of the total). The ion transmission from both an auxiliary cathode and the
filament itself was dependent on the parameters used. Maximum ion signals were
obtained with a constant voltage discharge and the maximum filament current, so
these parameters were used for the majority of the studies. The method showed
linearity in the limited concentration range studied for these experiments, and the
optimum operating pressure was found to be about 1.2 Torr argon, which is
consistent with that used for a typical glow discharge application. A major
difference that was observed with this source is the cathode-to-ion exit orifice
distance that produces the maximum ion signal. For the electrothermal source,
the maximum signal was observed with the filament 3.5 mm from the ion exit
orifice. This difference may be a result of larger electron production from the
filament than observed for a typical glow discharge, or the atoms in the plasma
may be at different energy levels than those of the typical discharge. The
positioning of the sample on the filament is a critical parameter as well, with the
maximum ion signal obtained when the sample is applied at the end of the
filament nearest the ion exit orifice. The ETV/GD method shows no significant
memory effects when a blank sample is used, which is important since many
analyses of this type suffer from memory effects. Multi-element samples may be

216
analyzed to some extent. Only binary elements were demonstrated In the data
presented here, since the elements must be in a narrow mass range.
Improvements would be expected with a fast scanning system and the use of
internal standards to improve quantitation. Binary samples were separated by
ramping the filament current, but this method requires elements that are far apart
in their atomization temperatures unless the power supply is computer controlled.
Internal and external ETV introduction schemes were used and compared. The
internal method is preferred, due to its simplicity and better sensitivity. However,
internal ETV introduction suffers from higher relative standard deviations and
erosion of the filament material. The overall inconsistencies of this technique have
been attributed to the continual introduction of contaminant air and water vapor
with each subsequent sample.
Effects of Water Vapor on GDMS
Water vapor effects in the plasma can occur to various degrees depending
upon the cathode material that is used, the purity of the discharge gas, and the
integrity of the vacuum system. The reduction in ion signal, observed with the
mass spectrometer, is most likely due to a combination of inefficient sputtering,
oxidation of the sample surface, loss of analyte atoms through gas phase
reactions, and quenching of argon metastable atoms that are responsible for
ionization. Many surface and gas-phase reactions are occurring in the ion source
as the water vapor content is changed in the plasma. It was found that during the
initial period, after the steady state introduction of water vapor was suspended,
the greatest detrimental effects were attributed to the surface interactions of the

217
cathode. Elements with a greater reactivity toward water vapor in the ion source
form a more tenacious oxide layer and result in a longer period of time before an
ion signal is detected. Once sputtering is initiated and the surface begins to be
cleaned, then gas phase reactions seem to dominate. In this realm, the cathode
materials that are more active toward water vapor species will react with them and
remove them from the system, resulting in a cleaner spectrum. Water dissociation
products and hydrogenated species (i.e., N2H+ and COH+) show similar effects
and are considered to be in an interactive equilibrium with one another. The
same is true for the various argon species in the plasma that exhibit very similar
behavior toward the water vapor content in the plasma. The use of a liquid
nitrogen coil was demonstrated to alleviate the effects of added water vapor by
freezing out the contaminants and results in a mass spectrum that is dominated
by the elemental ion signals.
Pulsed introduction of water vapor was studied to observe the time-
dependent effects a small short-lived water pulse would have on the plasma
processes. With this introduction device, the amount of water vapor introduced
into the plasma can be limited and should not affect the surface of the cathode.
Gas-phase reactions of the cathode material and the sputtering processes should
not allow a surface oxide layer to form as long as the solenoid open time is kept
short (this time will vary with the cathode material). The gettering elements, such
as titanium, were found to be affected the least while water vapor had the greatest
detrimental effects on copper. Upon pulsing water vapor into the source, the
matrix species and the argon species were reduced, while the metal-oxide

218
species, water species, and water dissociation products ion intensities were
increased for a short period of time. The operation of the discharge in the
constant current and constant voltage modes were compared and found to give
similar results. However, when operating at a constant current, the return to an
equilibrium condition after water introduction is enhanced due to the increase in
discharge voltage.
The effects observed in these experiments will become important in the
analysis of an alloy or other real-world sample that contains a variety of elements.
Since ion signals of different elements are affected to different degrees in the
presence of water, calibration data for quantitative analysis (e.g., RSF values) that
are obtained at unknowingly different water concentrations in the plasma can no
longer be reliably used. Therefore, it is especially crucial that the water content
in a glow discharge ion source be controlled to a constant and minimal level for
quantitative analysis.
Future Directions
Aqueous Solution Samples
Aqueous solution samples in the glow discharge suffer most from the
introduction of water vapor into the ion source. If the time required to analyze a
solution sample were not ideally in a short time frame, the results could most
likely be improved. Future studies for the analysis of solutions in GDMS will
probably involve mixing the sample with a powder and drying the mixture,
followed by pressing the sample into a pin or disk for analysis, or the application
of the solution to a cathode that is dried and subsequently analyzed. These two

219
techniques do not give the improvement in sensitivity observed with the current
ETV/GD method, but will result in a more reproducible analysis since the drying
of the sample is assured. If the surface deposition on the cathode is used, then
the analysis must occur in the early times of discharge sputtering and might result
in deviations similar to those seen for the ETV/GD technique. Using the mixing
method results in a long lived signal that may be stabilized over the first few
minutes of sputtering and give more reproducible results.
As for the ETV/GD method, some improvements in the experimental
method are needed. First, there is the need to use internal standards for
quantitative analyses so that any changes in the plasma conditions from run to
run may be corrected for. However, in the light of the studies conducted with the
water vapor, the internal standard must be ideally chosen to interact with water
and other contaminants in a similar manner as the analyte. Second, mass
spectral scanning needs to be faster. If the internal standards method is to be
useful or multi-element species are to be analyzed, then repetitive rapid scanning
over a large mass range is needed.
Effects of Water Vapor on GDMS
Water vapor and its reactions in the plasma remain an intriguing focus of
glow discharge research, since this contaminant species is always present to
some extent in the discharge ion source. Almost any analysis will require some
period of "pre-sPuttenn9" to remove the water contaminants from the sample
surface and the ion source. This is of particular importance for powdered
samples that are pressed into a cathode disk, since water is often occluded into

220
the sample itself and will migrate out of the cathode over the experiment time.
Many of the reactions that were illustrated in this dissertation are of interest in fully
understanding the mechanisms of water interactions in the glow discharge
plasma. To understand these phenomena better, the introduction of isotopically
labeled water would be of interest. Using both H2180 and D20 would give further
insight into the plasma processes and the dissociation of water vapor species in
the glow discharge.

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Co., New York, 1942.
3) Howatson, A.M., An Introduction to Gas Discharges, 2nd ed.; Pergammon
Press, New York, 1976.
4) Loeb, L. B., Fundamental Processes of Electrical Discharge in Gases, Wiley,
Chapman, and Hall, London, 1939.
5) Hunt, D.F., McEwen, C.N., and Harvey, T.M., Anal. Chem., 47,1730 (1975).
6) Barnes, R.M., ed., Emission Spectroscopy, Dowden, Hutchinson, and Ross,
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7) Lindors, P.A., Mulaire, W.M., and Wehner, G.K., Surf. Coat. Technol., 29,
275 (1986).
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1986.

BIOGRAPHICAL SKETCH
Philip Ratliff completed his Bachelor of Arts degree in chemistry at East
Tennessee State University in May 1987. In the fall of that year, his graduate
career began at the University of Virginia under the direction of Dr. Willard W.
Harrison. With the relocation of Dr. Harrison’s research group, he transferred to
the Chemistry Department at the University of Florida in January 1989. He
completed his graduate studies with a Doctor of Philosophy in August, 1992.
228

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
Willard W. Harrison, Chairman
Professor of Chemistry
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
ü (á
Uir^-1
/ames D. Wineforcmer
Graduate Research Professor of Chemistry
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
Richard A. Yost ^
Professor of Chemistry

I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
I certify that I have read this study and that in my opinion it conforms to
acceptable standards of scholarly presentation and is fully adequate, in scope
and quality, as a dissertation for the degree of Doctor of Philosophy.
Stephen E. Nagler
Associate Professor of Physics
This dissertation was presented to the Graduate Faculty of the Department
of Chemistry in the College of Liberal Arts and Sciences and to the Graduate
School and was accepted as partial fulfillment of the requirements for the degree
of Doctor of Philosophy.
August 1992
Dean, Graduate School

UNIVERSITY OF FLORIDA
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