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A detailed study of laser enhanced ionization with electrothermal vaporization-flame atomization for trace element analysis

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A detailed study of laser enhanced ionization with electrothermal vaporization-flame atomization for trace element analysis
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Riter, Ken Lynn, 1970-
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English
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xiii, 216 leaves : ill. ; 29 cm.

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Subjects / Keywords:
Atoms ( jstor )
Electrodes ( jstor )
Flames ( jstor )
Furnaces ( jstor )
Graphite ( jstor )
Ionization ( jstor )
Laser beams ( jstor )
Lasers ( jstor )
Leis ( jstor )
Signals ( jstor )
Chemistry thesis, Ph. D ( lcsh )
Dissertations, Academic -- Chemistry -- UF ( lcsh )
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bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1996.
Bibliography:
Includes bibliographical references (leaves 205-215).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Ken Lynn Riter.

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University of Florida
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A DETAILED STUDY OF LASER ENHANCED IONIZATION
WITH ELECTROTHERMAL VAPORIZATION-FLAME
ATOMIZATION FOR TRACE ELEMENT ANALYSIS














By

KEN LYNN RITER



















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

1996




























Dedicated to the loving memories of my mother, Namiko

(Tamanaha) Riter (September 16, 1936 July 9, 1983), and

father, Roger Lynn Riter (July 12, 1945 August 10, 1996).

Without their love, encouragement, and support none of this

would have been written.














ACKNOWLEDGMENTS


First, I would like to thank Dr. Jim Winefordner for the

opportunity to research in his lab. Although I have learned

much about laser spectroscopy in Jim's lab, his example of how

to be a decent person and to treat others with respect is what

will always be with me. I would like to thank Dr. Ben Smith

for all of his advice and help in the lab. Setting up the

instrumentation would have been an enormous task without Ben's

help. I would like to thank Dr. Oleg Matveev for working with

me and lending his expertise in laser enhanced ionization to

the project. I have learned so much about LEI from Oleg in a

very short period of time. Oleg's kindness, patience, and

modesty allowed him to explain very difficult concepts rather

easily.

I would like to thank Leah Mordoh and Wendy Clevenger for

assisting with and performing many of the experiments for the

LEIS of Mg. Others I would like to thank include Rob Guenard

for his help with the ultrasonic nebulizer, Chester Eastman in

the machine shop for building the new LEI burner, and all of

the Winefordner and Harrison group members for their help and

friendship. I would like to thank the National Institutes of










Health for funding this research (Grant # 5-R01-GM49638-03).

On a more personal note, I would like to thank my family

for their love and encouragement. Thanks to Jon DeGnore, Dr.

Bill Walden, and Dr. Wei Hang for being such good friends and

making my stay in Gainesville enjoyable. Finally, I would

like to thank my fiancee, Leah. Without her love and support,

I do not know how I would have made it through these final few

months.

All thanks be to God.

















TABLE OF CONTENTS




ACKNOWLEDGMENTS ... iii

LIST OF TABLES .... .viii

LIST OF FIGURES ... .ix

ABSTRACT ... xii

CHAPTER 1

INTRODUCTION ... 1

Absolute/Standardless Analysis 1
Analysis of Real Samples ............ 5
Determination of Lead in Whole Blood 6
Intent of Dissertation .. 7

CHAPTER 2

INTRODUCTION TO LASER ENHANCED IONIZATION 9

The Optogalvanic Effect 9
General Principles of LEI .. 10
Atomization of Sample .. 11
Excitation of Atoms .. .16
Ionization ................. 20
Charge Collection .. .20

CHAPTER 3

THEORY OF LASER ENHANCED IONIZATION .. .25

Introduction ..... .25
Thermal Ionization ... .25
Processes Reponsible for Thermal Ionization 27
Thermal Ionization Rate of an Atom in a Flame 29
Modeling of Laser Enhanced Ionization .. .30
Rate-Equation Formalism .. .30










Degree of Ionization for Two-Step
Excitation .
Density-Matrix Formalism ....
Detection of the Ionization Signal .. ..
One-Dimensional Approximation .. ..
Point Charge Model .....
Electrothermal Vaporization ...
Absolute Analysis ......


CHAPTER 4

REVIEW OF LASER ENHANCED IONIZATION ... .47

Analytical Performance of Flame-LEI ... .47
Limits of Detection and Sensitivities ... 47
Noise and Interferences 58
Applications of LEI to Real Samples .. 61
Determinations Without Interferant Removal 62
Determinations With Interferant Removal 66
Hybrid Techniques and Non-Flame Atom Reservoirs 71
Electrothermal Vaporizers ... 71
Hybrid Combinations of Flame and
Electrothermal Vaporizers ... 74
LEI in the Inductively Coupled Plasma ... 79
Other Methods and Reservoirs .. .80

CHAPTER 5

EXPERIMENTAL 83

LEI ..... 83
Burners .. 83
Graphite Furnace .... ... 88
Procedure and Conditions .. .97
Flame Gas Flows, Velocity, and Temperature 103
Noise Study .... 105
Fluorescence Dip and Fluorescence Profile of Flame 108
Transport Efficiency 113
Transimpedance Amplifier Calibration ...... 118
Atomization Efficiency Measurement .. .119

CHAPTER 6

RESULTS AND DISCUSSION 124

LEI of Magnesium 124
Magnesium As Analyte .. .124
Mg LEI Signal ... 127
System Parameter Optimizations for Old Burner











Design .
Flame Profile With Old Burner .. ...
Analytical Curve With Old Burner
New Burner Design .. .....
System Parameter Optimizations With New
Burner ...
Flame Profile With New Burner .. ..
Matrix Modifier/Carrier .. ...
Analytical Curve With New Burner .
Flame Temperature and Flame Gas Velocity
With the New Burner .. ....
Absolute Analysis ......
Vaporization Efficiency .. ...
Transport Efficiency ....
Probing Efficiency ......
Detection Efficiency ....
Atomization Efficiency ...
LEI of Lead .........
Excitation Scheme for Lead ..
Carrier ......................
Calibration Behavior ....


CHAPTER 7


CONCLUSIONS .

Absolute Analysis .
Pb in Blood .
Future Work .


REFERENCES ..........

BIOGRAPHICAL SKETCH .........


. 202

. 202
. 203
. 203

















LIST OF TABLES


Table aage

1. LEI limits of detection ... 48

2. Graphite furnace temperature program for
magnesium .... .. 99

3. Graphite furnace temperature program for lead in
blood .... .. 102

4. Values used for flame temperature calculation .175

5. Mg concentration in different cotton samples 177

6. Enhancement of LEI signal for different two-step
excitation schemes for lead ... .192


viii

















LIST OF FIGURES


Figure page

1. Processes needed for laser enhanced ionization
spectrometry .... 13

2. Typical experimental setup for flame-LEI ... .15

3. Typical excitation schemes for LEI spectroscopy,
a) one-step excitation using visible light, b)
one-step excitation using ultraviolet light, c)
two-step excitation (direct), d) two-step
excitation (indirect), and e) three-step
excitation .... 19

4. Various electrode arrangements used for LEI
spectroscopy, a) split-cathode rod arrangement,
b) split-cathode plate arrangement, c)water-
cooled, immersed cathode arrangement, d) water-
cooled coiled cathode arrangement ... .22

5. Schematic representation of various excitation
and deexcitation processes in a three-level atom:
nj, n2, and n3 are the number densitites of the
three levels, respectively; k21 is the sum of the
collisional deexcitation and spontaneous emission
rates between levels 2 and 1; k12 is the
collisional excitation rate between levels 1 and
2; k3,ion (k2,ion) is the collisional ionization rate
from level 3 (2); Bi2Uv(v12) (B23U,(v23)) and B21U(v21)
(B32U, v32)) are the absorption and stimulated
emission rates and U.(v21) (=U,(v12)) and U,(v23)
(=U,(V32)) are the spectral irradiances of the
laser light 32

6. Block diagram of the experimental setup for LEIS 85

7. First burner design used for LEIS ... .87










8. Diagram of new burner design with relative
position of the high voltage electrode and laser
beams .. 90

9. Detailed drawing of the new burner design .... 92

10. Cut-away view of the graphite furnace showing the
tantalum sample extraction interface ... 96

11. Sketch of laboratory constructed ultrasonic
nebulizer used ... 107

12. Block diagram of experimental setup for
monitoring of both fluorescence and LEI signals 112

13. Sketch of experimental setup for transport
efficiency measurement ... 116

14. Block diagram of the experimental setup for the
determination of the atomization efficiency for
Mg by atomic absorption ... 122

15. Oscilloscope trace of the laser beam timing 126

16. Typical LEI signal for magnesium with older
burner 129

17. Argon flow rate optimization for Mg with older
burner .... 133

18. Burner-to-electrode distance optimization for Mg
with older burner ... 135

19. Applied voltage optimization for Mg with older
burner ... 137

20. Horizontal profile of the flame with the older
burner 140

21. Analytical curve for Mg with the older burner .142

22. Log-Log plot of the analytical curve of Figure 21 144

23. Noises for the hydrogen/air and acetylene/air
flames with the older burner design ...... 147

24. Comparison of the noises for the new and old
burner designs .150










25. Argon flow rate optimization with new burner 152

26. Acetylene flow rate optimization with new burner 154

27. Air flow rate optimization for new burner 156

28. Burner-to-electrode distance optimization for new
burner ... .. 159

29. Electrode voltage optimization with new burner .161

30. Horizontal profile of Mg atoms in flame with new
burner .. 163

31. Fluorescence profile of Mg atoms in the flame
with the new burner .. 165

32. LEI signal for xylene while scanning dye laser
for X. ... .. 168

33. Effect of methanol on LEI signal for Mg ... 171

34. Analytical curve for Mg with new burner and
methanol 173

35. Mg LEI signal with increasing laser repetition
rate ... 181

36. Dye laser conversion efficiency with increasing
laser repetition rate for .. .183

37. Dye laser conversion efficiency with increasing
laser repetition rate for X2 ... 185

38. Analytical curve for aqueous lead standards with
and without NaC1 addition ... 194

39. Log-Log plot of analytical curves for aqueous
lead and blood lead ... 197

40. Analytical curve for diluted blood lead standards 199















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


A DETAILED STUDY OF LASER ENHANCED IONIZATION
WITH ELECTROTHERMAL VAPORIZATION-FLAME
ATOMIZATION FOR TRACE ELEMENT ANALYSIS

By

Ken Lynn Riter

December 1996

Chairperson: James D. Winefordner
Major Department: Chemistry

A system coupling electrothermal vaporization with flame-

laser enhanced ionization (ETV-FL-LEI) was examined for the

possibility of "absolute" analysis. For a method to be

considered absolute, analytical matrix interference must be

eliminated, the stability of the calibration over time must be

established, and the theoretical equation relating the signal

to the quantity of analyte must be known. For our system, the

LEI signal for magnesium is equal to the product of the

Faraday number (96,485 C/mol), moles of Mg, and overall system

efficiency. In our case, the overall system efficiency is the

product of the vaporization efficiency of the ETV, the

transport efficiency of Mg from the ETV to the flame, the

atomization efficiency of Mg in the flame, the probing

xii









efficiency of the laser beams, and the detection efficiency.

Ideally, these efficiencies should be unity. However, it was

found that all of these except the vaporization efficiency was

less than unity. Also, the LEI signal deviated from linearity

at low Mg concentrations and required the addition of a matrix

modifier to restore the signal. This indicates a dependence

of the LEI signal on the sample matrix. Therefore, ETV-FL-LEI

should not be considered an absolute method.

A second project involved the application of our ETV-FL-

LEI system to the determination of lead concentration in whole

blood. Blood standards from the Centers for Disease Control

(CDC) and the National Institute of Standards and Technology

(NIST) were diluted 21:1 with ultra pure water and analyzed.

Good agreement was found between the CDC and NIST standards.

A linear analytical curve was obtained with a detection limit

(30) of 8.9 X 10-3 pg/dL (890 fg absolute) for lead in whole

blood. This compares favorably with other current methods for

blood lead determinations including isotope dilution

inductively coupled plasma mass spectrometry (ID-ICP-MS) and

graphite furnace atomic absorption spectrometry (GFAAS).


xiii















CHAPTER 1
INTRODUCTION



Absolute/Standardless Analysis



There are no analytical procedures which are "absolute"

in the strictest sense of the word, because to analyze

absolutely i.e. without any presuppositions it would be

necessary to identify the atoms and the molecules of the

sample, to sort them out, and to count them individually and

completely [1].

However, if a theoretical expression is known for the

function relating the signal to the absolute quantity of

analyte present that is sufficiently reliable to allow a

direct calculation of the quantity of analyte from a single

measurement, then this method is called "an absolute method of

analysis" [2]. The most complete program for the development

of absolute methods of analysis includes [2]: (1) elimination

of analytical matrix interference, (2) stabilization of the

calibration over time, and (3) theoretical calculation of the

calibration based on fundamental parameters and actual

measurement conditions. This should be distinguished from













"standardless" analysis where the calibration curve is stable

over time. Therefore, once the system has been calibrated for

a particular sample, the calibration should need to be checked

only occasionally (such as once every 8 hours) with a

standard (concentration = 100 to 1000 times the limit of

detection).

Many classical methods, such as precipitation reactions,

titrimetry, and coulometry, are considered absolute. When

considering modern instrumental methods for absolute analysis,

usually atomic absorption, where relative measurements are

made, is considered the best candidate rather than emission or

fluorescence procedures where absolute radiometric

measurements are required. According to L'vov [2], this is

not surprising since the atomic absorption method of measuring

the analytical signal is free from many of the uncontrolled or

difficult-to-control factors typical of emission/fluorescence

methods. Also, the stability and consistency of calibration

for modern graphite furnace atomic absorption spectroscopy

(GF-AAS) have brought GF-AAS close to achieving absolute

analysis. However, the sensitivity of GF-AAS, although high,

is still well within the possibility of preparation of

accurate standard reference solutions with minimal

contamination and loss problems. In this respect, absolute

analysis by GF-AAS will most likely never be a necessity.

However, GF-AAS is very amenable to standardless analysis












because of the reproducibility and consistency of the

calibration. Electrothermal vaporization-inductively coupled

plasma-mass spectrometry has achieved, in some cases, low or

even sub-femtogram detection limits. However, it is essential

that standard samples or solutions be used for calibration,

since the transport efficiencies of analyte ions are unknown

and vary significantly with analyte type, matrix type, gas

flow rates, sampling cone and skimmer cone geometries and

electrostatic lens configuration and conditions.

Laser induced fluorescence with graphite furnace

atomization (GF-LIF) is one of the most sensitive methods for

trace element analysis. However, absolute analysis by GF-LIF

involves a difficult and time consuming calibratrion process

relating the signal level to the mass of the analyte and

requires corrections for laser induced ionization, thermal

ionization, and matrix background, and evaluation of or

knowledge of the diffusion coefficient of analyte atoms at the

furnace atomization temperature.

Analytical methods based upon ionization are potentially

the best candidates for absolute analysis. Ions can be

produced with great selectivity and, once an ion is produced,

the probability of detection is generally high. Moreover,

most of the complexities associated with absolute fluorescence

measurements are avoided; the relationship between amount of

analyte and the measured signal is substantially simpler. A













new analytical approach using laser enhanced ionization with

sub-fg (and sub-pg/mL) detection limits has been described by

Smith et al. [3,4] which shows great promise as a standardless

or absolute method. This method involves coupling of a

graphite furnace for sample vaporization with an acetylene/air

flame for laser enhanced ionization (LEI) detection. In

essence, this method can be considered as the analog of

coulometry (i.e. each atom will produce a charge of 1.6 X 10'-

C or 96,487 C/mol). It is hoped that the combination of high

sensitivity and simplicity of detection will make LEI a good

candidate for absolute analysis.

There are several fields in which extreme sensitivity

(sub-fg) combined with absolute analysis would be useful since

the preparation of standards at very low analyte

concentrations is difficult because of sample loss and

contamination problems. This makes the development of an

absolute analytical protocol a pressing need [5]. In medical

research, the determination of the microdistribution of trace

metals in pg amounts of human tissues is essential and

concentration levels at or below pg/g are common [6].

However, the preparation of standards is difficult or

impossible because of the complexity of the sample matrix.

Similar detection capability is required for the determination

of trace elements in small amounts of solid samples of












interest in criminalistic, expert legal, and forensic medical

cases [7]. In situations where the preparation of hazardous

standards needs to be avoided, e.g. the analysis of

radioactive species, a means of quantitation using an absolute

approach would be desirable. These examples are indicative of

the situations where absolute analysis by graphite furnace-

flame-LEI spectroscopy would be desirable: (i) when the

quantities of sample are limited and (ii) when standards are

difficult or impossible to prepare due to the very low

concentrations being used, the difficulty of simulating very

complex sample matrices, or the danger of handling hazardous

analytes.


Analysis of Real Samples by LEI Spectrometrv



Laser enhanced ionization spectrometry (LEIS) is a

sensitive technique for trace element analysis and has become

well established since its discovery in 1976 [8].

Unfortunately, the application of LEIS has been limited mostly

to simple aqueous systems because of ionization interference

encountered in complex matrices. Thus, the application of

LEIS to complex real samples, such as biological fluids and

environmental samples, has remained largely undeveloped.

The combination of the graphite furnace, used as an

electrothermal vaporizer (ETV), with a flame for LEI detection










6

has many advantages for the analysis of real samples. By

separation of the graphite furnace vaporization and LEI

detection processes, this system overcomes the problems of

thermionic emission from the graphite tube and poor

collisional ionization encountered with LEIS in the graphite

furnace. This system also allows for the removal of many

interfering matrix species by temperature programming of the

graphite furnace. As a result, ionization interference that

have plagued LEIS of real samples may be reduced or even

eliminated.


Determination of Lead in Whole Blood



Since the Centers for Disease Control (CDC) lowered its

pediatric level of concern for blood lead to 10 jg/dL in 1991,

there has been increased interest in more sensitive methods

for the determination of lead concentrations in blood. There

also exists a need in research laboratories for accurate and

precise measurements of substantially lower blood lead levels

(< 10 g/dL) to establish the levels for chronic lead toxicity

in humans [9]. Some studies indicate that there may be no

threshold for lead toxicity in humans [10,11], and so the need

for more sensitive methods for determining blood lead levels

is clear.

Isotope dilution inductively coupled plasma mass












spectrometry (ID-ICP-MS) and graphite furnace atomic

absorption spectrometry (GFAAS) are both methods currently

used to measure low levels of lead in blood. However, both of

these methods require extensive sample preparation with a

matrix modifier, and have detection limits of only around 1

jg/dL for lead in whole blood [12,13]. With the combination

of graphite furnace with flame-LEIS, little sample preparation

is needed (21:1 dilution with ultra pure water) unlike with

ID-ICP-MS and GFAAS. This not only reduces the complexity of

sample preparation but also reduces sample contamination from

matrix modifiers. This becomes especially important at very

low blood lead levels, so improved detection limits may be

expected. Another advantage of the ETV-LEIS combination is

the high sensitivity afforded by LEIS.


Intent of Dissertation



The aim of the present work was to satisfy the third

condition for absolute analysis (stated earlier) by

characterizing the efficiencies associated with our

experimental setup (consisting of the combination of a

graphite furnace with an acetylene/air flame for LEI

detection) to obtain the theoretical equation relating the

analyte quantity to the LEI signal. This equation and the

efficiencies will be discussed in more detail in Chapter 2.












A second objective was to explore the possibility of

applying this technique to the analysis of real samples with

complex matrices. We chose to analyze lead concentrations in

a blood matrix.











CHAPTER 2
INTRODUCTION TO LASER ENHANCED IONIZATION



The Optogalvanic Effect



The optogalvanic effect (OGE) is the term for the process

whereby the collisional ionization rate for an element is

enhanced by optical excitation to a higher electronic energy

state [8,14]. This mechanism was first postulated by Foote

and Mohler _n 192E 15]. The first purely optical/collisional

effect, however, was not actually observed until 1928 by

Penning [161. Penning observed the effect as a change in the

voltage drop across a neon discharge when illuminated by a

second neon discharge.

Unfortunately, optical enhancement of collisional

L;nizatis _- too .-eak of an effect to be seen in usual atom

reservoirs with conventionall light sources. Therefore, it

would not be until the advent of tunable lasers that the OGE

could be easily observed and used as a spectroscopic method.

In 1976, researchers at the National Bureau of Standards

'BS, now -he Nati-'al Institute of Standards and Technology,

7IST) decided to investigate the hollow cathode lamp as a

reservoir for laser induced fluorescence (LIF) after

:isappointing results for LIF in a flame [17]. These

researchers fortuitously discovered a change in the voltage










10

across the hollow cathode lamp when the dye laser was tuned to

an electronic transition of one of the atomic species in the

discharge [14]. Shortly afterwards, the same group decided to

look for a related effect in flames. The effect was observed

and a sub-ppb detection limit for sodium, equivalent to their

results with LIF in a flame, was quickly obtained [8].

The term, laser enhanced ionization (LEI), was first

introduced in 1978 by the group at NBS [18]. Today, LEI is

used to describe the optogalvanic effect in flames and other

reservoirs except discharges. OGE or optogalvanic

spectroscopy (OGS) is now used exclusively for phenomena

taking place in discharges.



General Principles of LEI



Laser-enhanced ionization spectrometry (LEIS) can be

defined as a spectroscopic method where an enhancement in the

normal collisional (thermal) ionization is obtained by optical

excitation of the atoms under study by resonant laser light.

This enhancement is detected as a change in the current

passing through a medium (atom reservoir) between two

electrodes at different potentials.

LEI can be broken down into four distinct processes:

atomization of the sample, optical excitation of the analyte

atoms by resonant laser light, collisional ionization of the












excited atoms, and collection of the charges produced

(Figure 1).


Atomization of Sample


The conversion of the sample into an atomic vapor or

atomization of the sample is dependent upon the particular

characteristics of the atom reservoir used. To date, many

different atom reservoirs have been used for LEI including

atmospheric flames, graphite furnaces, and inductively coupled

plasmas (ICPs). However, the large majority of LEI studies

have been done in atmospheric flames. This is because of the

combination of good qualities that flames exhibit for

atomization, ionization, and the detection processes. A

typical flame-LEI setup is shown in Figure 2 and consists of

a flame, laser system, electrodes, and electronic detection

equipment.

In conventional flame-LEI, the sample is aspirated by a

pneumatic nebulizer into a spray chamber. The sample leaves

the spray chamber as a spray or mist of fine droplets and

enters the flame. The flame heats this spray and causes the

solvent to vaporize leaving dry aerosol particles. Further





































Figure 1. Processes needed for laser enhanced ionization
spectrometry [19]





A





o at
'o~y^ M^ GS



























Figure 2. Typical experimental setup for flame-LEI [19]


















Flame


Dye Dye
Laser Laser Sample
1 2



:i::i :i:'.r!::: : !: Pum p Laser


:::. .








16

heating in the flame volatilizes these dry aerosol particles

producing atomic, molecular, and ionic species.

Many different flames have been used for LEIS including

acetylene-based flames such as CzH,/air and C2H2/N20 and

hydrogen-based flames such as H2/NO, and Hj/Oz/Ar. The cooler

hydrogen-based flames have little flame-ion production so the

background noise from the flame is low. However, the flame

temperature and flame composition have been found to be the

most important factors in obtaining strong LEI signals,

because these factors greatly affect the atomization

efficiency. Therefore, the hotter acetylene-based flames are

most commonly used as atomic reservoirs in LEI. On the other

hand, if the flame temperature is too high, thermal ionization

of the analyte may be considerable resulting in poor detection

limits for atomic LEI. The most common flame used for LEI has

been the air/acetylene flame in which a large number of

elements can be conveniently analyzed. Hotter flames, such as

CH2/N2O, are mostly used for refractory elements.


Excitation of Atoms


Optical excitation of the analyte atoms is usually

performed by a pulsed dye laser. The dye laser may be pumped

by a variety of sources including flashlamps, Nd:YAG, excimer,

and N2 lasers. Continuous-wave (cw) lasers have been used

infrequently for LEI because cw techniques are often too










17

complicated for analytical applications. This is mostly due

to the ability of pulsed laser sources to produce much higher

intensity light, especially in the ultra-violet region.

Another advantage of pulsed lasers for LEI is that the excess

charge created can be collected during a shorter period of

time, which reduces the influence of background current noise.

Flashlamp-pumped dye lasers usually have pulse durations

in the microsecond range while excimer, Nd:YAG, and N2-pumped

dye lasers usually have pulse durations around 5-20 ns. Pulse

energies vary typically from 0.1 to 100 mJ in the visible and

1 mJ to 10 mJ in the ultra-violet region depending on the

laser system, dyes, and crystals used. The wavelength region

covered by dye laser systems typically ranges from 220 nm to

1000 nm. Repetition rates used are normally around 5-100 Hz.

Many different excitation schemes may be used for LEI

spectroscopy, some of which are shown in Figure 3. One-step

excitation using either visible or ultra-violet light has been

used extensively in flame-LEIS. For elements that are more

easily ionized, a single-step scheme is sufficient to achieve

low limits of detection. However, for many elements, it is

favorable to use a two-step excitation scheme. Most two-step

schemes share an intermediate level although this is not

necessary if the upper level of the first step and the lower

level of the second step are sufficiently coupled by


























Figure 3. Typical excitation schemes for LEI spectroscopy, a) one-step excitation
using visible light, b) one-step excitation using ultraviolet light, c) two-step
excitation (direct), d) two-step excitation (indirect), and e) three-step excitation.


























Eo I I L
a) b) c) d) e)










20

collisions. The addition of the second step usually results

in a significant increase in the signal strength (up to three

orders of magnitude) compared to one-step excitation. Three-

step excitation schemes have also been used but are not common

in flame-LEIS.

Ionization


The common atomic reservoirs (flames, furnaces, and

plasmas) result in thermal ionization. Most atoms ionize

through collisions with thermally excited molecules in the

reservoir. Therefore, the thermal ionization rate depends on

the reservoir constituents, temperature of the medium, and the

ionization potential of the element of interest.

When an element is excited by laser radiation, the atomic

population of that element is greatly altered. Excited atoms

are more easily ionized by collisions than those in the ground

state, so an increased ionization rate results.


Charge Collection


The additional charges produced by optical excitation are

collected by applying an electrical field across the flame (or

other atomic reservoir) using an electrode arrangement and

measuring the current change. Many different electrode

arrangements have been used, some of which are shown in Figure



























Figure 4. Various electrode arrangements used for LEI spectroscopy, a) split-cathode
rod arrangement, b) split-cathode plate arrangement, c) water-cooled, immersed
cathode arrangement, d) water-cooled coiled cathode arrangement.












H.V. -H.V. -H.V. H.V.


Electrode


II



Flame




Burner



a) b) c) d)








23

In all four schemes, the burner head acts as the anode

and is connected to the detection electronics. In the split-

cathode rod arrangement (Figure 4a), two metal rods are

positioned on opposite sides of the flame and biased to the

same negative high voltage with respect to the burner head.

This arrangement was found to be very sensitive to easily

ionized matrix species [20,21]. This led to the development

of the split-cathode plate arrangement (Figure 4b) which was

used for several years [22,23]. This was the case because of

its stable electric field distribution in the flame,

relatively high contamination resistance, and long lifetime.

The stainless steel, water-cooled, immersed cathode

arrangement (Figure 4c) was developed to locate the cathode as

close to the laser excitation zone as possible [24]. The

maximum signal strength and optimal resistance to electrical

matrix interference should occur with the laser excitation as

close to the cathode as possible. This electrode is also easy

to fabricate, robust, and contributes negligible memory

effects. The saturation current is also reduced with respect

to that of the split-cathode plate, which results in a lower

background current. The water-cooled coiled cathode (Figure

4d) has

equivalent sensitivity to the immersed cathode and appears to

have even greater resistance to easily ionized matrix species

[25]












Depending on other experimental parameters such as

electrode configuration, the applied voltage varies from about

300 V to ~3000 V. The current drawn through the flame is

usually in the pA range and the amount of charge detected is

in the fC range or larger. A typical LEI signal has a

duration of 300 ns up to 1 4s. The very small LEI current is

directed through a current-to-voltage amplifier which is

usually placed very close to the flame to minimize radio

frequency noise from the pulsed laser. The d.c. current is

usually filtered out using a d.c. blocking capacitor.

The current pulse is detected synchronously using a

boxcar integrator with pulsed laser excitation or a lock-in

amplifier with a chopped continuous wave laser. The signal is

then collected and processed by a computer.























CHAPTER 3

THEORY OF LASER ENHANCED IONIZATION


Introduction



The basic principle of laser enhanced ionization (LEI) is

to enhance the existing thermal ionization rate in the flame

(or alternate atom reservoir) by optical (laser) excitation

and then to detect this increase in the ionization rate as an

increase in the current passing through the flame between two

electrodes. A discussion of the theory for two-step LEI in

flames will be covered here. A more general and in-depth

discussion of LEI theory is given by Axner and Rubinsztein-

Dunlop [26] and Travis and Turk [27].




Thermal Ionization



A combustion flame possesses a small but not always

insignificant amount of thermal ionization [5]. If an atomic

25










26

system is retained in a flame, we can define a thermal

ionization rate constant, kh.on for each specific process in

the flame. One such process is the collisional ionization

between a species M and a thermally excited collisional

partner X*:


M + X* -kt M* + e- + X (1)


where M' is the positive ion, e- is an electron, X is the

deexcited collisional partner.

Similarly, there exists a recombination rate constant,

keco,., for the reverse process, given by:


MI + e- + X krecomb M + X* (2)


When the system is in thermal equilibrium, the thermal

ionization and recombination rates balance exactly. This

enables us to write an expression for the relation between the

concentration of the species M, its ions, and the electrons.

This relation is known as the Saha equation [28] and is stated


as:


[M'] [e-] = Kion [M] (3)


where K.o. is the ionization constant given by:



K on = th.ion
K to b (4)
krecomb










27

It should be noted that although a third body (collision

partner) is required for the ionization and recombination

processes and the rates for each of these processes is

dependent upon the concentration of the third body, the

ionization constant (and degree of ionization of the atomic

system) is independent of the concentration of this third body

as long as thermal equilibrium conditions prevail in the

flame.

As has already been mentioned, in LEI, an electric field

is applied to the volume of interaction within the flame to

separate the created charges and make detection of those

charges possible. Consequently, the electric field will

minimize recombination from occurring at any significant rate.

Therefore, the only remaining process will be thermal

ionization, so the Saha equation is no longer valid.

Processes Responsible for Thermal Ionization


In a flame, a variety of different interactions between

atoms, molecules, and light can take place. The major

processes that lead to ionization can be divided into physical

and chemical ionization processes. The physical processes and

be further divided into collisional and radiative ionization

processes.

Collisional ionization processes are most often

considered to be responsible for ionization of atoms in













flames. An example would be the collisional ionization of

sodium atoms in the flame:

Na + X* Na' + e- + X (5)

where X represents any flame molecule [5,29].

Radiative ionization processes are usually

photoionization processes where atoms are irradiated with high

intensity, short wavelength light which results in the

ejection of an electron, such as:


Na + hv Na' + e- (6)


or by the interaction between blackbody radiation and excited

atoms.

Chemical ionization processes are most easily

characterized as those in which the formation of a new

chemical bond takes place. Most alkaline earth atoms are

believed to ionize in this way. Associative ionization, such

as for the calcium atom:


Ca + OH CaOH' + e- (7)


represents such a process. There are also many charge

distribution processes that may be of importance since they

can constitute one reaction in a chain leading to a net

ionization rate.












Thermal Ionization Rate of an Atom in a Flame


To model thermal ionization, some assumptions must be

made. It is assumed that collisional ionization dominates

other ionization processes in the flame. It is also assumed

that the flame, with all of its atomic and molecular species,

is in a state of thermal equilibrium. This implies that the

concept of detailed balance between all atomic levels is

valid. In other words, the atomic energy levels are populated

according to Boltzmann's distribution:


gi exp(-E,/kT)
n = Z nato (8)



where ni is the population of the ith level (m-3), g, is the

degeneracy of the ith level, Ei is the energy of the i1h level,

n,to,, is the total number density of neutral atoms (m-3), and

Z is the electronic partition function:


Z = Fgi exp(-Ei/kT) (9)



where k is the Boltzmann constant and T is the flame

temperature.

However, the thermal ionization rate will be

overestimated unless we assume that the condition of detailed

balance is not valid for the highest lying states in the atom.

If we assume that detailed balance is only valid for states up










30

to a certain level, then the thermal ionization rate, dn,,n/dt

(mW3 s '), can be expressed by:


dnt. 8 kT exp(-Eon/kT) n (10)
dt Z n. 91g..


where is the reduced mass of the system [g =

mato*mx/ (m.to~+mx) where mt.om and m. are the masses of the atom

under consideration and collision species X, respectively],

aion, is the ionization cross section for the species X, Eion is

the energy of the ionization limit, and nx is the

concentration of species X.

Although, with the above simplification, the thermal

ionization rate depends greatly upon what cutoff level is

chosen, the relationship between the enhanced ionization rate

due to laser excitation and the thermal ionization rate is not

very sensitive to the specific cutoff level chosen.


Modeling of Laser Enhanced Ionization



Rate-Equation Formalism


Let us assume a three-level atom illuminated by two laser

beams. For simplicity, it is assumed that the atoms consist

of only three bound levels (see Figure 5) among which laser























Figure 5. Schematic representation of various excitation and deexcitation processes
in a three-level atom: ni, n2, and n3 are the number densities of the three levels,
respectively; k21 is the sum of the collisional deexcitation and spontaneous emission
rates between levels 2 and 1; k12 is the collisional excitation rate between levels 1
and 2; k3,,. (k2,ion) is the collisional ionization rate from level 3 (2); B2U,(v12)
(B23U,(v23)) and B2iU,(vz2) (B32U,(v32)) are the absorption and stimulated emission rates
and U(v21) (=U(v12) ) and Uv(v23) (=Uv(v32)) are the spectral irradiances of the laser
light.




















B23U, (v23)


B12 v (12)


B21Uv (v21)


B32U ( v32)


n
ion


k2,o
2, lon


k13
k13


ni










excitation takes place in two steps, weakly coupled to an

ionization continuum.

Degree of Ionization for Two-Step Excitation

The degree of ionization here is defined as the fraction

of atoms in the interaction volume that ionize during the

laser pulse. The expression for the degree of ionization is

considerably simplified by assigning an effective collisional

ionization rate from the uppermost laser-coupled level, kfion

With these assumptions, the ionization rate can be written as:


dni _,, eO f f (11)
dt = 3 3,ok io (ntot- ion) )


where

SBuU,(v12) 23U, (v2)
c =-(C3%) (12)
3 (BU, (v,) +a) (B3U (v23) +b) +d12


g3 (13)
(C, ) = g1+tg+, 13



a = (C3 )tak3i + (Cn) ~k (14)



(C, )
b = -T-- (k3+k32) (15)
(C2 ) sa




d = [ (C) a.2 (k,+k, ) (k21- k f- )t (16)
(C2 ) sat














(C f) ga (17)
2 ) g1+g2



o(C, :g (18)
(C2a g)g1+g2+13



not is the total number density of the analyte (neutral atoms

and ions), nion is the number density of analyte ions, and the

designations of "on" and "off" refer to the second laser beam

(%2) being on or off.

Time dependent solutions to these equations for an

interaction time, -, of laser light can then be readily

expressed as:

njn( ) = aion ( ) nt (19)


where a," represents the degree of ionization of the atomic

system and can be expressed as:


al(T)= l-exp(-C)n k~ i,,0) (20)




Density-Matrix Formalism


The rate-equation formalism, however, is primarily valid

only for one-step excitation so an effective model for two-

step excitation is needed. Effects not taken into account are

those primarily caused by intense light fields, such as two-













photon excitation and dynamic Stark broadening, splitting, and

shifting. In order to account for these effects while keeping

the model as simple as possible, a theory for LEI based on the

density-matrix formalism was proposed by Axner et al.

[26,30,31,32,33].

The density-matrix formalism of light and matter

interactions and its theoretical assumptions have been

discussed in detail in many sources previously

[30,34,35,36,37,38,39,40,41,42,43]. Therefore, only a brief

overview will be given here.

Assuming that the laser frequency profile is Lorentzian,

the system of density matrix equations can be solved under

steady-state conditions when the time dependencies of the

level populations are neglected. The steady-state

simplification is valid if the pulse duration is substantially

longer than the inverse of the deexcitation collision rate.

The following expressions result for the fractions of atoms

excited, C2" and C3DM (where D denotes the Density Matrix

formalism), for the density-matrix formalism:


CD_ n2 1 R12(R23+k31+k32) + (R13 (R12+R23+k32) ( )
C, an (21) D



Ca = nt- -- (22
C3 3 (22)
ntom 3


where the denominator, 0, is given by:












S= R12R23+R13 (Ri2+R,) +2/3R12 (k31+k32)
+V/R23 (k21+k31) +1/R13 (2k21+k32) +1/k21 (k3l+k32) (23)

and where R12, R23, and Ri, are the excitation rates for the

atoms given by:

1 2 2 1 1 2 ) + 3( ( 2(
O ) 4 A23A 13 (24) 23)2


2 23 )2 (0 23 2 + 1 2 2)
R 23 2 m D R (2)

0) 1A 2 1 2 ( ) 2 3)2 )


R 2( R ) (25)
R 3= 2 Im (26)


D = 4A,2A23Aa3 (R2)2A -( 232A23 (27)


where Im() denotes the imaginary component, aR12 and OR23 are

the Rabi flop frequencies and A12, A23, and A13 are the complex

detunings given by:


R'= [ (As,12 312) / (2ihc) ]" (28)

)R"= [ (A32X233123) / (2ihc) ]" (29)

A12=)1,2-ni2- i722 (30)

A23=23 -- 23- 732 (31)

Al3= (0)2+023) ( 12+f23) IY31 (32)

where 012 and 0)3 are the energy between the levels in angular

frequency units, i12 and 023 are the angular frequencies

(rad/s) of the laser light related to the wavelength by













f=2tc/A, and y12, 713, and Y23 are the "off-diagonal" decay rates

between levels 1 and 2, 1 and 3,and 2 and 3, respectively,

given by:


Y21=k21+Yc+YL (33)

Y31=B (k31+k32) +Yc+2yL (34)

Y32=S ( k21+k31+k32) +Yc+YL (35)


where k21, k31, and k32 are the inelastic collision rates

between the levels, Yc is the elastic collision rate, and yL is

the half-width half-maximum laser bandwidths in angular

frequency units.

These equations are analyzed in more detail by Axner and

Ljungberg [36]. Some of the main properties of these

equations are briefly discussed below.

Computer simulations based on this theory reproduce the

main features of experimental curves fairly well, although

exact lineshapes and peak heights are not always

satisfactorily reproduced [5]. In other words, there is a

qualitative agreement between experiment and theory in

predicting the LEI signal lineshapes. However, the

quantitative agreement is still unsatisfactory and a

refinement of the theory is required. Several studies have

been published in the areas where the theory is not

satisfactory [31,32,33,44,45,46], such as the description of










38

the properties of the laser light and the collisional

broadening and ionization mechanisms. Consequently, these

will not be discussed here.

It should be noted, however, that many assumptions were

made in solving the density-matrix equations. Some of these

approximations included: neglect of the mode structure of the

laser light, assuming Lorentzian-shaped wings for the laser

light, reduction of the system to a three-level non-

degenerative system, and neglect of Doppler broadening,

depletion, and the influence of other non-laser-connected

levels. Consequently, the above theoretical model will not

adequately describe experimental results in certain situations

where these assumptions do not hold. However, in general, the

theoretical model is more than adequate in most analytical

situations.



Detection of the Ionization Signal



As has been noted by other authors [27,47], a good theory

for LEI signal detection in the air/acetylene flame has

remained largely underdeveloped. Therefore, a theory of LEI

signal detection and optimization will not be presented in

detail. A brief overview of the concepts and simple

approaches to modeling the detection of natural and laser-












produced ions and electrons will be given.

One-Dimensional Approximation


A one-dimensional model of the distributions and dynamics

of ions, electrons, and fields in flames has been described by

Travis and Turk [27]. In this model, the only axis

accommodated mathematically is perpendicular to the plane of

the electrodes assuming that plate electrodes are used. Other

assumptions include: the flame uniformly fills the space

between the plates and that both the flame and laser light

extend infinitely in the directions parallel to the plates

(this results in a laser beam shape of a plane rather than a

more realistic line).

Point Charae Model


The point charge model can be used to describe the LEI

signal when pulsed laser excitation is used. This theory is

described in detail elsewhere [48,49,50]. This model assumes

that pulsed laser excitation instantaneously deposits charges

(ions and electrons) in the flame. Travis and Turk [27] have

written a computer program to numerically model the LEI signal

by convolving the point charge model with charge distribution

functions.

The simulated current vs voltage curves and experimental

results for the air/acetylene flame (with the immersed

electrode configuration) are not in good agreement. The










40

reasons for this have been discussed by Travis and Turk [27]

and will not be presented here.

Therefore, there still exists a need for development of

an adequate model for LEI signal detection in the

air/acetylene flame and with the immersed electrode

configuration. Many opportunities exist for the extension of

the existing theory to higher dimensionalities and to

different geometries.



Electrothermal Vaporization



Electrothermal vaporizers have been studied extensively

for sample introduction into the inductively-coupled plasma

(ICP) and other plasmas. A detailed theory for electrothermal

vaporization and vapor transport has been described by others

[51,52,53,54]. A brief discussion will be presented here.

The production of dry aerosols by high temperature

processes, such as in the graphite furnace, is known as

thermal dispersion. Once the vapor is produced, condensation

of the vapor is needed for effective transport to the

observation well. This can be attained by physical

condensation of the vapor, vapor condensation on foreign

particles, or chemical condensation of the vapor.

Physical condensation is the attainment of












supersaturation by cooling the vapor or vapor-gas mixture

[52]. The simplest method of effective cooling is the mixing

of vapor with a turbulent stream of cold gas. When

condensation nuclei are generated by the vapor itself, the

process is known as homogeneous nucleation or self-nucleation

[55]. When a high concentration of stable nuclei is attained,

condensation of vapor takes place on existing particles which

is called heterogeneous condensation [55]. When a high

concentration of fine particles is reached, their growth is

governed by Brownian coagulation. Coagulation occurs when two

particles collide and adhere to form a single particle.

The condensation of the vapor of a volatile analyte on

the stable nuclei formed from another substance in the furnace

is likely to take place under ETV conditions. If adequate

mixing is achieved, a component of low volatility can increase

the transport efficiency of a volatile analyte, even if the

evaporated masses of the two components are equally small.

Condensation of the analyte vapor can also take place on

stable nuclei produced from organic substances as solid sample

matrices or additives. In general, the condensation of

analyte vapor on foreign particles of different origin is

expected to be one of the most important processes in the ETV

methods. It is also expected that this process enhances the

transport efficiency of the analyte. In other words, a

carrier effect will result.












Chemical condensation takes place when the vapor

undergoes a reaction by which a compound is formed which is

less volatile than the vaporized substance [40]. A good

example of this is the formation of a metal oxide. Therefore,

it should be kept in mind that the chemical form of the

analyte vapor as released initially is not necessarily

relevant from the point of view of aerosol formation.



Absolute Analysis



As stated in the introduction, for a method to be

considered absolute, the following conditions should be met:

(i) elimination of analytical matrix interference, (ii)

stabilization of calibration over time, and (iii) theoretical

calculation of the calibration function based on fundamental

parameters and actual measurement conditions [2]. The third

condition, a theoretical expression for the LEI signal, will

be discussed below.

The integrated LEI signal, S, in coulombs for our flame-

LEI system with ETV (graphite furnace) sample introduction is

given by:


96, 4 85 W (3
S =h m EyEpE (36)


where W, is the mass (g) of analyte in the sample, A, is the













atomic weight of the analyte (g mol-1), e is the vaporization

efficiency of the graphite furnace, s, is the transport

efficiency between the furnace and the flame, es is the

atomization efficiency (or the free atom fraction, 0) of the

analyte in the flame, e, is the laser beam probing efficiency,

and ed is the detection efficiency (efficiencies are all

dimensionless). All of these efficiencies must be measured

before considering the possibility of using this technique as

an absolute method. Ideally these efficiencies should all be

unity for absolute analysis, because, if they are not, there

is a greater chance that they will vary from sample to sample.

Also, the sensitivity of the method will be reduced if the

efficiencies are not unity.

The transport efficiency, ET, describes how effectively

the analyte vaporized in the furnace is transported to the

flame. This value must be experimentally determined, as it is

a characteristic of the experimental setup. The probing

efficiency of the laser beams, pE, is a product of the spatial

probing efficiency, E,, and the temporal probing efficiency,

Et. The spatial probing efficiency describes what portion of

the flame the laser beams encompass. If the lasers encompass

the entire flame, then E, = 1. The temporal probing

efficiency is given by:













D~f
E, =(37)
V


where Db is the diameter of the laser beam (cm), f is the

frequency of the pulsed laser (Hz), and v is the flame

velocity (cm s-1). The vaporization efficiency of the furnace,

,, is assumed to be unity, provided the proper temperature

and ramp times are chosen to control the graphite furnace.

The atomization efficiency, a, describes the portion of the

sample that is actually present in the flame as free atoms

capable of being ionized. This is a function of the analyte

and the flame conditions and can be found in tables or

experimentally determined [5,56,57].

The detection efficiency, Ed, describes the charge

collection by the electrode and the ionization efficiency

induced by the laser in the flame. It is given by:


Ed = YEo (38)


where Yi is the ionization yield and !, describes the

efficiency of charge collection. The actual signal detected

in our case is the induced charge, Qi, which is related to the

total charge, QTot, by the equation:


QI= Qo V (39)


where AV is the actual potential in the flame at the point of

ionization and V is the potential applied to the electrode










45

[58]. The fraction AV/V describes the efficiency of charge

collection, F., in our case, is given by:


AE = (40)
V

The ionization yield describes the fraction of atoms

which become ionized in the flame due to the laser induced

process. According to Omenetto et al. [59], when the second

excitation transition reaches a Rydberg level from which

collisional ionization occurs very rapidly, the ionization

yield can be determined by measuring what is known as the

fluorescence dip. This parameter describes the decrease in

resonance fluorescence from the first excited level occurring

when adding the second pumping process. This second

excitation process depletes the atomic population of the first

excited level such that they do not return to the ground

state. The resonance fluorescence signal is always

proportional to the number density of excited atoms in the

first excited state [60]. As it has been shown from simple

theoretical considerations [20,48,61], the ionization yield

will approach unity when an optically saturating laser pulse

has a duration that significantly exceeds the reciprocal of

the effective ionization rate of the laser populated excited

state. However, before using the fluorescence measurement to

evaluate directly the ion yield, one must be sure that there










46

is no loss due to quenching collisions into a metastable

level. In the absence of collisional quenching, the

ionization yield can be calculated as:

I,, (?only) -1, addede)
Y = (41)
I2, (%only)



where I21 is the signal intensity for the resonance

fluorescence.

















CHAPTER 4
REVIEW OF LASER ENHANCED IONIZATION



Analytical Performance of Flame-LEI



Limits of Detection and Sensitivities


The primary advantage LEI offers over other analytical

methods is its very high sensitivity. With LEI, detection

limits in the sub-ppb range have been achieved for many

elements. There are several reasons for the high sensitivity

of LEI: (i) electrical detection implies an almost 100%

signal collection efficiency in a substantial part of the

flame, (ii) high ionization efficiency in the interaction

region of the flame, (iii) no background from scattered light,

and (iv) molecular formation of the analyte atoms may be of

less importance than in other methods since the sensitivity is

high enough to compensate for this.

Table 1 lists over 200 experimentally measured limits of

detection for flame-LEI of 34 elements. For the best cases,

the measured limits of detection are within a factor of three

of the theoretical detection limit. There are a number of

elements which have detection limits in the sub-ppb range.

47











Table 1. LEI limits of detection [62]


El. X, (nm) X2 (nm) Laser Flame Time LOD (ppb) Ref
Con.


Ag 328.068 421.094 E AA 0.075 63
Ag 328.068 546.549 Y AA 0.3 64
Ag 328.068 547.155 Y AA 0.4 65
Ag 328.068 Y AA 2 65
Ag 328.068 F AA 1.1 1 65
Al 265.248 E AA 1 3 66
Al 266.039 E AA 1 2 67
Al 308.215 F AN 1 0.2 67
Al 309.271 F AN 1 0.2 68
As 278.022 E AA 1 3000 67
As 286.044 E AA 1 50000 67
Au 242.795 479.266 Y AA 1.4 1 68
Au 242.795 Y AA 1.4 1000 69
Au 264.148 E AA 1 4 67
Au 267.595 E AA 1 1.2 67
Au 267.595 294 E AA rod 0.02 69

Au 274.825 E AA 1 200 67
Ba 270.263 E AA 1 0.6 67
Ba 307.158 F AA 1.1 0.2 66
Bi 302.464 E AA 3 45 70
Bi 306.772 F AA 1.1 2 66
Ca 272.165 E AA 1 0.4 67
Ca 300.686 F AA 1.1 0.1 66
Ca 422.673 468.527 N AA 1 0.05 71
Ca 422.673 468.527 N MA 1 0.5 72
Ca 422.673 585.745 Y AA 1.5 0.03 72











Table 1 Continued


Ca 422.673 518.89 N AA 1 0.02 72

Ca 422.673 518.89 N MA 1 0.1 72

Ca 422.673 518.89 N AA 1 0.02 73

Ca 422.673 Y AA 1.5 15 73

Ca 422.673 518.885 N HA 3 100 74
Ca 422.673 Kr cw AA 0.3 1 75
Ca 422.673 N HA 3 30000 75

Ca 422.673 N AA 1 1 72

Ca 422.673 N MA 1 10 72
Cd 228.802 466.235 Y AA 1.4 0.1 69

Cd 228.802 Y AA 1.4 100 69

Co 252.136 591.680 Y AA 1.4 0.08 69
Co 252.136 Y AA 1.4 10 69

Co 273.112 E AA 1 50 67

Co 274.046 E AA 1 25 67
Co 276.419 E AA 1 6 67

Co 281.556 E AA 1 7 67

Co 304.400 E AA 3 6 71
Co 315.878 531.678 Y AA 0.2 65

Co 315.878 534.339 Y AA 0.2 65

Co 315.878 Y AA 2 65

Co 321.915 515.405 Y AA 0.3 65
Co 321.915 Y AA 4 65
Cr 272.651 E AA 1 0.9 67

Cr 278.070 E AA 1 1.5 67
Cr 298.600 F AA 15 2 76
Cr 301.492 E AA 3 36 71

Cr 301.757 F AA 15 2 77











Table 1 Continued


Cr 427.480 529.74 E AA 0.5 77

Cs 455.531 N PBA 0.004 78

Cs 455.531 N PBA 1 0.004 79
Cs 455.531 N PBA 0.1 0.1 80

Cs 455.500 N AA 0.002 81

Cs 852.124 Diode HA 1 0.25 82

Cu 276.637 E AA 1 50 67

Cu 282.437 F AA 15 100 77

Cu 282.437 E AA 1 40 67

Cu 296.116 E AA 1 600 67

Cu 324.754 453.078 E AA rod 0.02 70

Cu 324.754 453.078 Y AA 1.4 0.07 69

Cu 324.754 Y AA 1.4 3 69

Cu 324.754 F AA 15 100 18

Cu 324.754 Y AA 1 2 83

Cu 324.754 Y AA tc 1 2 84

Cu 510.600 453.078 N HA 3 500 75

Eu 459.404 564.02 N AA 0.1 4000 84

Fe 271.902 E AA 1 0.1 67

Fe 273.358 E AA 1 2 67
Fe 273.548 E AA 1 3 67

Fe 274.698 E AA 1 30 67

Fe 278.810 E AA 1 1.5 67

Fe 281.329 E AA 1 5 67

Fe 298.357 F AA 15 4 77

Fe 302.064 E AA 3 0.12 71
Fe 302.064 F AA 15 2 77

Fe 318.490 Y AA 100 65











Table 1 Continued


Fe 319.166 Y AA 4 65

Fe 319.323 Y AA 3 65

Fe 321.440 Y AA 200 65
Fe 364.784 538.337 N HA 3 100 75

Fe 364.784 N HA 3 2000 75

Ga 265.987 E AA 1 0.1 67

Ga 271.965 E AA 1 0.04 71

Ga 287.424 F AA 15 0.07 77

Ga 287.424 E AA 1 0.06 67
Ga 294.364 E AA 3 0.06 71

Ga 294.364 F AA 15 0.1 77
Ga 417.200 Kr cw AA 0.3 60 76

In 271.026 E AA 1 0.001 67

In 271.394 E AA 1 0.008 67

In 275.388 E AA 1 0.005 67

In 293.263 E AA 1 0.03 67

In 303.936 532 Y AA 75 0.0004 85

In 303.936 786.4 Y AA 0.03 4

In 303.936 F AA 1.1 0.006 66

In 303.936 F AA tc 1 0.1 84

In 303.936 F AA 1 0.02 84

In 303.936 Y AA tc 1 0.1 86

In 303.936 Y AA 1 0.02 87
In 303.936 F AA 15 0.008 77

In 303.936 Y AA 0.007 82

In 303.936 E AA 3 0.03 71

In 410.176 Kr cw AA 0.3 20 76

In 451.131 571.0 E AA rod 0.0004 87










Table 1 Continued


In 451.131 501.8 N HA 3 0.6 75

In 451.131 501.8 N AA 0.007 82

In 451.131 502.3 N AA 0.03 82

In 451.131 525.4 N AA 0.003 82

In 451.131 526.3 Y AA 0.01 82

In 451.131 571.0 N AA 0.001 82

In 451.131 572.8 N AA 0.03 88

In 451.131 572.8 N AA 0.003 82

In 451.131 Kr cw AA 0.3 0.1 76

In 451.131 N HA 3 100 75

Ir 266.479 562.004 E AA 1 0.3 89
+ 642.0

K 294.268 F AA 15 1 77

K 296.321 E AA 1 1.5 67
K 404.414 N PBA 0.1 79

K 580.200 E AA 0.1 90
K 766.490 Kr cw HA 0.3 0.1 76

Li 274.119 E AA 1 0.005 67

Li 460.286 Kr cw AA 0.3 20 76

Li 610.362 F AA 1.1 0.01 66

Li 639.146 639.146 F AA 1.1 0.4 66

Li 670.784 460.286 E AA 0.0003 64
Li 670.784 610.362 N HA 3 0.04 75

Li 670.784 610.36 Y AA 0.1 0.03 81

Li 670.784 F AA 1.1 0.001 66

Li 670.784 N HA 3 4 75

Mg 285.213 435.2 Y AA 0.002 4
Mg 285.213 470.3 N AA 0.1 0.4 91

Mg 285.213_ F AA 15 0.1 18










Table 1 Continued


Mg 285.213 E AA 3 0.005 71

Mn 279.482 E AA 1 0.04 67

Mn 279.482 521.482 Y AA 1 0.02 92

Mn 279.482 F AA 15 0.3 18

Mn 279.827 E AA 1 0.05 67
Mn 279.984 F AA 15 5 18

Mn 280.106 E AA 1 0.08 67

Mn 292.557 E AA 1 3 67

Mn 292.557 E AA 3 3 71

Mn 403.076 602.180 N HA 3 5 75

Mn 403.076 N HA 3 30 75

Mo 267.985 F AN 1 30 68
Mo 306.428 F AN 1 400 68

Mo 307.437 F AN 1 500 68
Mo 308.562 F AN 1 500 68

Mo 311.212 F AN 1 900 68

Mo 313.259 F AN 1 70 68

Mo 315.816 F AN 1 70 68

Mo 317.035 F AN 1 20 68

Mo 319.397 F AN 1 10 68

Mo 320.883 F AN 1 50 68

Na 268.037 E AA 1 0.1 67

Na 268.046 E AA 1 0.1 67
Na 285.281 E AA 3 0.0015 71

Na 285.301 F AA 15 0.05 18

Na 540 540 Y PBA 70 79

Na 550 550 Y PBA 3 79

Na 578.732 578.732 E AA 0.001 93










Table 1 Continued


Na 578.732 578.732 Y PBA 0.9 79

Na 588.995 568.266 E AA 0.003 64

Na 588.995 568.821 N HA 3 0.04 75

Na 588.995 568.821 Y AA 0.012 94

Na 588.995 568.821 Y PBA 0.002 79

Na 588.995 568.821 N AA 0.0006 82

Na 588.995 616.075 Y PBA 0.01 79

Na 588.995 Kr cw AA 0.3 0.03 76

Na 588.995 N HA 3 6 75

Na 588.995 E AL HA 10 0.3 95

Na 588.995 E AA 0.02 94

Na 588.995 F AA 0.01 96

Na 588.995 F AA -20 97
Na 588.995 Y HA 1 0.8 98

Na 588.995 Y AA 1 0.6 99

Na 589.592 568.263 N AA 0.001 82

Na 589.592 568.26 N AA 0.1 0.005 92

Na 589.000 449 E AA rod 0.0002 70

Na 588.995 F AA 15 0.1 18

Ni 269.649 E AA 1 24 67

Ni 279.865 E AA 1 0.4 67

Ni 282.129 E AA 1 0.3 67

Ni 300.249 F AA 1.1 7 66

Ni 300.249 F AA 15 8 77

Ni 300.249 576.755 Y AA 1.4 0.08 69

Ni 300.249 Y AA 1.4 8 69

Ni 301.200 E AA 3 1.5 71

Ni 324.846 Y AA 2 65











Table 1 Continued


Pb 280.199 E AA 1 0.4 67

Pb 280.199 F AA 15 0.6 18
Pb 282.320 E AA 1 0.5 67

Pb 282.320 600.193 Y AA 75 0.0007 86
+ 1064

Pb 282.320 F AA 15 3 18

Pb 283.306 600.193 Y AA 1.4 0.09 69

Pb 283.306 600.193 E AA 0.3 64
Pb 283.306 Y AA 1.4 3 69
Pb 283.306 E AA 1 0.2 67

Pb 287.331 E AA 3 3 71

Pb 287.331 E AA 1 0.6 67

Rb 420.185 Kr cw HA 0.3 0.7 76

Rb 420.185 N PBA 0.1 79

Rb 420.185 N AA 1 0.0006 74
Rb 780.023 K HA 0.3 0.09 76

Rb 780.023 Diode HA 1 0.3 83

Sb 276.995 E AA 1 90 67

Sb 287.792 E AA 1 50 67

Si 288.158 F AN 1 40 68
Sn 266.124 E AA 1 30 67

Sn 270.651 E AA 1 8 67

Sn 270.651 F AN 1 2 68

Sn 283.999 597.028 Y HA 1.4 0.3 69

Sn 283.999 Y HA 1.4 8 69

Sn 283.999 E AA 1 2 67
Sn 283.999 F AN 1 0.4 68

Sn 283.999 F AA 15 6 77

Sn 286.333 F AN 1 2 68











Table 1 Continued


Sn 286.333 F AA 15 10 77
Sn 286.333 E AA 3 20 71

Sn 286.333 E AA 1 3 67

Sn 300.914 F AN 1 10 68
Sn 303.412 F AN 1 6 68
Sn 317.505 F AN 1 3 68
Sn 326.234 F AN 1 2 68
Sr 293.183 E AA 1 0.01 67
Sr 459.513 E AA 15 99

Sr 460.733 K AA 0.3 0.4 76
Sr 460.733 Y HA 1 3 99

Sr 460.733 Y AA 1 1 99
Sr 460.733 554.336 E AA 0.3 100
Ti 294.200 F AN 1 10 68
Ti 294.826 F AN 1 8 68
Ti 295.613 F AN 1 6 68

Ti 300.087 F AN 1 20 68

Ti 318.645 F AN 1 1 68
Ti 319.199 F AN 1 1 68
Ti 319.992 F AN 1 1 68

Ti 331.442 F AN 1 3 68

Ti 334.188 F AN 1 2 68
Ti 335.469 F AN 1 3 68
Ti 337.145 F AN 1 4 68
T1 276.787 377.572 E AA 1 0.008 100
T1 276.787 E AA 10 0.02 101
T1 276.787 E AA 1 0.006 67
T1 291.832 E AA 3 0.02 71










Table 1 Continued


T1 291.832 E AA 1 0.008 67
T1 291.832 F AA 15 0.09 77
T1 377.572 E AL HA 10 3 96

T1 377.572 655.6 Y AA 0.01 4
V 292.362 F AN 1 20 68
V 305.633 F AN 1 6 68

V 306.046 F AN 1 4 68
V 306.638 F AN 1 3 68
V 318.398 F AN 1 0.9 68
V 318.540 F AN 1 0.9 68
W 283.138 E AA 1 300 67

Yb 267.198 E AA 1 1.7 67
Yb 555.647 581.2 Y AA 0.1 82
Zn 213.856 396.545 Y AA 1 1 101
Zn 213.856 Y AA 1 3 102
Zn 307.590 472.216 Y AA 1 15 102

E=excimer pumped dye laser, E AL=excimer pumped atomic line
laser, F=flashlamp pumped dye laser, Kr cw=krypton ion
pumped cw dye laser, N=nitrogen pumped dye laser, Y=Nd:YAG
pumped dye laser, AA=acetylene/air, AN=acetylene/nitrous
oxide, HA=hydrogen/air, MA=methane/air, PBA=propane/butane/
air, rod=graphite rod in flame, tc=total consumption burner.













The lowest detection limits are obtained for elements with

good atomization in the flame and low ionization potentials

such as Li, Na, In, and Tl. This demonstrates that

collisional ionization is very efficient from excited states

close to the ionization limit. The low limits of detection

obtained for one-step LEI for some of the other elements with

comparatively higher ionization limits, such as Mg and Cd,

suggest that alternative ionizing routes may exist.

Unfortunately, in many cases, the measured detection

limits are much worse than the theoretical values [63]. This

is due to a variety of reasons: poor atomization fractions,

high contamination levels in blanks, radio frequency

interference, low repetition rate lasers, high thermal

ionization fractions, and the use of non-optimum excitation

wavelengths.

Noise and Interferences


LEI detection limits are usually limited by noise or

spectral interference during the measurement. Sources of

noise can be separated into two categories: multiplicative

and additive. Multiplicative noises in LEI arise from the

fluctuations in atomic population, fluctuations in the

ionization yield, and fluctuations in the detection

efficiency. Sources of additive noise include fluctuations in










59

the thermal background ionization, fluctuations in the laser-

induced background ionization, and electronic noise.

Fluctuations of the atomic population within the

irradiated volume of the flame result from fluctuations in the

nebulization rate and in the flame gas flows. Fluctuations in

the ionization yield are a result of changes in the laser

output properties, such as the pulse-to-pulse power variation

in the dye laser output. A 4% RSD is typical but may be worse

in some circumstances. This is further complicated by the

variation in laser power across the beam profile, where the

power is higher at the center of the beam than at the edges.

However, this problem may be minimized by saturation of the

atomic transitions. Fluctuations in the temperature of the

atom reservoir also contribute to fluctuations in the

ionization yield.

Fluctuations in the detection efficiency can result from

fluctuations in the high voltage power source for the

electrode, variations in the flame composition, and spatial

fluctuations of the laser beam.

As the concentration of the analyte decreases,

multiplicative noise decreases; however, additive noise

remains even in the absence of analyte. Therefore, it is

usually the additive noises that ultimately limit the

detection capability of the system.

Fluctuations in thermal ionization additive noise are a










60

result of fluctuations in the number of natural flame ions and

sample matrix ions. These fluctuations may be a result of the

flame flow fluctuations and nebulizer-induced noise.

Electronic additive noise results from the noise of the

various electronic components used to measure the LEI current.

Of the detection electronics used, the current preamplifier is

the noisiest. Another source of electronic noise that may be

significant is radio frequency (rf) noise. The LEI electrodes

and preamplifier seem to act as an excellent antenna and

detector for rf noise, so care should be taken to shield and

ground the LEI instrumentation.

Random fluctuations in the laser-induced background may

result from laser-induced ionization of spectral

interference. Spectral interference can be caused by any

matrix element but are most often a result of easily ionized

elements. Also, spectral interference may result from the

overlapping of atomic lines but are usually a result of an

overlap between the analyte line and some broadband spectral

feature of a matrix component.

Line overlaps are rare and are easy to eliminate when

two-step LEI is used. However, one disadvantage of using two-

step excitation is that interference could occur at one or

both wavelengths.

Overlaps between analyte lines and broadband spectral

features of matrix constituents are, again, much more likely










61

to be encountered than direct spectral line overlaps. Such

interference include line wings, molecular bands, and

thermionic ionization of particles. The wings of atomic lines

are easily observed in laser spectroscopy, including LEI. An

example of this is the line wing interference from Na on the

determination of Ni by two-step (300.249 nm and 561.479 nm)

LEI [63]. The most common molecular band interference

encountered in LEI is due to LEI of CaOH [102]. The

prevalence of Ca in many sample matrices, the incomplete

dissociation of CaOH in the air/acetylene flame, the rather

low ionization potential of CaOH (5.7 eV), the broad spectrum

from green to red wavelengths, and the location of many

second-step LEI stepwise excitation lines in this wavelength

range combine to make this a common problem. Laser-induced

particle ionization, may occur when a fuel-rich flame is being

used or when certain organic solvents are aspirated. The

mechanism for this is thought to be thermionic in nature

[103]. It may be possible to correct for these by scanning

the laser wavelength across the analyte line and performing

the appropriate background correction [104]. Wavelength

modulation has also been used to deal with these interference

[105].


Applications of LEI to Real Samples










62

LEI is one of the most sensitive analytical methods for

trace element analysis. Unfortunately, LEI has found limited

applications to real samples because of its susceptibility to

easily ionized matrix elements. The inherent ease of

collecting and sensing ions that contribute to the simplicity

of the LEI detection scheme also makes it vulnerable to these

easily ionizable elements (EIEs) [106]. So, although the

laser affords a good amount of selectivity, it cannot

compensate for an undiscriminating detector.

Sample dilution was the first solution to matrix

interference. It was often possible to dilute sufficiently

the sample matrix and still detect the analyte because of the

high sensitivity afforded by LEI. The use of an immersed

electrode also helped to reduce the loss of LEI signal due to

ion collection interference [107], but did not reduce the dc

background current from EIEs.

Today, approaches to analyze real samples by LEI can be

categorized as involving interferant removal or those without

interferant removal.

Determinations Without Interferant Removal


LEI is particularly amenable to samples of high purity

with small amounts of EIEs. In these cases, little

accommodation for interference is necessary. Alloy samples

are particularly well suited to LEI because they typically












contain low levels of sodium and potassium. The determination

of indium in nickel-based, high-temperature alloys [22] is an

early example of application of LEI spectrometry to a

difficult analytical problem. An acetylene/air flame on a

slot burner and plate electrodes produced satisfactory results

because of the low levels of EIEs. Similar samples usually

require a time-consuming extraction before conventional

furnace atomic absorption analysis, in contrast to LEI where

the alloy samples were successfully analyzed without sample

preparation. The results were also in close agreement with

values obtained with furnace atomic absorption.

Lowering the temperature of the atom reservoir is also a

potential solution for analytes with low atomization

temperatures such as cesium. Using a solid stainless-steel

rod immersed in a low temperature propane/butane/air flame,

researchers were able to determine accurately low

concentrations (ng/mL) of cesium in tap water samples by LEI

even with tens of mg/mL of sodium, potassium, and calcium

present [80].

Natural water samples are also ideal for LEI. The

concentrations of several elements at pg/mL levels were

validated in a simulated rainwater Standard Reference Material

(SRM 2694) by researchers at NBS (now NIST) using LEI

spectroscopy [73]. LEI was one of the unrelated methods used

to certify the concentration of the standards at NBS. Some










64

spectral interference due to excitation in the wings of

nearby peaks were corrected by standardization using matrix

matched standards.

Two-step excitation has been used to determine zinc in

SRM 1643a, trace elements in water, in the presence of a

background interference [102]. The experimental value for

zinc was slightly high, but no attempt was made to remove

potential interference beyond using an immersed electrode and

sample dilution. It was felt that matrix matching of the

standards would have improved the accuracy of the measurement.

As part of an environmental monitoring program, lead was

determined in unpolluted waters from mountainous regions and

compared with results for natural waters impacted by

industrial development [108]. Many spectroscopic techniques

do not have adequate sensitivity to determine species which

are naturally present at very low background levels. These

pristine waters presented little difficulty because of the

very low levels of impurities. In the case of water impacted

by industry, the concentrations of calcium, potassium, sodium,

and magnesium impurities were 4-5 orders of magnitude larger

than the lead concentrations and produced broad background

signals. It was found that CaOH molecules were responsible

for the interference at both excitation wavelengths but, by

tuning off the resonance lines, it was possible to use

background subtraction successfully.










65

Several elements have been determined in rock samples by

LEI spectrometry [109]. Most of the other analytical methods

require the use of complicated procedures prior to analysis

unless the sample is preconcentrated or interference are

removed. However, for LEI the dissolved samples were

aspirated into a propane/butane/air flame with an immersed

electrode used for detection. Although a broad ionization

background was found (due to CaOH), by reduction of the laser

powers used, good agreement with certified values was obtained

using aqueous standards. Detection limits of 0.002, 0.001,

and 0.5 ig/g were obtained for cesium, lithium and rubidium,

respectively.

LEI spectrometry has also been demonstrated as a viable

approach for detecting dopants and impurities in acid-

dissolved bulk gallium arsenide [110]. By using a two-step

excitation scheme, background subtraction was possible. Trace

amounts of chromium, iron, nickel, indium, manganese, and

cobalt were detected. Two-step LEI has also been used to

determine sodium in semiconductor silicon [111].

LEI has been used for determination of trace amounts of

nickel in petroleum products because nickel poisons the

catalysts used in petroleum processing [112]. Samples of both

heavy-oil flash distillate and an oil-based SRM were diluted

with a xylene/n-butanol solvent mixture and aspirated into an










66

air/acetylene flame. Nickel determination in the SRM was in

good agreement with the NIST certified value. Because of the

high sensitivity of LEI, it was possible to dilute the samples

considerably which nearly eliminated the need for matrix

matching of the standards.

Determination of indium in a CdHgTe alloy was

accomplished in both liquid solutions and solid samples

without sample preparation [70,113]. Electrothermal

atomization was coupled with LEI spectrometry by inserting a

resistively heated graphite rod in a premixed flame of a slot

burner. Propane/butane/air and acetylene/air flames were

used. No matrix interference was found for the samples and

aqueous standards were used for calibration. A good

correlation between results for liquid and solid samples

indicated analytical accuracy and an absence of analyte losses

for solid sampling.

Determinations With Interferant Removal


Preionization has been used for removal of spectral

interference and is described in more detail elsewhere [114].

Magnesium was chosen as the analyte since it is very

susceptible to interference from sodium (atomic wing

absorption). Several preionization schemes were investigated

using up to three photons of different energies. Up to an 83%

sodium depletion in the flame was achieved. A probe laser












then interrogated the preionized "hole" with 285 nm photons to

enhance thermal ionization of the analyte. Although

satisfactory results were achieved, the technique will

probably not be widely utilized because of the cost involved

for the two separate laser systems, the complexity in timing

the arrival of the ionization laser beam(s) and the probe

laser beam, and because signal collection interference are

related to the bulk flame environment and are not relieved by

laser preionization.

Chekalin and others determined copper and sodium in

concentrated orthophosphoric acid using their rod-flame system

[70,114]. The sodium interferant was removed by selective

volatilization from the dried sample at 1000"C. When the

temperature was raised to 2000'C, the copper signal could be

detected in the absence of noise. Detection limits were

determined by the purity of the rod material.

The determination of lead in a blood matrix has also been

reported [1151. A graphite furnace, used for sample

vaporization, was coupled with an acetylene/air miniature

flame for the analysis. With only a 21:1 dilution using ultra

pure water and temperature programming of the graphite

furnace, a detection limit of 0.089 ng/mL (890 fg absolute)

for lead in whole blood was obtained.

Solvent extraction has been shown to be effective for the

determination of trace amounts of manganese using a single-










68

step excitation scheme [116]. Manganese was completed in

water with sodium diethyldithiocarbamate and extracted into

diisobuytl ketone. The extraction resulted in a 10-fold

increase in the concentration of the manganese as well as

interferant removal. This method was successfully applied to

the analysis of ng/mL of manganese in groundwater, river and

lake waters, seawater, tap water, and wastewater.

An extraction also made the determination of 0.001%

calcium in aluminum alloys possible [72]. The separation was

based on the different solubilities of calcium and aluminum

chlorides in methanol. The detection limit was determined by

the purity of the methanol used.

Chromatography using a chelating resin to separate the

interfering elements from the analyte was used to determine

trace amounts of copper in a sulfate plating solution and

seawater [65]. The removal of EIEs was accomplished using

Chelex 100 [117]. In this process, transition and heavy

metals were chelated in the 5.2-5.6 pH range while ammonium

acetate was used to selectively elute the alkali and alkaline

earth metals by ion exchange. In the final step, the trace

metals were eluted with nitric acid and introduced into a

burner for LEI spectrometry. A microsampling cup coupled to

a premix burner (acetylene/air) permitted absolute

determinations of copper as low as 50 pg. Silver, cobalt,

iron, and nickel were also detected.










69

Turk and Kingston have combined automated chelation

chromatography with computer-controlled LEI spectrometry to

determine a large number of elements in a wide range of NIST

SRMs [105]. Chelex 100 resin was used for the separation

which was automated with a laboratory robot after preliminary

work. The elements determined and the reference materials

analyzed included the following: Cd, Co, Cu, Mn, Ni, and Pb

in Trace Elements in Water (SRM 1643b), Mn and Ni in Inorganic

Constituents in Bovine Serum (SRM 1598), Ni and Pb in Buffalo

River Sediment (SRM 2704), Cu, Mn, and Ni in Total Diet (SRM

1548), and Mn and Ni in Apple Leaves (SRM 1512' and in Peach

Leaves (SRM 1547). Concentrations determined ranged from the

mg/g to the ng/g range while precisions were in the range from

0.8% to 36% RSD. It was hoped that this impressive display of

technology would go a long way towards establishing LEI

spectrometry as a practical analytical method.

Alkyltins in sediment were determined by ion-exchange

chromatography coupled with LEI detection [106,118].

Tributyltin was extracted into 1-butanol and two-step (284.0

nm and 603.8 nm) LEI was performed in an acetylene/air flame.

The detection limit determined with the LEI detector was 3

ng/mL tin as tributyltin or 60 pg of tin.

The coupling of liquid chromatography with LEI has also

been reported for the measurement of organolead species [119].

Two Nd:YAG pumped dye lasers were used to optically excite










70

lead at 283.31 nm and 600.19 nm. A reversed phase LC column

was attached to the LEI acetylene/air flame. A detection

limit of 0.9 ng/mL (20 pg Pb for 20 pL injection) for

tetraethyllead was calculated. Oyster tissue samples (SRM

1566a) were analyzed. Trace levels of trimethyllead were

observed in the Oyster tissue, but concentrations varied among

the samples tested. This seemed to indicate that the

extraction and digestion procedures used were inadequate.

The use of flow injection analysis to reduce the

electrical interference from a sodium matrix for LEI has been

reported [120]. A frequency doubled Nd:YAG laser provided

optical excitation at 325.62 nm for LEI of indium. A typical

slot burner and nebulizer were used. A flow injection

apparatus was used to handle the solution prior to the

nebulizer. The standard addition method was also used with

this system to recover the original In concentration. This

combination of flow injection analysis and LEI was capable of

detecting In in a Na matrix of over 40 ppm, which is about 20-

fold more than the conventional LEI apparatus could tolerate

alone. This system also exhibited a larger linear dynamic

range for In, which was extended to 30 ppm with a mixture of

8 ppm Na matrix, up from 5 ppm for the conventional LEI

system.

Although the future for applications of LEI spectrometry












looks good, for LEI to become more widespread will require

continued evolution of hardware and software to accommodate

routine analysis by LEI. Coupling of LEI to other techniques

has also enhanced its ability to handle real samples. It has

been suggested that perhaps a multi-capability "laser

spectrometer" may be considered more promising for commercial

LEI instrumentation [121].


Hybrid Techniques and Non-Flame Atom Reservoirs



Although the flame is the most commonly used atom

reservoir for trace element analysis by LEI, the flame

atomizer alone has some serious drawbacks for trace element

analysis: (i) dilution of sample vapors by flame gas

combustion products, (ii) limited range of temperatures used,

(iii) a small (0.1-0.15) sample utilization factor (fraction

of the sample that reaches the flame), (iv) the impossibility

of separating the processes of sample evaporation and

atomization, (v) problems associated with handling

microvolumes of liquids and solid samples, and (vi) combustion

products of flames may hinder some of the spectral regions for

successful implementation of LEI [122]. These limitations

have prompted consideration for hybrid techniques and

alternate atom reservoirs for LEI spectrometry.

Electrothermal Vaoorizers










72

The first attempts to use a graphite furnace for LEI

determinations of elements failed [108]. Later, Gonchakov et

al. reported the successful application of electrothermal

atomization for determination of small amounts of sodium using

a three-step ionization scheme [123]. A graphite cup in an

argon atmosphere was used as an atomizer and a tungsten loop,

positioned 2 cm above the graphite cup, was used as an

electrode. A detection limit of 1 fg was calculated for Na.

Torres used a Varian-Techtron CRA-90 electrothermal tube

atomizer for LEI spectrometry [124]. The electrode and

graphite tube were positioned end-to-end on the same axis with

the laser beams passing through the graphite tube. A

detection limit of 5 pg was achieved for Cs. However, other

metals could not be determined because of thermionic emission

at higher temperatures and also because of arcing between the

electrode and graphite tube.

Graphite furnace-LEI was also used to determine the

presence of sodium and indium at the fg level [82]. A

tungsten wire was place axially inside the graphite tube. The

LEI signal was found to be 10-100 times larger than in a

flame, but the reproducibility was poor. Arcing between the

electrode and graphite tube also occurred when the temperature

was increased.

The most detailed investigations of LEI in the graphite










73

furnace were made by Magnusson and others [125,126,127].

However, they were faced with the same problems encountered in

the previously mentioned studies. Thermionic emission from

the graphite tube and electrode made it impossible to detect

elements which atomized at high temperatures. Although the

sampling efficiency (fraction of sample introduced that

reaches the probing area) was 2-3 orders of magnitude higher

than that in a flame, the limits of detection were on the same

order of magnitude. This was partially due to the lower rate

of collisional ionization in the argon atmosphere of the

graphite furnace.

A T-shaped furnace was suggested by Magnusson and

demonstrated by Sjdstrom to solve the thermionic emission

problem by spatially separating the regions of atomization and

detection [128]. A flow of argon was used to transport the

atoms to an external cavity adjacent to the tube, in which

laser excitation and LEI detection were performed. Detection

limits in the pg range were obtained for manganese and

strontium. However, this system had some drawbacks. As a

result of the temperature gradient between the center of the

graphite tube and the detection region, relatively few of the

atoms in the sample actually reached the detection region.

The use of modern furnace technology with probe

atomization for LEI has been described by Butcher et al.










74

[129]. In this system, a graphite probe was used for both

sample introduction and as the high-voltage electrode (-50 V).

The sample was vaporized off of the probe into an already

isothermal, furnace environment, which reduced the matrix

interference mentioned above and prevented atom condensation

onto the probe. For elements such as Tl, In and Li, detection

limits were between 0.7 and 2 pg. For Pb, Mg, and Fe,

detection limits were between 10 and 60 pg. The linear

dynamic range was between 3 and 4 orders of magnitude with a

precision between 12 and 16% for aqueous solutions. Sodium

matrix effects were also investigated and found to suppress

the LEI signal in the same manner as in the flame, so similar

difficulties would be encountered in the graphite furnace with

complex matrices as in the flame.

A novel design for a graphite furnace atomizer-ionizer

was examined by Chekalin and Vlasov [70]. Their outer

electrode design along with careful selection of the applied

electrode voltage suppressed interference from thermionic

emission of the heated graphite tube. Detection limits of

0.08 pg/mL for In and 100 pg/mL for Yb were obtained. This

system seems very promising for analysis of high purity

materials.

Hybrid Combinations of Flame and Electrothermal Vaporizers


Hybrid combinations of the flame and electrothermal










75

vaporizer were developed in order to alleviate the problems

associated with LEI in the graphite furnace alone, which were

mentioned above. The combination of flame and electrothermal

vaporizer/atomizer was first proposed by Chaplygin et al.

[130]. Their system consisted of a specially designed

cylindrical burner which contained an electrically heated wire

loop or filament in the central channel. An argon flow up the

central channel carried the sample vaporized off the wire loop

up into the flame. This design helped to minimize the sample

matrix by using small injection volumes and by temperature

programming of the sample vaporization loop. Detection in the

flame eliminated interference from thermionic emission from

heating of the sample loop and also maximized the LEI signal

because the flame region irradiated by the laser beams was

located much higher than the combustion zone. A detection

limit of 0.5 pg for Cs with reproducibility of 5-6% was

obtained.

Miyazaki and Tao have reported the use of a commercial

electrothermal vaporizer as a sample introduction system for

LEI spectrometry [131]. The commercial electrothermal

vaporizer (ETV) used (Seiko Instruments Inc., Tokyo) consisted

of a tungsten boat and a glass chamber. A Nd:YAG pumped dye

laser provided optical excitation at 276.79 nm and 291.83 nm

for one-step LEI of thallium. EIEs were found to interfere at










76

a 10-fold excess or more. Tl was extracted into 2,6-dimethyl-

4-heptanone (DIBK) with ammonium tetramethylene

dithiocarbamate (APDC) and hexamethyleneammonium

hexamethylenedithiocarbamate (HMAHMDC) at pH 6 to remove EIE

interference. Detection limits of 0.043 ng/mL (276.79 nm)

for Tl with the extraction and 0.11 ng/mL (291.83 nm) for Tl

without the extraction were obtained. This system was then

applied to and found to be useful for the analysis of

different types of natural water samples.

In order to acquire the advantages of both graphite

furnace vaporization and flame ionization detection while

maintaining independent control of each process, Smith et al.

have reported the coupling of a graphite furnace with a

miniature acetylene/air flame for LEI detection [132]. The

graphite furnace was used to vaporize samples which were

transported to the flame by a flow of argon gas. Two Nd:YAG

pumped dye lasers provided optical excitation at 285.2 and

435.2 nm for Mg, 377.6 and 655.6 nm for Tl, and 303.9 and

786.4 nm for In. Limits of detection of 0.0017 ng/mL (17 fg

absolute) for Mg, 0.012 ng/mL (118 fg absolute) for Tl, and

0.026 ng/mL (260 fg absolute) for In were obtained.

Riter et al. have reported the use of a modification of

the above system for trace element analysis [133]. A

commercially available graphite furnace (Finnigan MAT/SOLA,










77

Bremen, Germany) was used to vaporize samples which were

subsequently transported to a redesigned miniature burner,

which supported an acetylene/air flame, by a flow of argon

gas. Two XeCl excimer pumped dye lasers were used to provide

optical excitation at 285.213 and 435.191 nm for the analysis

of Mg. A complete determination of the instrumental

characteristics of the above system for the analysis of Mg was

performed. A blank-limited detection limit of 2 ng/mL (20 pg

absolute) was obtained for Mg. A detection limit of 590 fg/mL

(5.9 fg absolute) was calculated in the absence of a blank

signal and a reduction of the radio frequency noise.

Riter et al. also evaluated the above system for the

determination of trace Pb concentrations in whole blood [116].

Optical excitation at 283.3 and 509.0 nm was provided by two

XeCl excimer pumped dye lasers. Bovine blood samples from the

Centers for Disease Control (CDC) and NIST (SRM 955a) were

analyzed. With temperature programming of the graphite

furnace, there appeared to be no interference from the blood

matrix, and, instead, the matrix appeared to produce a carrier

effect, increasing the transfer efficiency between the furnace

and the flame over that for aqueous standards. The authors

concluded that there was sufficient matrix removal from

temperature programming of the graphite furnace and that the

matrix remaining acted as a carrier. A detection limit of










78

0.089 ng/mL (890 fg absolute) was calculated for Pb in whole

blood. This technique appears to have great promise for the

analysis of samples with complex matrices.

Marunkov reported the first LEI experiments with sample

vapors being introduced into a flame by an electrically heated

graphite rod and by diffusion through the wall of an

electrically heated closed graphite tube [134]. Detailed

experiments were not carried out, but results appeared to be

promising.

A hybrid "rod-flame" arrangement was proposed by Chekalin

et al. to combine the advantages of flame and electrothermal

atomizers [70,88,114,135]. In their system, the sample is

evaporated by an electrically heated graphite rod into the

flame where the analyte is atomized, laser excited, ionized,

and then detected. Advantages of this system included an

increase of the sample utilization factor (fraction of the

sample introduced that reaches the probing region), the

analysis of microsamples, and the direct analysis of solid

samples. However, the researchers encountered problems with

nonselective background ionization from the compounds

evaporating from the heated graphite rod and impurities in the

graphite were contributing to a blank signal. Good detection

limits were obtained for many elements including Au (2 pg/mL),

Co (100 pg/mL), Cr (20 pg/mL), Cu (2 pg/mL), In (0.04 pg/mL),










79

Mn (30 pg/mL), Na (0.02 pg/mL), and Ni (8 pg/mL). The

analysis of high-purity substances, such as Na and Cu in

orthophosphoric acid, Cu in germanium, In in Cd-Hg-Te alloy,

and Cr, Co, Mn, and Ni in fluorine-containing materials for

optical fibers, was also demonstrated. Detection limits

ranged from 0.1 to 7,000 ng/g for the different elements.

LEI in the Inductively Coupled Plasma


The first measurements of LEI in the ICP were made by

Turk and Watters [136]. Resonant LEI was detected for Fe, Mn,

Na, and Cu, however, the population of free atoms was too

small so the sensitivity was very poor. Turk et al. used a

power modulated ICP to reduce the rf interference from the

plasma [137]. With their approach, detection limits of 80

ng/mL for Fe and 20 ng/mL for Ga were achieved which are still

poorer than for the flame.

Ng et al. reported significant improvements in the

detection limits by using an extended-torch ICP, modifying the

torch and electrode designs, and using a continuous wave laser

[138]. The smaller electrode dimensions and smaller

separation between electrodes used by Ng et al. probably

accounted for the 2 orders of improvement in the detection

limit over Turk and Watters. Limits of detection ranged from

30 ng/mL for Ca to 810 ng/mL for Sr. Although these are an

improvement, they are still worse than the detection limits












for flame and furnace LEI.

The coupling of ICP and flame LEI with mass spectrometric

detection has been reported by Turk and others [139,140,141].

With a modified commercial ICP-mass spectrometer (ICP-MS), Sr

was examined employing laser-induced ionization (460.733 nm

and 308 nm). An enhancement of only 11% in the Sr' ion signal

was observed with the addition of laser excitation. The flame

was found to offer a much better environment for laser-induced

ionization or LEI than the ICP. A hydrogen/air flame was used

for Na, K, and Fe while an acetylene/air flame was used for

Ca. A 350 times increase in the signal was observed for Na'

with the addition of two-step optical excitation (589.0 nm and

498.3 nm). A detection limit of 0.05 ng/mL for Na was

calculated. Detection limits of 0.14 ng/mL for K (766.5 nm

and 580.2 nm), 35 ng/mL for Fe (302.1 nm), and 61 ng/mL for Ca

(422.7 nm and 585.7 nm) were obtained. The poor detection

limit for iron was believed to be due to one-step excitation,

and the detection limit for Ca was thought to be due to the

poor atomization efficiency of Ca in the acetylene/air flame.

Other Methods and Reservoirs


Gorbatenko et al. have reported the use of a Nd:YAG laser

as a laser microprobe solid sampling device for flame LEI

[142]. The sample was positioned at the edge of the burner

head to allow for the direct transport of the sample vapors to












the combustion zone. Lithium was chosen as the analyte and

was ablated from an aluminum alloy sample. Optical excitation

was provided by two Nd:YAG pumped dye lasers at 670.8 nm and

610.4 nm. It was found that the atomization efficiency was

determined by flame temperature and composition regardless of

how the sample was introduced into the flame. It was also

found that this technique allowed for the study of the

distribution of Li impurities over the surface of a solid

sample with a spatial resolution of about 100 pm. A detection

limit of around 30 ug/g for Li in solid samples was achieved

with a very short (s 20 s) analysis time.

Churchwell et al. have investigated the atmospheric-

pressure microarc atomizer as an atom reservoir for LEI [143].

Microvolumes of analyte solution were deposited on the tip of

the tungsten cathode loop and dried with a heat gun. LEI

measurements were performed directly in the plasma above the

microarc discharge. A detection limit of 3 ng for Na was

estimated. Preliminary results indicate that the helium-

microarc-induced plasma may be feasible for LEI spectrometry

but further studies are needed.

From the above examples, it can be seen that alternate

atom reservoirs and hybrid techniques have been used with

varying degrees of success for LEI spectrometry. The flame

still appears to be the optimal atom reservoir for LEI.











82

However, the hybrid techniques appear to be the most promising

of those discussed, especially for dealing with the matrix

interference from real samples. Further research on their

application to samples with complex matrices is warranted.

















CHAPTER 5
EXPERIMENTAL



LEL



A block diagram of the experimental setup used for

graphite furnace-LEI is shown in Figure 6. An excimer laser

(Model LPX-240i, Lambda Physik, Acton, MA), operated with

XeCl, was used to pump two dye lasers (Model Scanmate 1,

Lambda Physik, Acton, MA). The output of the dye lasers was

directed into an air/acetylene flame for LEI spectrometry. A

repetition rate of 30 Hz was used for all LEI experiments.

Burners


Two different burners (both designed in this laboratory)

were examined in this work. The first burner design (Figure

7) consisted of a Teflon base and a 2 7/8 in. diameter brass

top. A flow of argon carried the vaporized sample up through

a central stainless-steel capillary (o.d.=1/8 in, i.d.=1/16

in) where the sample was injected up into the flame. Premixed

air and acetylene flowed up in stainless-steel capillaries

(o.d.=3/32 in, i.d.=1/32 in) surrounding the central capillary

and were ignited to form the flame.


83



























Figure 6. Block diagram of the experimental setup for LEIS









Computer


Transfer Line


Boxcar


Amp


Preamp


Electrode


ETV





























Figure 7. First burner design used for LEIS













Flame


Brass






< Ar Sheath
Gas





SCH,/Air


S! Flame Gas o


: __ Ar Sheath
Ar + Sample

Teflon
Ar +
Sample
Top View
Side View




Full Text
A DETAILED STUDY OF LASER ENHANCED IONIZATION
WITH ELECTROTHERMAL VAPORIZATION-FLAME
ATOMIZATION FOR TRACE ELEMENT ANALYSIS
By
KEN LYNN RITER
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
1996

Dedicated to the loving memories of my mother, Namiko
(Tamanaha) Riter (September 16, 1936 - July 9, 1983), and
father, Roger Lynn Riter (July 12, 1945 - August 10, 1996).
Without their love, encouragement, and support none of this
would have been written.

ACKNOWLEDGMENTS
First, I would like to thank Dr. Jim Winefordner for the
opportunity to research in his lab. Although I have learned
much about laser spectroscopy in Jim's lab, his example of how
to be a decent person and to treat others with respect is what
will always be with me. I would like to thank Dr. Ben Smith
for all of his advice and help in the lab. Setting up the
instrumentation would have been an enormous task without Ben's
help. I would like to thank Dr. Oleg Matveev for working with
me and lending his expertise in laser enhanced ionization to
the project. I have learned so much about LEI from Oleg in a
very short period of time. Oleg's kindness, patience, and
modesty allowed him to explain very difficult concepts rather
easily.
I would like to thank Leah Mordoh and Wendy Clevenger for
assisting with and performing many of the experiments for the
LEIS of Mg. Others I would like to thank include Rob Guenard
for his help with the ultrasonic nebulizer, Chester Eastman in
the machine shop for building the new LEI burner, and all of
the Winefordner and Harrison group members for their help and
friendship. I would like to thank the National Institutes of

Health for funding this research (Grant # 5-R01-GM49638-03).
On a more personal note, I would like to thank my family
for their love and encouragement. Thanks to Jon DeGnore, Dr.
Bill Walden, and Dr. Wei Hang for being such good friends and
making my stay in Gainesville enjoyable. Finally, I would
like to thank my fiancée, Leah. Without her love and support,
I do not know how I would have made it through these final few
months.
All thanks be to God.
IV

TABLE OF CONTENTS
ACKNOWLEDGMENTS iii
LIST OF TABLES viii
LIST OF FIGURES ix
ABSTRACT xii
CHAPTER 1
INTRODUCTION 1
Absolute/Standardless Analysis 1
Analysis of Real Samples 5
Determination of Lead in Whole Blood 6
Intent of Dissertation 7
CHAPTER 2
INTRODUCTION TO LASER ENHANCED IONIZATION 9
The Optogalvanic Effect 9
General Principles of LEI 10
Atomization of Sample 11
Excitation of Atoms 16
Ionization 20
Charge Collection 20
CHAPTER 3
THEORY OF LASER ENHANCED IONIZATION 25
Introduction 25
Thermal Ionization 25
Processes Reponsible for Thermal Ionization . . 27
Thermal Ionization Rate of an Atom in a Flame . 29
Modeling of Laser Enhanced Ionization 30
Rate-Equation Formalism 30
v

Degree of Ionization for Two-Step
Excitation 33
Density-Matrix Formalism 34
Detection of the Ionization Signal 38
One-Dimensional Approximation 39
Point Charge Model 39
Electrothermal Vaporization 40
Absolute Analysis 42
CHAPTER 4
REVIEW OF LASER ENHANCED IONIZATION 47
Analytical Performance of Flame-LEI 47
Limits of Detection and Sensitivities 47
Noise and Interferences 58
Applications of LEI to Real Samples 61
Determinations Without Interferant Removal . . 62
Determinations With Interferant Removal .... 66
Hybrid Techniques and Non-Flame Atom Reservoirs . .71
Electrothermal Vaporizers 71
Hybrid Combinations of Flame and
Electrothermal Vaporizers 74
LEI in the Inductively Coupled Plasma 79
Other Methods and Reservoirs 80
CHAPTER 5
EXPERIMENTAL 83
LEI 83
Burners 83
Graphite Furnace 88
Procedure and Conditions 97
Flame Gas Flows, Velocity, and Temperature .... 103
Noise Study 105
Fluorescence Dip and Fluorescence Profile of Flame 108
Transport Efficiency 113
Transimpedance Amplifier Calibration 118
Atomization Efficiency Measurement 119
CHAPTER 6
RESULTS AND DISCUSSION 124
LEI of Magnesium 124
Magnesium As Analyte 124
Mg LEI Signal 127
System Parameter Optimizations for Old Burner
vi

Design 130
Flame Profile With Old Burner 138
Analytical Curve With Old Burner 138
New Burner Design 145
System Parameter Optimizations With New
Burner 148
Flame Profile With New Burner 157
Matrix Modifier/Carrier 166
Analytical Curve With New Burner 169
Flame Temperature and Flame Gas Velocity
With the New Burner 174
Absolute Analysis 176
Vaporization Efficiency 176
Transport Efficiency 176
Probing Efficiency 178
Detection Efficiency 186
Atomization Efficiency 187
LEI of Lead 190
Excitation Scheme for Lead 190
Carrier 191
Calibration Behavior 200
CHAPTER 7
CONCLUSIONS 202
Absolute Analysis 202
Pb in Blood 203
Future Work 203
REFERENCES 205
BIOGRAPHICAL SKETCH 216
vii

LIST OF TABLES
Table page
1. LEI limits of detection 48
2. Graphite furnace temperature program for
magnesium 99
3. Graphite furnace temperature program for lead in
blood 102
4. Values used for flame temperature calculation . . 175
5. Mg concentration in different cotton samples . . 177
6. Enhancement of LEI signal for different two-step
excitation schemes for lead 192
viii

LIST OF FIGURES
Figure Eaa£
1. Processes needed for laser enhanced ionization
spectrometry 13
2. Typical experimental setup for flame-LEI 15
3. Typical excitation schemes for LEI spectroscopy,
a) one-step excitation using visible light, b)
one-step excitation using ultraviolet light, c)
two-step excitation (direct), d) two-step
excitation (indirect), and e) three-step
excitation 19
4. Various electrode arrangements used for LEI
spectroscopy, a) split-cathode rod arrangement,
b) split-cathode plate arrangement, c)water-
cooled, immersed cathode arrangement, d) water-
cooled coiled cathode arrangement 22
5. Schematic representation of various excitation
and deexcitation processes in a three-level atom:
nx, n2, and n3 are the number densitites of the
three levels, respectively; k21 is the sum of the
collisional deexcitation and spontaneous emission
rates between levels 2 and 1; k12 is the
collisional excitation rate between levels 1 and
2; k3iion (k2iio„) is the collisional ionization rate
from level 3 (2) ; B32Uv(v12) (B23Uv(v23) ) and B23Uv(v21)
(B32Uv (v32) ) are the absorption and stimulated
emission rates and Uv(v21) (=Uv(v12)) and Uv(v23)
(=Uv(v32)) are the spectral irradiances of the
laser light 32
6. Block diagram of the experimental setup for LEIS . 85
7. First burner design used for LEIS 87
ix

8. Diagram of new burner design with relative
position of the high voltage electrode and laser
beams 90
9. Detailed drawing of the new burner design 92
10. Cut-away view of the graphite furnace showing the
tantalum sample extraction interface 96
11. Sketch of laboratory constructed ultrasonic
nebulizer used 107
12. Block diagram of experimental setup for
monitoring of both fluorescence and LEI signals . 112
13. Sketch of experimental setup for transport
efficiency measurement 116
14. Block diagram of the experimental setup for the
determination of the atomization efficiency for
Mg by atomic absorption 122
15. Oscilloscope trace of the laser beam timing . . . 126
16. Typical LEI signal for magnesium with older
burner 129
17. Argon flow rate optimization for Mg with older
burner 133
18. Burner-to-electrode distance optimization for Mg
with older burner 135
19. Applied voltage optimization for Mg with older
burner 137
20. Horizontal profile of the flame with the older
burner 140
21. Analytical curve for Mg with the older burner . . 142
22. Log-Log plot of the analytical curve of Figure 21 144
23. Noises for the hydrogen/air and acetylene/air
flames with the older burner design 147
24. Comparison of the noises for the new and old
burner designs 150
x

25. Argon flow rate optimization with new burner . . 152
26. Acetylene flow rate optimization with new burner 154
27. Air flow rate optimization for new burner .... 156
28. Burner-to-electrode distance optimization for new
burner 159
29. Electrode voltage optimization with new burner . 161
30. Horizontal profile of Mg atoms in flame with new
burner 163
31. Fluorescence profile of Mg atoms in the flame
with the new burner 165
32. LEI signal for xylene while scanning dye laser
for A.! 168
33. Effect of methanol on LEI signal for Mg 171
34. Analytical curve for Mg with new burner and
methanol 173
35. Mg LEI signal with increasing laser repetition
rate 181
36. Dye laser conversion efficiency with increasing
laser repetition rate for 183
37. Dye laser conversion efficiency with increasing
laser repetition rate for \2 185
38. Analytical curve for aqueous lead standards with
and without NaCl addition 194
39. Log-Log plot of analytical curves for aqueous
lead and blood lead 197
40. Analytical curve for diluted blood lead standards 199
xi

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
A DETAILED STUDY OF LASER ENHANCED IONIZATION
WITH ELECTROTHERMAL VAPORIZATION-FLAME
ATOMIZATION FOR TRACE ELEMENT ANALYSIS
By
Ken Lynn Riter
December 1996
Chairperson: James D. Winefordner
Major Department: Chemistry
A system coupling electrothermal vaporization with flame-
laser enhanced ionization (ETV-FL-LEI) was examined for the
possibility of "absolute" analysis. For a method to be
considered absolute, analytical matrix interferences must be
eliminated, the stability of the calibration over time must be
established, and the theoretical equation relating the signal
to the quantity of analyte must be known. For our system, the
LEI signal for magnesium is equal to the product of the
Faraday number (96,485 C/mol), moles of Mg, and overall system
efficiency. In our case, the overall system efficiency is the
product of the vaporization efficiency of the ETV, the
transport efficiency of Mg from the ETV to the flame, the
atomization efficiency of Mg in the flame, the probing
xii

efficiency of the laser beams, and the detection efficiency.
Ideally, these efficiencies should be unity. However, it was
found that all of these except the vaporization efficiency was
less than unity. Also, the LEI signal deviated from linearity
at low Mg concentrations and required the addition of a matrix
modifier to restore the signal. This indicates a dependence
of the LEI signal on the sample matrix. Therefore, ETV-FL-LEI
should not be considered an absolute method.
A second project involved the application of our ETV-FL-
LEI system to the determination of lead concentration in whole
blood. Blood standards from the Centers for Disease Control
(CDC) and the National Institute of Standards and Technology
(NIST) were diluted 21:1 with ultra pure water and analyzed.
Good agreement was found between the CDC and NIST standards.
A linear analytical curve was obtained with a detection limit
(3cr) of 8.9 X 10~3 pg/dL (890 fg absolute) for lead in whole
blood. This compares favorably with other current methods for
blood lead determinations including isotope dilution
inductively coupled plasma mass spectrometry (ID-ICP-MS) and
graphite furnace atomic absorption spectrometry (GFAAS).
xiii

CHAPTER 1
INTRODUCTION
Absolute/Standardless Analysis
There are no analytical procedures which are "absolute"
in the strictest sense of the word, because to analyze
absolutely - i.e. without any presuppositions - it would be
necessary to identify the atoms and the molecules of the
sample, to sort them out, and to count them individually and
completely [1].
However, if a theoretical expression is known for the
function relating the signal to the absolute quantity of
analyte present that is sufficiently reliable to allow a
direct calculation of the quantity of analyte from a single
measurement, then this method is called "an absolute method of
analysis" [2]. The most complete program for the development
of absolute methods of analysis includes [2] : (1) elimination
of analytical matrix interferences, (2) stabilization of the
calibration over time, and (3) theoretical calculation of the
calibration based on fundamental parameters and actual
measurement conditions. This should be distinguished from
l

2
"standardless" analysis where the calibration curve is stable
over time. Therefore, once the system has been calibrated for
a particular sample, the calibration should need to be checked
only occassionally (such as once every 8 hours) with a
standard (concentration = 100 to 1000 times the limit of
detection).
Many classical methods, such as precipitation reactions,
titrimetry, and coulometry, are considered absolute. When
considering modern instrumental methods for absolute analysis,
usually atomic absorption, where relative measurements are
made, is considered the best candidate rather than emission or
fluorescence procedures where absolute radiometric
measurements are required. According to L'vov [2], this is
not surprising since the atomic absorption method of measuring
the analytical signal is free from many of the uncontrolled or
difficult-to-control factors typical of emission/fluorescence
methods. Also, the stability and consistency of calibration
for modern graphite furnace atomic absorption spectroscopy
(GF-AAS) have brought GF-AAS close to achieving absolute
analysis. However, the sensitivity of GF-AAS, although high,
is still well within the possibility of preparation of
accurate standard reference solutions with minimal
contamination and loss problems. In this respect, absolute
analysis by GF-AAS will most likely never be a necessity.
However, GF-AAS is very amenable to standardless analysis

3
because of the reproducibility and consistency of the
calibration. Electrothermal vaporization-inductively coupled
plasma-mass spectrometry has achieved, in some cases, low or
even sub-femtogram detection limits. However, it is essential
that standard samples or solutions be used for calibration,
since the transport efficiencies of analyte ions are unknown
and vary significantly with analyte type, matrix type, gas
flow rates, sampling cone and skimmer cone geometries and
electrostatic lens configuration and conditions.
Laser induced fluorescence with graphite furnace
atomization (GF-LIF) is one of the most sensitive methods for
trace element analysis. However, absolute analysis by GF-LIF
involves a difficult and time consuming calibratrion process
relating the signal level to the mass of the analyte and
requires corrections for laser induced ionization, thermal
ionization, and matrix background, and evaluation of or
knowledge of the diffusion coefficient of analyte atoms at the
furnace atomization temperature.
Analytical methods based upon ionization are potentially
the best candidates for absolute analysis. Ions can be
produced with great selectivity and, once an ion is produced,
the probability of detection is generally high. Moreover,
most of the complexities associated with absolute fluorescence
measurements are avoided; the relationship between amount of
analyte and the measured signal is substantially simpler. A

4
new analytical approach using laser enhanced ionization with
sub-fg (and sub-pg/mL) detection limits has been described by
Smith et al. [3,4] which shows great promise as a standardless
or absolute method. This method involves coupling of a
graphite furnace for sample vaporization with an acetylene/air
flame for laser enhanced ionization (LEI) detection. In
essence, this method can be considered as the analog of
coulometry (i.e. each atom will produce a charge of 1.6 X 10'19
C or 96,487 C/mol). It is hoped that the combination of high
sensitivity and simplicity of detection will make LEI a good
candidate for absolute analysis.
There are several fields in which extreme sensitivity
(sub-fg) combined with absolute analysis would be useful since
the preparation of standards at very low analyte
concentrations is difficult because of sample loss and
contamination problems. This makes the development of an
absolute analytical protocol a pressing need [5]. In medical
research, the determination of the microdistribution of trace
metals in (jg
amounts
of
human tissues
is essential
and
concentration
levels
at
or below pg/g
are common
[6] .
However, the
preparation
of standards
is difficult
or
impossible because of the complexity of the sample matrix.
Similar detection capability is required for the determination
of trace elements in small amounts of solid samples of

5
interest in criminalistic, expert legal, and forensic medical
cases [7]. In situations where the preparation of hazardous
standards needs to be avoided, e.g. the analysis of
radioactive species, a means of quantitation using an absolute
approach would be desirable. These examples are indicative of
the situations where absolute analysis by graphite furnace-
flame-LEI spectroscopy would be desirable: (i) when the
quantities of sample are limited and (ii) when standards are
difficult or impossible to prepare due to the very low
concentrations being used, the difficulty of simulating very
complex sample matrices, or the danger of handling hazardous
analytes.
Analysis of Real Samples bv LEI Spectrometry
Laser enhanced ionization spectrometry (LEIS) is a
sensitive technique for trace element analysis and has become
well established since its discovery in 1976 [8] .
Unfortunately, the application of LEIS has been limited mostly
to simple aqueous systems because of ionization interferences
encountered in complex matrices. Thus, the application of
LEIS to complex real samples, such as biological fluids and
environmental samples, has remained largely undeveloped.
The combination of the graphite furnace, used as an
electrothermal vaporizer (ETV), with a flame for LEI detection

6
has many advantages for the analysis of real samples. By
separation of the graphite furnace vaporization and LEI
detection processes, this system overcomes the problems of
thermionic emission from the graphite tube and poor
collisional ionization encountered with LEIS in the graphite
furnace. This system also allows for the removal of many
interfering matrix species by temperature programming of the
graphite furnace. As a result, ionization interferences that
have plagued LEIS of real samples may be reduced or even
eliminated.
Determination of Lead in Whole Blood
Since the Centers for Disease Control (CDC) lowered its
pediatric level of concern for blood lead to 10 (ig/dL in 1991,
there has been increased interest in more sensitive methods
for the determination of lead concentrations in blood. There
also exists a need in research laboratories for accurate and
precise measurements of substantially lower blood lead levels
(« 10 pg/dL) to establish the levels for chronic lead toxicity
in humans [9]. Some studies indicate that there may be no
threshold for lead toxicity in humans [10,11], and so the need
for more sensitive methods for determining blood lead levels
is clear.
Isotope dilution inductively coupled plasma mass

7
spectrometry (ID-ICP-MS) and graphite furnace atomic
absorption spectrometry (GFAAS) are both methods currently
used to measure low levels of lead in blood. However, both of
these methods require extensive sample preparation with a
matrix modifier, and have detection limits of only around 1
Hg/dL for lead in whole blood [12,13], With the combination
of graphite furnace with flame-LEIS, little sample preparation
is needed (21:1 dilution with ultra pure water) unlike with
ID-ICP-MS and GFAAS. This not only reduces the complexity of
sample preparation but also reduces sample contamination from
matrix modifiers. This becomes especially important at very
low blood lead levels, so improved detection limits may be
expected. Another advantage of the ETV-LEIS combination is
the high sensitivity afforded by LEIS.
Intent of Dissertation
The aim of the present work was to satisfy the third
condition for absolute analysis (stated earlier) by
characterizing the efficiencies associated with our
experimental setup (consisting of the combination of a
graphite furnace with an acetylene/air flame for LEI
detection) to obtain the theoretical equation relating the
analyte quantity to the LEI signal. This equation and the
efficiencies will be discussed in more detail in Chapter 2.

A second objective was to explore the possibility of
applying this technique to the analysis of real samples with
complex matrices. We chose to analyze lead concentrations in
a blood matrix.

CHAPTER 2
INTRODUCTION TO LASER ENHANCED IONIZATION
The Optoqalvanic Effect
The optogalvanic effect (OGE) is the term for the process
whereby the collisional ionization rate for an element is
enhanced by optical excitation to a higher electronic energy
state [8,14], This mechanism was first postulated by Foote
and Mohler in 1925 [15]. The first purely optical/collisional
effect, however, was not actually observed until 1928 by
Penning [16]. Penning observed the effect as a change in the
voltage drop across a neon discharge when illuminated by a
second neon discharge.
Unfortunately, optical enhancement of collisional
ionization is too weak of an effect to be seen in usual atom
reservoirs with conventional light sources. Therefore, it
would not be until the advent of tunable lasers that the OGE
could be easily observed and used as a spectroscopic method.
In 1976, researchers at the National Bureau of Standards
NBS, now the National Institute of Standards and Technology,
MIST) decided to investigate the hollow cathode lamp as a
reservoir for laser induced fluorescence (LIF) after
disappointing results for LIF in a flame [17], These
researchers fortuitously discovered a change in the voltage
9

10
across the hollow cathode lamp when the dye laser was tuned to
an electronic transition of one of the atomic species in the
discharge [14]. Shortly afterwards, the same group decided to
look for a related effect in flames. The effect was observed
and a sub-ppb detection limit for sodium, equivalent to their
results with LIF in a flame, was quickly obtained [8].
The term, laser enhanced ionization (LEI), was first
introduced in 1978 by the group at NBS [18]. Today, LEI is
used to describe the optogalvanic effect in flames and other
reservoirs except discharges. OGE or optogalvanic
spectroscopy (OGS) is now used exclusively for phenomena
taking place in discharges.
General Principles of LEI
Laser-enhanced ionization spectrometry (LEIS) can be
defined as a spectroscopic method where an enhancement in the
normal collisional (thermal) ionization is obtained by optical
excitation of the atoms under study by resonant laser light.
This enhancement is detected as a change in the current
passing through a medium (atom reservoir) between two
electrodes at different potentials.
LEI can be broken down into four distinct processes:
atomization of the sample, optical excitation of the analyte
atoms by resonant laser light, collisional ionization of the

11
excited atoms, and collection of the charges produced
(Figure 1).
Atomization of Sample
The conversion of the sample into an atomic vapor or
atomization of the sample is dependent upon the particular
characteristics of the atom reservoir used. To date, many
different atom reservoirs have been used for LEI including
atmospheric flames, graphite furnaces, and inductively coupled
plasmas (ICPs). However, the large majority of LEI studies
have been done in atmospheric flames. This is because of the
combination of good qualities that flames exhibit for
atomization, ionization, and the detection processes. A
typical flame-LEI setup is shown in Figure 2 and consists of
a flame, laser system, electrodes, and electronic detection
equipment.
In conventional flame-LEI, the sample is aspirated by a
pneumatic nebulizer into a spray chamber. The sample leaves
the spray chamber as a spray or mist of fine droplets and
enters the flame. The flame heats this spray and causes the
solvent to vaporize leaving dry aerosol particles. Further

Figure 1. Processes needed for laser enhanced ionization
spectrometry [19]

13

Figure 2. Typical experimental setup for flame-LEI [19]

/
Dye
Dye
Laser
Laser
1
2

IS
heating in the flame volatilizes these dry aerosol particles
producing atomic, molecular, and ionic species.
Many different flames have been used for LEIS including
acetylene-based flames such as C2H2/air and C2H2/N20 and
hydrogen-based flames such as H2/N20, and H2/02/Ar. The cooler
hydrogen-based flames have little flame-ion production so the
background noise from the flame is low. However, the flame
temperature and flame composition have been found to be the
most important factors in obtaining strong LEI signals,
because these factors greatly affect the atomization
efficiency. Therefore, the hotter acetylene-based flames are
most commonly used as atomic reservoirs in LEI. On the other
hand, if the flame temperature is too high, thermal ionization
of the analyte may be considerable resulting in poor detection
limits for atomic LEI. The most common flame used for LEI has
been the air/acetylene flame in which a large number of
elements can be conveniently analyzed. Hotter flames, such as
C2H2/N20, are mostly used for refractory elements.
Excitation of Atoms
Optical excitation of the analyte atoms is usually
performed by a pulsed dye laser. The dye laser may be pumped
by a variety of sources including flashlamps, Nd:YAG, excimer,
and N2 lasers. Continuous-wave (cw) lasers have been used
infrequently for LEI because cw techniques are often too

17
complicated for analytical applications. This is mostly due
to the ability of pulsed laser sources to produce much higher
intensity light, especially in the ultra-violet region.
Another advantage of pulsed lasers for LEI is that the excess
charge created can be collected during a shorter period of
time, which reduces the influence of background current noise.
Flashlamp-pumped dye lasers usually have pulse durations
in the microsecond range while excimer, Nd:YAG, and N2-pumped
dye lasers usually have pulse durations around 5-20 ns. Pulse
energies vary typically from 0.1 to 100 mJ in the visible and
1 mJ to 10 mJ in the ultra-violet region depending on the
laser system, dyes, and crystals used. The wavelength region
covered by dye laser systems typically ranges from 220 nm to
1000 nm. Repetition rates used are normally around 5-100 Hz.
Many different excitation schemes may be used for LEI
spectroscopy, some of which are shown in Figure 3. One-step
excitation using either visible or ultra-violet light has been
used extensively in flame-LEIS. For elements that are more
easily ionized, a single-step scheme is sufficient to achieve
low limits of detection. However, for many elements, it is
favorable to use a two-step excitation scheme. Most two-step
schemes share an intermediate level although this is not
necessary if the upper level of the first step and the lower
level of the second step are sufficiently coupled by

Figure 3. Typical excitation schemes for LEI spectroscopy, a) one-step excitation
using visible light, b) one-step excitation using ultraviolet light, c) two-step
excitation (direct), d) two-step excitation (indirect), and e) three-step excitation.

d)
e)

20
collisions. The addition of the second step usually results
in a significant increase in the signal strength (up to three
orders of magnitude) compared to one-step excitation. Three-
step excitation schemes have also been used but are not common
in flame-LEIS.
Ionization
The common atomic reservoirs (flames, furnaces, and
plasmas) result in thermal ionization. Most atoms ionize
through collisions with thermally excited molecules in the
reservoir. Therefore, the thermal ionization rate depends on
the reservoir constituents, temperature of the medium, and the
ionization potential of the element of interest.
When an element is excited by laser radiation, the atomic
population of that element is greatly altered. Excited atoms
are more easily ionized by collisions than those in the ground
state, so an increased ionization rate results.
Charge Collection
The additional charges produced by optical excitation are
collected by applying an electrical field across the flame (or
other atomic reservoir) using an electrode arrangement and
measuring the current change. Many different electrode
arrangements have been used, some of which are shown in Figure
4.

Figure 4. Various electrode arrangements used for LEI spectroscopy, a) split-cathode
rod arrangement, b) split-cathode plate arrangement, c) water-cooled, immersed
cathode arrangement, d) water-cooled coiled cathode arrangement.

- H.V.
- H.V.
a) b)
- H.V.
H.V.

23
In all four schemes, the burner head acts as the anode
and is connected to the detection electronics. In the split-
cathode rod arrangement (Figure 4a) , two metal rods are
positioned on opposite sides of the flame and biased to the
same negative high voltage with respect to the burner head.
This arrangement was found to be very sensitive to easily
ionized matrix species [20,21]. This led to the development
of the split-cathode plate arrangement (Figure 4b) which was
used for several years [22,23]. This was the case because of
its stable electric field distribution in the flame,
relatively high contamination resistance, and long lifetime.
The stainless steel, water-cooled, immersed cathode
arrangement (Figure 4c) was developed to locate the cathode as
close to the laser excitation zone as possible [24] . The
maximum signal strength and optimal resistance to electrical
matrix interference should occur with the laser excitation as
close to the cathode as possible. This electrode is also easy
to fabricate, robust, and contributes negligible memory
effects. The saturation current is also reduced with respect
to that of the split-cathode plate, which results in a lower
background current. The water-cooled coiled cathode (Figure
4d) has
equivalent sensitivity to the immersed cathode and appears to
have even greater resistance to easily ionized matrix species
[25] .

24
Depending on other experimental parameters such as
electrode configuration, the applied voltage varies from about
300 V to ~3000 V. The current drawn through the flame is
usually in the |iA range and the amount of charge detected is
in the fC range or larger. A typical LEI signal has a
duration of 300 ns up to 1 |is. The very small LEI current is
directed through a current-to-voltage amplifier which is
usually placed very close to the flame to minimize radio
frequency noise from the pulsed laser. The d.c. current is
usually filtered out using a d.c. blocking capacitor.
The current pulse is detected synchronously using a
boxcar integrator with pulsed laser excitation or a lock-in
amplifier with a chopped continuous wave laser. The signal is
then collected and processed by a computer.

CHAPTER 3
THEORY OF LASER ENHANCED IONIZATION
Introduction
The basic principle of laser enhanced ionization (LEI) is
to enhance the existing thermal ionization rate in the flame
(or alternate atom reservoir) by optical (laser) excitation
and then to detect this increase in the ionization rate as an
increase in the current passing through the flame between two
electrodes. A discussion of the theory for two-step LEI in
flames will be covered here. A more general and in-depth
discussion of LEI theory is given by Axner and Rubinsztein-
Dunlop [26] and Travis and Turk [27].
Thermal Ionization
A combustion flame possesses a small but not always
insignificant amount of thermal ionization [5]. If an atomic
25

26
system is retained in a flame, we can define a thermal
ionization rate constant, kch.io„, for each specific process in
the flame. One such process is the collisional ionization
between a species M and a thermally excited collisional
partner X*:
]£
M + X* th'--°- ► M* + e- + X (1)
where M+ is the positive ion, e~ is an electron, X is the
deexcited collisional partner.
Similarly, there exists a recombination rate constant,
kreoomb., for the reverse process, given by:
M* + e- + X —kr*c°”,b—*• M + X* (2)
When the system is in thermal equilibrium, the thermal
ionization and recombination rates balance exactly. This
enables us to write an expression for the relation between the
concentration of the species M, its ions, and the electrons.
This relation is known as the Saha equation [28] and is stated
as:
[M+] [e‘] = Klon [M] (3)
where Klon is the ionization constant given by:
k
r e c o mb
(4)

27
It should be noted that although a third body (collision
partner) is required for the ionization and recombination
processes and the rates for each of these processes is
dependent upon the concentration of the third body, the
ionization constant (and degree of ionization of the atomic
system) is independent of the concentration of this third body
as long as thermal equilibrium conditions prevail in the
flame.
As has already been mentioned, in LEI, an electric field
is applied to the volume of interaction within the flame to
separate the created charges and make detection of those
charges possible. Consequently, the electric field will
minimize recombination from occurring at any significant rate.
Therefore, the only remaining process will be thermal
ionization, so the Saha equation is no longer valid.
Processes Responsible for Thermal Ionization
In a flame, a variety of different interactions between
atoms, molecules, and light can take place. The major
processes that lead to ionization can be divided into physical
and chemical ionization processes. The physical processes and
be further divided into collisional and radiative ionization
processes.
Collisional ionization processes are most often
considered to be responsible for ionization of atoms in

28
flames. An example would be the collisional ionization of
sodium atoms in the flame:
Na + X* - Na* + e" + X (5)
where X represents any flame molecule [5,29].
Radiative ionization processes are usually
photoionization processes where atoms are irradiated with high
intensity, short wavelength light which results in the
ejection of an electron, such as:
Na + hv - Na+ + e' (6)
or by the interaction between blackbody radiation and excited
atoms.
Chemical ionization processes are most easily
characterized as those in which the formation of a new
chemical bond takes place. Most alkaline earth atoms are
believed to ionize in this way. Associative ionization, such
as for the calcium atom:
Ca + OH - CaOH* + e' (7)
represents such a process. There are also many charge
distribution processes that may be of importance since they
can constitute one reaction in a chain leading to a net
ionization rate.

29
Thermal Ionization Rate of an Atom in a Flame
To model thermal ionization, some assumptions must be
made. It is assumed that collisional ionization dominates
other ionization processes in the flame. It is also assumed
that the flame, with all of its atomic and molecular species,
is in a state of thermal equilibrium. This implies that the
concept of detailed balance between all atomic levels is
valid. In other words, the atomic energy levels are populated
according to Boltzmann's distribution:
g, exp (-E1/kT)
n, = -i 2—1 na
(8)
where n± is the population of the ith level (m~3) , g¡, is the
degeneracy of the ith level, Et is the energy of the ith level,
nato„ is the total number density of neutral atoms (m‘3), and
Z is the electronic partition function:
Z = £gi exp (-E±/kT) (9)
i
where k is the Boltzmann constant and T is the flame
temperature.
However, the thermal ionization rate will be
overestimated unless we assume that the condition of detailed
balance is not valid for the highest lying states in the atom.
If we assume that detailed balance is only valid for states up

30
to a certain level, then the thermal ionization rate, dnion/dt
(m“3 s"1), can be expressed by:
dn
ion
dt
/8kT \1/!_x exp (~Elon/lcT)
\ n\i ) °io" Z
(10)
where p is the reduced mass of the system [p =
mato„*mx/ (mato„+mx) , where rnatom and mx are the masses of the atom
under consideration and collision species X, respectively],
oionx is the ionization cross section for the species X, Elon is
the energy of the ionization limit, and nx is the
concentration of species X.
Although, with the above simplification, the thermal
ionization rate depends greatly upon what cutoff level is
chosen, the relationship between the enhanced ionization rate
due to laser excitation and the thermal ionization rate is not
very sensitive to the specific cutoff level chosen.
Modeling of Laser Enhanced Ionization
Rate-Eauation Formalism
Let us assume a three-level atom illuminated by two laser
beams. For simplicity, it is assumed that the atoms consist
of only three bound levels (see Figure 5) among which laser

Figure 5. Schematic representation of various excitation and deexcitation processes
in a three-level atom: n1( n2, and n3 are the number densities of the three levels,
respectively; k21 is the sum of the collisional deexcitation and spontaneous emission
rates between levels 2 and 1; k12 is the collisional excitation rate between levels 1
and 2; k3,lt>„ (k2,io„) is the collisional ionization rate from level 3 (2); B12Uv(v12)
(B23Uv (v23) ) and B21Uv(v21) (B32Uv(v32)) are the absorption and stimulated emission rates
and Uy(v21) (=Uv(v12) ) and Uv(v23) (=Uv(v32)) are the spectral irradiances of the laser
light.

. :.: : . . .V/////////, •. x,: ,///////■'.'
^23^v ( ^23 )
Bi2Uv (V12)
®32^v ( V32 )
“■32
A
32
L23
i
L
^21
B21Uv ( V2l)
^12
L,
A2i
31
1 '
r
r
n;
2, ion
n
n.
k
13
n,

33
excitation takes place in two steps, weakly coupled to an
ionization continuum.
Degree of Ionization for Two-Step Excitation
The degree of ionization here is defined as the fraction
of atoms in the interaction volume that ionize during the
laser pulse. The expression for the degree of ionization is
considerably simplified by assigning an effective collisional
ionization rate from the uppermost laser-coupled level, kJffon .
With these assumptions, the ionization rate can be written as:
(11)
dt
where
B^Uy ( Vj2 ) B;3Uv (V;3 )
(12)
(13)
sat *-21
(14)
b =
sat
( k31 + k32)
(15)
d = [ (c°n)
sat
' (
(c
)
(16)

34
(C.
2 ' «t g1+g2
(17)
(C°n) = ^
1 2 '”t gi+g2+g3
(18)
ntot is the total number density of the analyte (neutral atoms
and ions), nlon is the number density of analyte ions, and the
designations of "on" and "off" refer to the second laser beam
(X2) being on or off.
Time dependent solutions to these equations for an
interaction time, x, of laser light can then be readily
expressed as:
n!o„(x> = «“ID ntot <19>
where og"1 represents the degree of ionization of the atomic
system and can be expressed as:
a1on(t)= 1-exp (-Ca" kj*Jont) (20)
Density-Matrix Formalism
The rate-equation formalism, however, is primarily valid
only for one-step excitation so an effective model for two-
step excitation is needed. Effects not taken into account are
those primarily caused by intense light fields, such as two-

35
photon excitation and dynamic Stark broadening, splitting, and
shifting. In order to account for these effects while keeping
the model as simple as possible, a theory for LEI based on the
density-matrix formalism was proposed by Axner et al.
[26, 30, 31, 32,33] .
The density-matrix formalism of light and matter
interactions and its theoretical assumptions have been
discussed in detail in many sources previously
[30,34,35, 36,37,38,39, 40, 41,42,43]. Therefore, only a brief
overview will be given here.
Assuming that the laser frequency profile is Lorentzian,
the system of density matrix equations can be solved under
steady-state conditions when the time dependencies of the
level populations are neglected. The steady-state
simplification is valid if the pulse duration is substantially
longer than the inverse of the deexcitation collision rate.
The following expressions result for the fractions of atoms
excited, C2m and Czm (where “ denotes the Density Matrix
formalism), for the density-matrix formalism:
(21)
1 ^12^23~*~^13 ( Rl2~*~^23+ ^21 )
(22)
atom
" 3
D
where the denominator, D, is given by:

36
D =
Rl2^23^"^13 (Rl2"^"R23) +^ARi2 (^31^^32)
+V6r23 (k21+k31) +1/3R13 (2k21+k32) +'/3k21 (k31+k32)
(23)
and where R12, R23, and R13 are the excitation rates for the
atoms given by:
( <2>2 j m /4 A ,¡A n - ( a r2); + ( r3 ) ^
(24)
(03 l3)2 /4 A,,A,,- ( co 2B3>2+ ( co i2)2\
R 2 3 2 1 m \ D )
(25)
R , , =
( to 12)2( co
I ml
(H
D = 4 A12A,,A
12í-*23‘-M3
z a / „ 2 3 \ 2 A
A,, (coR ) A2
(26)
(27)
where Im() denotes the imaginary component, coR12 and coR23 are
the Rabi flop frequencies and A12, A23/ and A13 are the complex
detunings given by:
<<>r12=[ (A21?c123I12)/(27thc) ]" (28)
o>r23=[ (A32X233I23) / (27thc) ]** (29)
A32=(0i2—012—iy21 (30)
^23~0O23 f^23 Í'Y32 (31)
a13= (©i2+co23) — where 23 are the energy between the levels in angular
frequency units, fl12 and CX>3 are the angular frequencies
(rad/s) of the laser light related to the wavelength by

37
íí=27tcA, and y12, y13, and y23 are the "off-diagonal" decay rates
between levels 1 and 2, 1 and 3,and 2 and 3, respectively,
given by:
y2i=^k21+Yc+YL
(33)
Y3i=ii (^3i+k32) +Yc+2Yl
(34)
Y32=ii ( ^21+ ^31+ k32 ) +Yc+Yl
(35)
and k32 are the inelastic
collision rates
between the levels, yc is the elastic collision rate, and yt is
the half-width half-maximum laser bandwidths in angular
frequency units.
These equations are analyzed in more detail by Axner and
Ljungberg [36] . Some of the main properties of these
equations are briefly discussed below.
Computer simulations based on this theory reproduce the
main features of experimental curves fairly well, although
exact lineshapes and peak heights are not always
satisfactorily reproduced [5]. In other words, there is a
qualitative agreement between experiment and theory in
predicting the LEI signal lineshapes. However, the
quantitative agreement is still unsatisfactory and a
refinement of the theory is required. Several studies have
been published in the areas where the theory is not
satisfactory [31,32,33,44,45,46], such as the description of

38
the properties of the laser light and the collisional
broadening and ionization mechanisms. Consequently, these
will not be discussed here.
It should be noted, however, that many assumptions were
made in solving the density-matrix equations. Some of these
approximations included: neglect of the mode structure of the
laser light, assuming Lorentzian-shaped wings for the laser
light, reduction of the system to a three-level non-
degenerative system, and neglect of Doppler broadening,
depletion, and the influence of other non-laser-connected
levels. Consequently, the above theoretical model will not
adequately describe experimental results in certain situations
where these assumptions do not hold. However, in general, the
theoretical model is more than adequate in most analytical
situations.
Detection of the Tonization Signa]
As has been noted by other authors [27,47], a good theory
for LEI signal detection in the air/acetylene flame has
remained largely underdeveloped. Therefore, a theory of LEI
signal detection and optimization will not be presented in
detail. A brief overview of the concepts and simple
approaches to modeling the detection of natural and laser-

39
produced ions and electrons will be given.
One-Dimensional Approximation
A one-dimensional model of the distributions and dynamics
of ions, electrons, and fields in flames has been described by
Travis and Turk [27] . In this model, the only axis
accommodated mathematically is perpendicular to the plane of
the electrodes assuming that plate electrodes are used. Other
assumptions include: the flame uniformly fills the space
between the plates and that both the flame and laser light
extend infinitely in the directions parallel to the plates
(this results in a laser beam shape of a plane rather than a
more realistic line).
Point Charge Model
The point charge model can be used to describe the LEI
signal when pulsed laser excitation is used. This theory is
described in detail elsewhere [48,49,50], This model assumes
that pulsed laser excitation instantaneously deposits charges
(ions and electrons) in the flame. Travis and Turk [27] have
written a computer program to numerically model the LEI signal
by convolving the point charge model with charge distribution
functions.
The simulated current vs voltage curves and experimental
results for the air/acetylene flame (with the immersed
electrode configuration) are not in good agreement. The

40
reasons for this have been discussed by Travis and Turk [27]
and will not be presented here.
Therefore, there still exists a need for development of
an adequate model for LEI signal detection in the
air/acetylene flame and with the immersed electrode
configuration. Many opportunities exist for the extension of
the existing theory to higher dimensionalities and to
different geometries.
Electrothermal Vaporization
Electrothermal vaporizers have been studied extensively
for sample introduction into the inductively-coupled plasma
(ICP) and other plasmas. A detailed theory for electrothermal
vaporization and vapor transport has been described by others
[51,52,53,54], A brief discussion will be presented here.
The production of dry aerosols by high temperature
processes, such as in the graphite furnace, is known as
thermal dispersion. Once the vapor is produced, condensation
of the vapor is needed for effective transport to the
observation well. This can be attained by physical
condensation of the vapor, vapor condensation on foreign
particles, or chemical condensation of the vapor.
Physical condensation is the attainment of

41
supersaturation by cooling the vapor or vapor-gas mixture
[52]. The simplest method of effective cooling is the mixing
of vapor with a turbulent stream of cold gas. When
condensation nuclei are generated by the vapor itself, the
process is known as homogeneous nucleation or self-nucleation
[55]. When a high concentration of stable nuclei is attained,
condensation of vapor takes place on existing particles which
is called heterogeneous condensation [55]. When a high
concentration of fine particles is reached, their growth is
governed by Brownian coagulation. Coagulation occurs when two
particles collide and adhere to form a single particle.
The condensation of the vapor of a volatile analyte on
the stable nuclei formed from another substance in the furnace
is likely to take place under ETV conditions. If adequate
mixing is achieved, a component of low volatility can increase
the transport efficiency of a volatile analyte, even if the
evaporated masses of the two components are equally small.
Condensation of the analyte vapor can also take place on
stable nuclei produced from organic substances as solid sample
matrices or additives. In general, the condensation of
analyte vapor on foreign particles of different origin is
expected to be one of the most important processes in the ETV
methods. It is also expected that this process enhances the
transport efficiency of the analyte. In other words, a
carrier effect will result.

42
Chemical condensation takes place when the vapor
undergoes a reaction by which a compound is formed which is
less volatile than the vaporized substance [40] . A good
example of this is the formation of a metal oxide. Therefore,
it should be kept in mind that the chemical form of the
analyte vapor as released initially is not necessarily
relevant from the point of view of aerosol formation.
Absolute Analysis
As stated in the introduction, for a method to be
considered absolute, the following conditions should be met:
(i) elimination of analytical matrix interferences, (ii)
stabilization of calibration over time, and (iii) theoretical
calculation of the calibration function based on fundamental
parameters and actual measurement conditions [2]. The third
condition, a theoretical expression for the LEI signal, will
be discussed below.
The integrated LEI signal, S, in coulombs for our flame-
LEI system with ETV (graphite furnace) sample introduction is
given by:
S
96,485
A,
Wa
£v£ TE a£ p£ d
(36)
where Wa is the mass (g) of analyte in the sample, Aa is the

43
atomic weight of the analyte (g mol'1), ev is the vaporization
efficiency of the graphite furnace, et is the transport
efficiency between the furnace and the flame, ea is the
atomization efficiency (or the free atom fraction, p) of the
analyte in the flame, sP is the laser beam probing efficiency,
and ed is the detection efficiency (efficiencies are all
dimensionless). All of these efficiencies must be measured
before considering the possibility of using this technique as
an absolute method. Ideally these efficiencies should all be
unity for absolute analysis, because, if they are not, there
is a greater chance that they will vary from sample to sample.
Also, the sensitivity of the method will be reduced if the
efficiencies are not unity.
The transport efficiency, tT, describes how effectively
the analyte vaporized in the furnace is transported to the
flame. This value must be experimentally determined, as it is
a characteristic of the experimental setup. The probing
efficiency of the laser beams, £p, is a product of the spatial
probing efficiency, and the temporal probing efficiency,
Sc. The spatial probing efficiency describes what portion of
the flame the laser beams encompass. If the lasers encompass
the entire flame, then e, = 1. The temporal probing
efficiency is given by:

44
C
D.f
(37)
v
where Db is the diameter of the laser beam (cm) , f is the
frequency of the pulsed laser (Hz) , and v is the flame
velocity (cm s-1) . The vaporization efficiency of the furnace,
sv, is assumed to be unity, provided the proper temperature
and ramp times are chosen to control the graphite furnace.
The atomization efficiency, £a, describes the portion of the
sample that is actually present in the flame as free atoms
capable of being ionized. This is a function of the analyte
and the flame conditions and can be found in tables or
experimentally determined [5,56,57].
The detection efficiency, £d, describes the charge
collection by the electrode and the ionization efficiency
induced by the laser in the flame. It is given by:
(38)
Ed = YíSd
where Yd is the ionization yield and £d describes the
efficiency of charge collection. The actual signal detected
in our case is the induced charge, Qt, which is related to the
total charge, QTot, by the equation:
(39)
where AV is the actual potential in the flame at the point of
ionization and V is the potential applied to the electrode

45
[58]. The fraction AV/V describes the efficiency of charge
collection, sD, in our case, is given by:
The ionization yield describes the fraction of atoms
which become ionized in the flame due to the laser induced
process. According to Omenetto et al. [59], when the second
excitation transition reaches a Rydberg level from which
collisional ionization occurs very rapidly, the ionization
yield can be determined by measuring what is known as the
fluorescence dip. This parameter describes the decrease in
resonance fluorescence from the first excited level occurring
when adding the second pumping process. This second
excitation process depletes the atomic population of the first
excited level such that they do not return to the ground
state. The resonance fluorescence signal is always
proportional to the number density of excited atoms in the
first excited state [60]. As it has been shown from simple
theoretical considerations [20,48,61], the ionization yield
will approach unity when an optically saturating laser pulse
has a duration that significantly exceeds the reciprocal of
the effective ionization rate of the laser populated excited
state. However, before using the fluorescence measurement to
evaluate directly the ion yield, one must be sure that there

46
is no loss due to quenching collisions into a metastable
level. In the absence of collisional quenching, the
ionization yield can be calculated as:
„ 12i (X,only) -I21 (X2added)
1 I21 (XjOnly)
where I21 is the signal intensity for the resonance
fluorescence.

CHAPTER 4
REVIEW OF LASER ENHANCED IONIZATION
Analytical Performance nf Flame-IEI
Limits of Detection and Sensitivities
The primary advantage LEI offers over other analytical
methods is its very high sensitivity. With LEI, detection
limits in the sub-ppb range have been achieved for many
elements. There are several reasons for the high sensitivity
of LEI: (i) electrical detection implies an almost 100%
signal collection efficiency in a substantial part of the
flame, (ii) high ionization efficiency in the interaction
region of the flame, (iii) no background from scattered light,
and (iv) molecular formation of the analyte atoms may be of
less importance than in other methods since the sensitivity is
high enough to compensate for this.
Table 1 lists over 200 experimentally measured limits of
detection for flame-LEI of 34 elements. For the best cases,
the measured limits of detection are within a factor of three
of the theoretical detection limit. There are a number of
elements which have detection limits in the sub-ppb range.
47

48
Table 1. LEI limits of detection [62]
El.
A.J (nm)
X2 (nm)
Laser
Flame
Time
Con.
(s)
LOD (ppb)
Ref
Ag
328.068
421.094
E
AA
-
0.075
63
Ag
328.068
546.549
Y
AA
-
0.3
64
Ag
328.068
547.155
Y
AA
-
0.4
65
Ag
328.068
Y
AA
-
2
65
Ag
328.068
F
AA
1.1
1
65
A1
265.248
E
AA
1
3
66
A1
266.039
E
AA
1
2
67
A1
308.215
F
AN
1
0.2
67
A1
309.271
F
AN
1
0.2
68
As
278.022
E
AA
1
3000
67
As
286.044
E
AA
1
50000
67
Au
242.795
479.266
Y
AA
1.4
1
68
Au
242.795
Y
AA
1.4
1000
69
Au
264.148
E
AA
1
4
67
Au
267.595
E
AA
1
1.2
67
Au
267.595
294
E
AA rod
-
0.02
69
Au
274.825
E
AA
1
200
67
Ba
270.263
E
AA
1
0.6
67
Ba
307.158
F
AA
1.1
0.2
66
Bi
302.464
E
AA
3
45
70
Bi
306.772
F
AA
1.1
2
66
Ca
272.165
E
AA
1
0.4
67
Ca
300.686
F
AA
1.1
0.1
66
Ca
422.673
468.527
N
AA
1
0.05
71
Ca
422.673
468.527
N
MA
1
0.5
72
Ca
422.673
585.745
Y
AA
1.5
0.03
72

49
Table 1 Continued
Ca
422
673
518.89
N
AA
1
0
02
72
Ca
422
673
518.89
N
MA
1
0
1
72
Ca
422
673
518.89
N
AA
1
0
02
73
Ca
422
673
Y
AA
1.5
15
73
Ca
422
673
518.885
N
HA
3
100
74
Ca
422
673
Kr cw
AA
0.3
1
75
Ca
422
673
N
HA
3
30000
75
Ca
422
673
N
AA
1
1
72
Ca
422
673
N
MA
1
10
72
Cd
228
802
466.235
Y
AA
1.4
0
1
69
Cd
228
802
Y
AA
1.4
100
69
Co
252
136
591.680
Y
AA
1.4
0
08
69
Co
252
136
Y
AA
1.4
10
69
Co
273
112
E
AA
1
50
67
Co
274
046
E
AA
1
25
67
Co
276
419
E
AA
1
6
67
Co
281
556
E
AA
1
7
67
Co
304
400
E
AA
3
6
71
Co
315
878
531.678
Y
AA
-
0
2
65
Co
315
878
534.339
Y
AA
-
0
2
65
Co
315
878
Y
AA
-
2
65
Co
321
915
515.405
Y
AA
-
0
3
65
Co
321
915
Y
AA
-
4
65
Cr
272
651
E
AA
1
0
9
67
Cr
278
070
E
AA
1
1
5
67
Cr
298
600
F
AA
15
2
76
Cr
301
492
E
AA
3
36
71
Cr
301
757
F
AA
15
2
77

50
Table 1 Continued
Cr
427.480
529.74
E
AA
-
0.5
77
Cs
455.531
N
PBA
-
0.004
78
Cs
455.531
N
PBA
i
0.004
79
Cs
455.531
N
PBA
0.1
0.1
80
Cs
455.500
N
AA
-
0.002
81
Cs
852.124
Diode
HA
1
0.25
82
Cu
276.637
E
AA
1
50
67
Cu
282.437
F
AA
15
100
77
Cu
282.437
E
AA
1
40
67
Cu
296.116
E
AA
1
600
67
Cu
324.754
453.078
E
AA rod
-
0.02
70
Cu
324.754
453.078
Y
AA
1.4
0.07
69
Cu
324.754
Y
AA
1.4
3
69
Cu
324.754
F
AA
15
100
18
Cu
324.754
Y
AA
1
2
83
Cu
324.754
Y
AA tc
1
2
84
Cu
510.600
453.078
N
HA
3
500
75
Eu
459.404
564.02
N
AA
0.1
4000
84
Fe
271.902
E
AA
1
0.1
67
Fe
273.358
E
AA
1
2
67
Fe
273.548
E
AA
1
3
67
Fe
274.698
E
AA
1
30
67
Fe
278.810
E
AA
1
1.5
67
Fe
281.329
E
AA
1
5
67
Fe
298.357
F
AA
15
4
77
Fe
302.064
E
AA
3
0.12
71
Fe
302.064
F
AA
15
2
77
Fe
318.490
Y
AA
-
100
65

51
Table 1 Continued
Fe
319.166
Y
AA
-
4
65
Fe
319.323
Y
AA
-
3
65
Fe
321.440
Y
AA
-
200
65
Fe
364.784
538.337
N
HA
3
100
75
Fe
364.784
N
HA
3
2000
75
Ga
265.987
E
AA
1
0.1
67
Ga
271.965
E
AA
1
0.04
71
Ga
287.424
F
AA
15
0.07
77
Ga
287.424
E
AA
1
0.06
67
Ga
294.364
E
AA
3
0.06
71
Ga
294.364
F
AA
15
0.1
77
Ga
417.200
Kr cw
AA
0.3
60
76
In
271.026
E
AA
1
0.001
67
In
271.394
E
AA
1
0.008
67
In
275.388
E
AA
1
0.005
67
In
293.263
E
AA
1
0.03
67
In
303.936
532
Y
AA
75
0.0004
85
In
303.936
786.4
Y
AA
-
0.03
4
In
303.936
F
AA
1.1
0.006
66
In
303.936
F
AA tc
1
0.1
84
In
303.936
F
AA
1
0.02
84
In
303.936
Y
AA tc
1
0.1
86
In
303.936
Y
AA
1
0.02
87
In
303.936
F
AA
15
0.008
77
In
303.936
Y
AA
-
0.007
82
In
303.936
E
AA
3
0.03
71
In
410.176
Kr cw
AA
0.3
20
76
In
451.131
571.0
E
AA rod
-
0.0004
87

52
Table 1 Continued
In
451.131
501.8
N
HA
3
0.6
75
In
451.131
501.8
N
AA
-
0.007
82
In
451.131
502.3
N
AA
-
0.03
82
In
451.131
525.4
N
AA
-
0.003
82
In
451.131
52 6.3
Y
AA
-
0.01
82
In
451.131
571.0
N
AA
-
0.001
82
In
451.131
572.8
N
AA
-
0.03
88
In
451.131
572.8
N
AA
-
0.003
82
In
451.131
Kr
cw
AA
0.3
0.1
76
In
451.131
N
HA
3
100
75
Ir
266.479
562.004
+ 642.0
E
AA
1
0.3
89
K
294.268
F
AA
15
1
77
K
296.321
E
AA
1
1.5
67
K
404.414
N
PBA
-
0.1
79
K
580.200
E
AA
-
0.1
90
K
766.490
Kr
cw
HA
0.3
0.1
76
Li
274.119
E
AA
1
0.005
67
Li
460.286
Kr
cw
AA
0.3
20
76
Li
610.362
F
AA
1.1
0.01
66
Li
639.146
639.146
F
AA
1.1
0.4
66
Li
670.784
460.286
E
AA
-
0.0003
64
Li
670.784
610.362
N
HA
3
0.04
75
Li
670.784
610.36
Y
AA
0.1
0.03
81
Li
670.784
F
AA
1.1
0.001
66
Li
670.784
N
HA
3
4
75
Mg
285.213
435.2
Y
AA
-
0.002
4
Mg
285.213
470.3
N
AA
0.1
0.4
91
Mg
285.213
F
AA
15
0.1
18

53
Table 1 Continued
Mg
285.213
E
AA
3
0.005
71
Mn
279.482
E
AA
1
0.04
67
Mn
279.482
521.482
Y
AA
1
0.02
92
Mn
279.482
F
AA
15
0.3
18
Mn
279.827
E
AA
1
0.05
67
Mn
279.984
F
AA
15
5
18
Mn
280.106
E
AA
1
0.08
67
Mn
292.557
E
AA
1
3
67
Mn
292.557
E
AA
3
3
71
Mn
403.076
602.180
N
HA
3
5
75
Mn
403.076
N
HA
3
30
75
Mo
267.985
F
AN
1
30
68
Mo
306.428
F
AN
1
400
68
Mo
307.437
F
AN
1
500
68
Mo
308.562
F
AN
1
500
68
Mo
311.212
F
AN
1
900
68
Mo
313.259
F
AN
1
70
68
Mo
315.816
F
AN
1
70
68
Mo
317.035
F
AN
1
20
68
Mo
319.397
F
AN
1
10
68
Mo
320.883
F
AN
1
50
68
Na
268.037
E
AA
1
0.1
67
Na
268.046
E
AA
1
0.1
67
Na
285.281
E
AA
3
0.0015
71
Na
285.301
F
AA
15
0.05
18
Na
540
540
Y
PBA
-
70
79
Na
550
550
Y
PBA
-
3
79
Na
578.732
578.732
E
AA
-
0.001
93

54
Table 1 Continued
Na
578.732
578.732
Y
PBA
-
0.9
79
Na
588.995
568.266
E
AA
-
0.003
64
Na
588.995
568.821
N
HA
3
0.04
75
Na
588.995
568.821
Y
AA
-
0.012
94
Na
588.995
568.821
Y
PBA
-
0.002
79
Na
588.995
568.821
N
AA
-
0.0006
82
Na
588.995
616.075
Y
PBA
-
0.01
79
Na
588.995
Kr cw
AA
0.3
0.03
76
Na
588.995
N
HA
3
6
75
Na
588.995
E AL
HA
10
0.3
95
Na
588.995
E
AA
-
0.02
94
Na
588.995
F
AA
-
0.01
96
Na
588.995
F
AA
-
20
97
Na
588.995
Y
HA
1
0.8
98
Na
588.995
Y
AA
1
0.6
99
Na
589.592
568.263
N
AA
-
0.001
82
Na
589.592
568.26
N
AA
0.1
0.005
92
Na
589.000
449
E
AA rod
-
0.0002
70
Na
588.995
F
AA
15
0.1
18
Ni
269.649
E
AA
1
24
67
Ni
279.865
E
AA
1
0.4
67
Ni
282.129
E
AA
1
0.3
67
Ni
300.249
F
AA
1.1
7
66
Ni
300.249
F
AA
15
8
77
Ni
300.249
576.755
Y
AA
1.4
0.08
69
Ni
300.249
Y
AA
1.4
8
69
Ni
301.200
E
AA
3
1.5
71
Ni
324.846
Y
AA
-
2
65

55
Table 1 Continued
Pb
280.199
E
AA
1
0.4
67
Pb
280.199
F
AA
15
0.6
18
Pb
282.320
E
AA
1
0.5
67
Pb
282.320
600.193
+ 1064
Y
AA
75
0.0007
86
Pb
282.320
F
AA
15
3
18
Pb
283.306
600.193
Y
AA
1.4
0.09
69
Pb
283.306
600.193
E
AA
-
0.3
64
Pb
283.306
Y
AA
1.4
3
69
Pb
283.306
E
AA
1
0.2
67
Pb
287.331
E
AA
3
3
71
Pb
287.331
E
AA
1
0.6
67
Rb
420.185
Kr cw
HA
0.3
0.7
76
Rb
420.185
N
PBA
-
0.1
79
Rb
420.185
N
AA
1
0.0006
74
Rb
780.023
K
HA
0.3
0.09
76
Rb
780.023
Diode
HA
1
0.3
83
Sb
276.995
E
AA
1
90
67
Sb
287.792
E
AA
1
50
67
Si
288.158
F
AN
1
40
68
Sn
266.124
E
AA
1
30
67
Sn
270.651
E
AA
1
8
67
Sn
270.651
F
AN
1
2
68
Sn
283.999
597.028
Y
HA
1.4
0.3
69
Sn
283.999
Y
HA
1.4
8
69
Sn
283.999
E
AA
1
2
67
Sn
283.999
F
AN
1
0.4
68
Sn
283.999
F
AA
15
6
77
Sn
286.333
F
AN
1
2
68

56
Table 1 Continued
Sn
286.333
F
AA
15
10
77
Sn
286.333
E
AA
3
20
71
Sn
286.333
E
AA
1
3
67
Sn
300.914
F
AN
1
10
68
Sn
303.412
F
AN
1
6
68
Sn
317.505
F
AN
1
3
68
Sn
326.234
F
AN
1
2
68
Sr
293.183
E
AA
1
0.01
67
Sr
459.513
E
AA
-
15
99
Sr
460.733
K
AA
0.3
0.4
76
Sr
460.733
Y
HA
1
3
99
Sr
460.733
Y
AA
1
1
99
Sr
460.733
554.336
E
AA
-
0.3
100
Ti
294.200
F
AN
1
10
68
Ti
294.826
F
AN
1
8
68
Ti
295.613
F
AN
1
6
68
Ti
300.087
F
AN
1
20
68
Ti
318.645
F
AN
1
1
68
Ti
319.199
F
AN
1
1
68
Ti
319.992
F
AN
1
1
68
Ti
331.442
F
AN
1
3
68
Ti
334.188
F
AN
1
2
68
Ti
335.469
F
AN
1
3
68
Ti
337.145
F
AN
1
4
68
TI
276.787
377.572
E
AA
1
0.008
100
TI
276.787
E
AA
10
0.02
101
TI
276.787
E
AA
1
0.006
67
TI
291.832
E
AA
3
0.02
71

57
Table 1 Continued
T1
291.832
E
AA
1
0.008
67
T1
291.832
F
AA
15
0.09
77
T1
377.572
E AL
HA
10
3
96
T1
377.572
655.6
Y
AA
-
0.01
4
V
292.362
F
AN
1
20
68
V
305.633
F
AN
1
6
68
V
306.046
F
AN
1
4
68
V
306.638
F
AN
1
3
68
V
318.398
F
AN
1
0.9
68
V
318.540
F
AN
1
0.9
68
w
283.138
E
AA
1
300
67
Yb
267.198
E
AA
1
1.7
67
Yb
555.647
581.2
Y
AA
-
0.1
82
Zn
213.856
396.545
Y
AA
1
1
101
Zn
213.856
Y
AA
1
3
102
Zn
307.590
472.216
Y
AA
1
15
102
E=excimer pumped dye laser, E AL=excimer pumped atomic line
laser, F=flashlamp pumped dye laser, Kr cw=krypton ion
pumped cw dye laser, N=nitrogen pumped dye laser, Y=Nd:YAG
pumped dye laser, AA=acetylene/air, AN=acetylene/nitrous
oxide, HA=hydrogen/air, MA=methane/air, PBA=propane/butane/
air, rod=graphite rod in flame, tc=total consumption burner

58
The lowest detection limits are obtained for elements with
good atomization in the flame and low ionization potentials
such as Li, Na, In, and Tl. This demonstrates that
collisional ionization is very efficient from excited states
close to the ionization limit. The low limits of detection
obtained for one-step LEI for some of the other elements with
comparatively higher ionization limits, such as Mg and Cd,
suggest that alternative ionizing routes may exist.
Unfortunately, in many cases, the measured detection
limits are much worse than the theoretical values [63], This
is due to a variety of reasons: poor atomization fractions,
high contamination levels in blanks, radio frequency
interference, low repetition rate lasers, high thermal
ionization fractions, and the use of non-optimum excitation
wavelengths.
Noise and Interferences
LEI detection limits are usually limited by noise or
spectral interferences during the measurement. Sources of
noise can be separated into two categories: multiplicative
and additive. Multiplicative noises in LEI arise from the
fluctuations in atomic population, fluctuations in the
ionization yield, and fluctuations in the detection
efficiency. Sources of additive noise include fluctuations in

59
the thermal background ionization, fluctuations in the laser-
induced background ionization, and electronic noise.
Fluctuations of the atomic population within the
irradiated volume of the flame result from fluctuations in the
nebulization rate and in the flame gas flows. Fluctuations in
the ionization yield are a result of changes in the laser
output properties, such as the pulse-to-pulse power variation
in the dye laser output. A 4% RSD is typical but may be worse
in some circumstances. This is further complicated by the
variation in laser power across the beam profile, where the
power is higher at the center of the beam than at the edges.
However, this problem may be minimized by saturation of the
atomic transitions. Fluctuations in the temperature of the
atom reservoir also contribute to fluctuations in the
ionization yield.
Fluctuations in the detection efficiency can result from
fluctuations in the high voltage power source for the
electrode, variations in the flame composition, and spatial
fluctuations of the laser beam.
As the concentration of the analyte decreases,
multiplicative noise decreases; however, additive noise
remains even in the absence of analyte. Therefore, it is
usually the additive noises that ultimately limit the
detection capability of the system.
Fluctuations in thermal ionization additive noise are a

60
result of fluctuations in the number of natural flame ions and
sample matrix ions. These fluctuations may be a result of the
flame flow fluctuations and nebulizer-induced noise.
Electronic additive noise results from the noise of the
various electronic components used to measure the LEI current.
Of the detection electronics used, the current preamplifier is
the noisiest. Another source of electronic noise that may be
significant is radio frequency (rf) noise. The LEI electrodes
and preamplifier seem to act as an excellent antenna and
detector for rf noise, so care should be taken to shield and
ground the LEI instrumentation.
Random fluctuations in the laser-induced background may
result from laser-induced ionization of spectral
interferences. Spectral interferences can be caused by any
matrix element but are most often a result of easily ionized
elements. Also, spectral interferences may result from the
overlapping of atomic lines but are usually a result of an
overlap between the analyte line and some broadband spectral
feature of a matrix component.
Line overlaps are rare and are easy to eliminate when
two-step LEI is used. However, one disadvantage of using two-
step excitation is that interference could occur at one or
both wavelengths.
Overlaps between analyte lines and broadband spectral
features of matrix constituents are, again, much more likely

61
to be encountered than direct spectral line overlaps. Such
interferences include line wings, molecular bands, and
thermionic ionization of particles. The wings of atomic lines
are easily observed in laser spectroscopy, including LEI. An
example of this is the line wing interference from Na on the
determination of Ni by two-step (300.249 nm and 561.479 nm)
LEI [63] . The most common molecular band interference
encountered in LEI is due to LEI of CaOH [102] . The
prevalence of Ca in many sample matrices, the incomplete
dissociation of CaOH in the air/acetylene flame, the rather
low ionization potential of CaOH (5.7 eV) , the broad spectrum
from green to red wavelengths, and the location of many
second-step LEI stepwise excitation lines in this wavelength
range combine to make this a common problem. Laser-induced
particle ionization, may occur when a fuel-rich flame is being
used or when certain organic solvents are aspirated. The
mechanism for this is thought to be thermionic in nature
[103] . It may be possible to correct for these by scanning
the laser wavelength across the analyte line and performing
the appropriate background correction [104], Wavelength
modulation has also been used to deal with these interferences
[105] .
Applications of LEI to Real Samples

62
LEI is one of the most sensitive analytical methods for
trace element analysis. Unfortunately, LEI has found limited
applications to real samples because of its susceptibility to
easily ionized matrix elements. The inherent ease of
collecting and sensing ions that contribute to the simplicity
of the LEI detection scheme also makes it vulnerable to these
easily ionizable elements (EIEs) [106]. So, although the
laser affords a good amount of selectivity, it cannot
compensate for an undiscriminating detector.
Sample dilution was the first solution to matrix
interferences. It was often possible to dilute sufficiently
the sample matrix and still detect the analyte because of the
high sensitivity afforded by LEI. The use of an immersed
electrode also helped to reduce the loss of LEI signal due to
ion collection interferences [107], but did not reduce the dc
background current from EIEs.
Today, approaches to analyze real samples by LEI can be
categorized as involving interferant removal or those without
interferant removal.
Determinations Without Interferant Removal
LEI is particularly amenable to samples of high purity
with small amounts of EIEs. In these cases, little
accommodation for interferences is necessary. Alloy samples
are particularly well suited to LEI because they typically

S3
contain low levels of sodium and potassium. The determination
of indium in nickel-based, high-temperature alloys [22] is an
early example of application of LEI spectrometry to a
difficult analytical problem. An acetylene/air flame on a
slot burner and plate electrodes produced satisfactory results
because of the low levels of EIEs. Similar samples usually
require a time-consuming extraction before conventional
furnace atomic absorption analysis, in contrast to LEI where
the alloy samples were successfully analyzed without sample
preparation. The results were also in close agreement with
values obtained with furnace atomic absorption.
Lowering the temperature of the atom reservoir is also a
potential solution for analytes with low atomization
temperatures such as cesium. Using a solid stainless-steel
rod immersed in a low temperature propane/butane/air flame,
researchers were able to determine accurately low
concentrations (ng/mL) of cesium in tap water samples by LEI
even with tens of mg/mL of sodium, potassium, and calcium
present [80].
Natural water samples are also ideal for LEI. The
concentrations of several elements at pg/mL levels were
validated in a simulated rainwater Standard Reference Material
(SRM 2694) by researchers at NBS (now NIST) using LEI
spectroscopy [73] . LEI was one of the unrelated methods used
to certify the concentration of the standards at NBS. Some

64
spectral interferences due to excitation in the wings of
nearby peaks were corrected by standardization using matrix
matched standards.
Two-step excitation has been used to determine zinc in
SRM 1643a, trace elements in water, in the presence of a
background interference [102]. The experimental value for
zinc was slightly high, but no attempt was made to remove
potential interferences beyond using an immersed electrode and
sample dilution. It was felt that matrix matching of the
standards would have improved the accuracy of the measurement.
As part of an environmental monitoring program, lead was
determined in unpolluted waters from mountainous regions and
compared with results for natural waters impacted by
industrial development [108]. Many spectroscopic techniques
do not have adequate sensitivity to determine species which
are naturally present at very low background levels. These
pristine waters presented little difficulty because of the
very low levels of impurities. In the case of water impacted
by industry, the concentrations of calcium, potassium, sodium,
and magnesium impurities were 4-5 orders of magnitude larger
than the lead concentrations and produced broad background
signals. It was found that CaOH molecules were responsible
for the interference at both excitation wavelengths but, by
tuning off the resonance lines, it was possible to use
background subtraction successfully.

65
Several elements have been determined in rock samples by
LEI spectrometry [109]. Most of the other analytical methods
require the use of complicated procedures prior to analysis
unless the sample is preconcentrated or interferences are
removed. However, for LEI the dissolved samples were
aspirated into a propane/butane/air flame with an immersed
electrode used for detection. Although a broad ionization
background was found (due to CaOH) , by reduction of the laser
powers used, good agreement with certified values was obtained
using aqueous standards. Detection limits of 0.002, 0.001,
and 0.5 pg/g were obtained for cesium, lithium and rubidium,
respectively.
LEI spectrometry has also been demonstrated as a viable
approach for detecting dopants and impurities in acid-
dissolved bulk gallium arsenide [110]. By using a two-step
excitation scheme, background subtraction was possible. Trace
amounts of chromium, iron, nickel, indium, manganese, and
cobalt were detected. Two-step LEI has also been used to
determine sodium in semiconductor silicon [111] .
LEI has been used for determination of trace amounts of
nickel in petroleum products because nickel poisons the
catalysts used in petroleum processing [112]. Samples of both
heavy-oil flash distillate and an oil-based SRM were diluted
with a xylene/n-butanol solvent mixture and aspirated into an

66
air/acetylene flame. Nickel determination in the SRM was in
good agreement with the NIST certified value. Because of the
high sensitivity of LEI, it was possible to dilute the samples
considerably which nearly eliminated the need for matrix
matching of the standards.
Determination of indium in a CdHgTe alloy was
accomplished in both liquid solutions and solid samples
without sample preparation [70,113]. Electrothermal
atomization was coupled with LEI spectrometry by inserting a
resistively heated graphite rod in a premixed flame of a slot
burner. Propane/butane/air and acetylene/air flames were
used. No matrix interference was found for the samples and
aqueous standards were used for calibration. A good
correlation between results for liquid and solid samples
indicated analytical accuracy and an absence of analyte losses
for solid sampling.
Determinations With Interferant Removal
Preionization has been used for removal of spectral
interferences and is described in more detail elsewhere [114] .
Magnesium was chosen as the analyte since it is very
susceptible to interference from sodium (atomic wing
absorption). Several preionization schemes were investigated
using up to three photons of different energies. Up to an 83%
sodium depletion in the flame was achieved. A probe laser

67
then interrogated the preionized "hole" with 285 nm photons to
enhance thermal ionization of the analyte. Although
satisfactory results were achieved, the technique will
probably not be widely utilized because of the cost involved
for the two separate laser systems, the complexity in timing
the arrival of the ionization laser beam(s) and the probe
laser beam, and because signal collection interferences are
related to the bulk flame environment and are not relieved by
laser preionization.
Chekalin and others determined copper and sodium in
concentrated orthophosphoric acid using their rod-flame system
[70,114]. The sodium interferant was removed by selective
volatilization from the dried sample at 1000°C. When the
temperature was raised to 2000°C, the copper signal could be
detected in the absence of noise. Detection limits were
determined by the purity of the rod material.
The determination of lead in a blood matrix has also been
reported [115]. A graphite furnace, used for sample
vaporization, was coupled with an acetylene/air miniature
flame for the analysis. With only a 21:1 dilution using ultra
pure water and temperature programming of the graphite
furnace, a detection limit of 0.089 ng/mL (890 fg absolute)
for lead in whole blood was obtained.
Solvent extraction has been shown to be effective for the
determination of trace amounts of manganese using a single-

68
step excitation scheme [116]. Manganese was complexed in
water with sodium diethyldithiocarbamate and extracted into
diisobuytl ketone. The extraction resulted in a 10-fold
increase in the concentration of the manganese as well as
interferant removal. This method was successfully applied to
the analysis of ng/mL of manganese in groundwater, river and
lake waters, seawater, tap water, and wastewater.
An extraction also made the determination of 0.001%
calcium in aluminum alloys possible [72]. The separation was
based on the different solubilities of calcium and aluminum
chlorides in methanol. The detection limit was determined by
the purity of the methanol used.
Chromatography using a chelating resin to separate the
interfering elements from the analyte was used to determine
trace amounts of copper in a sulfate plating solution and
seawater [65]. The removal of EIEs was accomplished using
Chelex 100 [117] . In this process, transition and heavy
metals were chelated in the 5.2-5.6 pH range while ammonium
acetate was used to selectively elute the alkali and alkaline
earth metals by ion exchange. In the final step, the trace
metals were eluted with nitric acid and introduced into a
burner for LEI spectrometry. A microsampling cup coupled to
a premix burner (acetylene/air) permitted absolute
determinations of copper as low as 50 pg. Silver, cobalt,
iron, and nickel were also detected.

69
Turk and Kingston have combined automated chelation
chromatography with computer-controlled LEI spectrometry to
determine a large number of elements in a wide range of NIST
SRMs [105] . Chelex 100 resin was used for the separation
which was automated with a laboratory robot after preliminary
work. The elements determined and the reference materials
analyzed included the following: Cd, Co, Cu, Mn, Ni, and Pb
in Trace Elements in Water (SRM 1643b), Mn and Ni in Inorganic
Constituents in Bovine Serum (SRM 1598), Ni and Pb in Buffalo
River Sediment (SRM 2704), Cu, Mn, and Ni in Total Diet (SRM
1548), and Mn and Ni in Apple Leaves (SRM 151^ and in Peach
Leaves (SRM 1547). Concentrations determined ranged from the
mg/g to the ng/g range while precisions were in the range from
0.8% to 36% RSD. It was hoped that this impressive display of
technology would go a long way towards establishing LEI
spectrometry as a practical analytical method.
Alkyltins in sediment were determined by ion-exchange
chromatography coupled with LEI detection [106,118].
Tributyltin was extracted into 1-butanol and two-step (284.0
nm and 603.8 nm) LEI was performed in an acetylene/air flame.
The detection limit determined with the LEI detector was 3
ng/mL tin as tributyltin or 60 pg of tin.
The coupling of liquid chromatography with LEI has also
been reported for the measurement of organolead species [119].
Two Nd:YAG pumped dye lasers were used to optically excite

70
lead at 283.31 nm and 600.19 nm. A reversed phase LC column
was attached to the LEI acetylene/air flame. A detection
limit of 0.9 ng/mL (20 pg Pb for 20 nL injection) for
tetraethyllead was calculated. Oyster tissue samples (SRM
1566a) were analyzed. Trace levels of trimethyllead were
observed in the Oyster tissue, but concentrations varied among
the samples tested. This seemed to indicate that the
extraction and digestion procedures used were inadequate.
The use of flow injection analysis to reduce the
electrical interference from a sodium matrix for LEI has been
reported [120]. A frequency doubled Nd:YAG laser provided
optical excitation at 325.62 nm for LEI of indium. A typical
slot burner and nebulizer were used. A flow injection
apparatus was used to handle the solution prior to the
nebulizer. The standard addition method was also used with
this system to recover the original In concentration. This
combination of flow injection analysis and LEI was capable of
detecting In in a Na matrix of over 40 ppm, which is about 20-
fold more than the conventional LEI apparatus could tolerate
alone. This system also exhibited a larger linear dynamic
range for In, which was extended to 30 ppm with a mixture of
8 ppm Na matrix, up from 5 ppm for the conventional LEI
system.
Although the future for applications of LEI spectrometry

71
looks good, for LEI to become more widespread will require
continued evolution of hardware and software to accommodate
routine analysis by LEI. Coupling of LEI to other techniques
has also enhanced its ability to handle real samples. It has
been suggested that perhaps a multi-capability "laser
spectrometer" may be considered more promising for commercial
LEI instrumentation [121].
Hybrid Techniques and Non-Flame Atom Reservoirs
Although the flame is the most commonly used atom
reservoir for trace element analysis by LEI, the flame
atomizer alone has some serious drawbacks for trace element
analysis: (i) dilution of sample vapors by flame gas
combustion products, (ii) limited range of temperatures used,
(iii) a small (0.1-0.15) sample utilization factor (fraction
of the sample that reaches the flame), (iv) the impossibility
of separating the processes of sample evaporation and
atomization, (v) problems associated with handling
microvolumes of liquids and solid samples, and (vi) combustion
products of flames may hinder some of the spectral regions for
successful implementation of LEI [122] . These limitations
have prompted consideration for hybrid techniques and
alternate atom reservoirs for LEI spectrometry.
Electrothermal Vaporizers

72
The first attempts to use a graphite furnace for LEI
determinations of elements failed [108]. Later, Gonchakov et
al. reported the successful application of electrothermal
atomization for determination of small amounts of sodium using
a three-step ionization scheme [123] . A graphite cup in an
argon atmosphere was used as an atomizer and a tungsten loop,
positioned 2 cm above the graphite cup, was used as an
electrode. A detection limit of 1 fg was calculated for Na.
Torres used a Varian-Techtron CRA-90 electrothermal tube
atomizer for LEI spectrometry [124] . The electrode and
graphite tube were positioned end-to-end on the same axis with
the laser beams passing through the graphite tube. A
detection limit of 5 pg was achieved for Cs. However, other
metals could not be determined because of thermionic emission
at higher temperatures and also because of arcing between the
electrode and graphite tube.
Graphite furnace-LEI was also used to determine the
presence of sodium and indium at the fg level [82]. A
tungsten wire was place axially inside the graphite tube. The
LEI signal was found to be 10-100 times larger than in a
flame, but the reproducibility was poor. Arcing between the
electrode and graphite tube also occurred when the temperature
was increased.
The most detailed investigations of LEI in the graphite

73
furnace were made by Magnusson and others [125,126,127].
However, they were faced with the same problems encountered in
the previously mentioned studies. Thermionic emission from
the graphite tube and electrode made it impossible to detect
elements which atomized at high temperatures. Although the
sampling efficiency (fraction of sample introduced that
reaches the probing area) was 2-3 orders of magnitude higher
than that in a flame, the limits of detection were on the same
order of magnitude. This was partially due to the lower rate
of collisional ionization in the argon atmosphere of the
graphite furnace.
A T-shaped furnace was suggested by Magnusson and
demonstrated by Sjdstrom to solve the thermionic emission
problem by spatially separating the regions of atomization and
detection [128]. A flow of argon was used to transport the
atoms to an external cavity adjacent to the tube, in which
laser excitation and LEI detection were performed. Detection
limits in the pg range were obtained for manganese and
strontium. However, this system had some drawbacks. As a
result of the temperature gradient between the center of the
graphite tube and the detection region, relatively few of the
atoms in the sample actually reached the detection region.
The use of modern furnace technology with probe
atomization for LEI has been described by Butcher et al.

74
[129]. In this system, a graphite probe was used for both
sample introduction and as the high-voltage electrode (-50 V).
The sample was vaporized off of the probe into an already
isothermal, furnace environment, which reduced the matrix
interferences mentioned above and prevented atom condensation
onto the probe. For elements such as Tl, In and Li, detection
limits were between 0.7 and 2 pg. For Pb, Mg, and Fe,
detection limits were between 10 and 60 pg. The linear
dynamic range was between 3 and 4 orders of magnitude with a
precision between 12 and 16% for aqueous solutions. Sodium
matrix effects were also investigated and found to suppress
the LEI signal in the same manner as in the flame, so similar
difficulties would be encountered in the graphite furnace with
complex matrices as in the flame.
A novel design for a graphite furnace atomizer-ionizer
was examined by Chekalin and Vlasov [70]. Their outer
electrode design along with careful selection of the applied
electrode voltage suppressed interference from thermionic
emission of the heated graphite tube. Detection limits of
0.08 pg/mL for In and 100 pg/mL for Yb were obtained. This
system seems very promising for analysis of high purity
materials.
Hybrid Combinations of Flame and Electrothermal Vaporizers
Hybrid combinations of the flame and electrothermal

75
vaporizer were developed in order to alleviate the problems
associated with LEI in the graphite furnace alone, which were
mentioned above. The combination of flame and electrothermal
vaporizer/atomizer was first proposed by Chaplygin et al.
[130]. Their system consisted of a specially designed
cylindrical burner which contained an electrically heated wire
loop or filament in the central channel. An argon flow up the
central channel carried the sample vaporized off the wire loop
up into the flame. This design helped to minimize the sample
matrix by using small injection volumes and by temperature
programming of the sample vaporization loop. Detection in the
flame eliminated interference from thermionic emission from
heating of the sample loop and also maximized the LEI signal
because the flame region irradiated by the laser beams was
located much higher than the combustion zone. A detection
limit of 0.5 pg for Cs with reproducibility of 5-6% was
obtained.
Miyazaki and Tao have reported the use of a commercial
electrothermal vaporizer as a sample introduction system for
LEI spectrometry [131]. The commercial electrothermal
vaporizer (ETV) used (Seiko Instruments Inc., Tokyo) consisted
of a tungsten boat and a glass chamber. A Nd:YAG pumped dye
laser provided optical excitation at 276.79 nm and 291.83 nm
for one-step LEI of thallium. EIEs were found to interfere at

76
a 10-fold excess or more. T1 was extracted into 2,6-dimethyl-
4-heptanone (DIBK) with ammonium tetramethylene
dithiocarbamate (APDC) and hexamethyleneammonium
hexamethylenedithiocarbamate (HMAHMDC) at pH 6 to remove EIE
interferences. Detection limits of 0.043 ng/mL (276.79 nm)
for T1 with the extraction and 0.11 ng/mL (291.83 nm) for T1
without the extraction were obtained. This system was then
applied to and found to be useful for the analysis of
different types of natural water samples.
In order to acquire the advantages of both graphite
furnace vaporization and flame ionization detection while
maintaining independent control of each process, Smith et al.
have reported the coupling of a graphite furnace with a
miniature acetylene/air flame for LEI detection [132]. The
graphite furnace was used to vaporize samples which were
transported to the flame by a flow of argon gas. Two Nd:YAG
pumped dye lasers provided optical excitation at 285.2 and
435.2 nm for Mg, 377.6 and 655.6 nm for Tl, and 303.9 and
786.4 nm for In. Limits of detection of 0.0017 ng/mL (17 fg
absolute) for Mg, 0.012 ng/mL (118 fg absolute) for Tl, and
0.026 ng/mL (260 fg absolute) for In were obtained.
Riter et al. have reported the use of a modification of
the above system for trace element analysis [133]. A
commercially available graphite furnace (Finnigan MAT/SOLA,

77
Bremen, Germany) was used to vaporize samples which were
subsequently transported to a redesigned miniature burner,
which supported an acetylene/air flame, by a flow of argon
gas. Two XeCl excimer pumped dye lasers were used to provide
optical excitation at 285.213 and 435.191 nm for the analysis
of Mg. A complete determination of the instrumental
characteristics of the above system for the analysis of Mg was
performed. A blank-limited detection limit of 2 ng/mL (20 pg
absolute) was obtained for Mg. A detection limit of 590 fg/mL
(5.9 fg absolute) was calculated in the absence of a blank
signal and a reduction of the radio frequency noise.
Riter et al. also evaluated the above system for the
determination of trace Pb concentrations in whole blood [116].
Optical excitation at 283.3 and 509.0 nm was provided by two
XeCl excimer pumped dye lasers. Bovine blood samples from the
Centers for Disease Control (CDC) and NIST (SRM 955a) were
analyzed. With temperature programming of the graphite
furnace, there appeared to be no interference from the blood
matrix, and, instead, the matrix appeared to produce a carrier
effect, increasing the transfer efficiency between the furnace
and the flame over that for aqueous standards. The authors
concluded that there was sufficient matrix removal from
temperature programming of the graphite furnace and that the
matrix remaining acted as a carrier. A detection limit of

78
0.089 ng/mL (890 fg absolute) was calculated for Pb in whole
blood. This technique appears to have great promise for the
analysis of samples with complex matrices.
Marunkov reported the first LEI experiments with sample
vapors being introduced into a flame by an electrically heated
graphite rod and by diffusion through the wall of an
electrically heated closed graphite tube [134] . Detailed
experiments were not carried out, but results appeared to be
promising.
A hybrid "rod-flame" arrangement was proposed by Chekalin
et al. to combine the advantages of flame and electrothermal
atomizers [70,88,114,135]. In their system, the sample is
evaporated by an electrically heated graphite rod into the
flame where the analyte is atomized, laser excited, ionized,
and then detected. Advantages of this system included an
increase of the sample utilization factor (fraction of the
sample introduced that reaches the probing region), the
analysis of microsamples, and the direct analysis of solid
samples. However, the researchers encountered problems with
nonselective background ionization from the compounds
evaporating from the heated graphite rod and impurities in the
graphite were contributing to a blank signal. Good detection
limits were obtained for many elements including Au (2 pg/mL),
Co (100 pg/mL), Cr (20 pg/mL), Cu (2 pg/mL), In (0.04 pg/mL),

79
Mn (30 pg/mL), Na (0.02 pg/mL) , and Ni (8 pg/mL) . The
analysis of high-purity substances, such as Na and Cu in
orthophosphoric acid, Cu in germanium, In in Cd-Hg-Te alloy,
and Cr, Co, Mn, and Ni in fluorine-containing materials for
optical fibers, was also demonstrated. Detection limits
ranged from 0.1 to 7,000 ng/g for the different elements.
LEI in the Inductively Coupled Plasma
The first measurements of LEI in the ICP were made by
Turk and Watters [136]. Resonant LEI was detected for Fe, Mn,
Na, and Cu, however, the population of free atoms was too
small so the sensitivity was very poor. Turk et al. used a
power modulated ICP to reduce the rf interference from the
plasma [137]. With their approach, detection limits of 80
ng/mL for Fe and 20 ng/mL for Ga were achieved which are still
poorer than for the flame.
Ng et al. reported significant improvements in the
detection limits by using an extended-torch ICP, modifying the
torch and electrode designs, and using a continuous wave laser
[138]. The smaller electrode dimensions and smaller
separation between electrodes used by Ng et al. probably
accounted for the 2 orders of improvement in the detection
limit over Turk and Watters. Limits of detection ranged from
30 ng/mL for Ca to 810 ng/mL for Sr. Although these are an
improvement, they are still worse than the detection limits

80
for flame and furnace LEI.
The coupling of ICP and flame LEI with mass spectrometric
detection has been reported by Turk and others [139,140,141],
With a modified commercial ICP-mass spectrometer (ICP-MS), Sr
was examined employing laser-induced ionization (460.733 nm
and 308 nm). An enhancement of only 11% in the Sr* ion signal
was observed with the addition of laser excitation. The flame
was found to offer a much better environment for laser-induced
ionization or LEI than the ICP. A hydrogen/air flame was used
for Na, K, and Fe while an acetylene/air flame was used for
Ca. A 350 times increase in the signal was observed for Na*
with the addition of two-step optical excitation (589.0 nm and
498.3 nm) . A detection limit of 0.05 ng/mL for Na was
calculated. Detection limits of 0.14 ng/mL for K (766.5 nm
and 580.2 nm), 35 ng/mL for Fe (302.1 nm) , and 61 ng/mL for Ca
(422.7 nm and 585.7 nm) were obtained. The poor detection
limit for iron was believed to be due to one-step excitation,
and the detection limit for Ca was thought to be due to the
poor atomization efficiency of Ca in the acetylene/air flame.
Other Methods and Reservoirs
Gorbatenko et al. have reported the use of a Nd:YAG laser
as a laser microprobe solid sampling device for flame LEI
[142] . The sample was positioned at the edge of the burner
head to allow for the direct transport of the sample vapors to

81
the combustion zone. Lithium was chosen as the analyte and
was ablated from an aluminum alloy sample. Optical excitation
was provided by two Nd:YAG pumped dye lasers at 670.8 nm and
610.4 nm. It was found that the atomization efficiency was
determined by flame temperature and composition regardless of
how the sample was introduced into the flame. It was also
found that this technique allowed for the study of the
distribution of Li impurities over the surface of a solid
sample with a spatial resolution of about 100 pm. A detection
limit of around 30 pg/g for Li in solid samples was achieved
with a very short (i 20 s) analysis time.
Churchwell et al. have investigated the atmospheric-
pressure microarc atomizer as an atom reservoir for LEI [143] .
Microvolumes of analyte solution were deposited on the tip of
the tungsten cathode loop and dried with a heat gun. LEI
measurements were performed directly in the plasma above the
microarc discharge. A detection limit of 3 ng for Na was
estimated. Preliminary results indicate that the helium-
microarc-induced plasma may be feasible for LEI spectrometry
but further studies are needed.
From the above examples, it can be seen that alternate
atom reservoirs and hybrid techniques have been used with
varying degrees of success for LEI spectrometry. The flame
still appears to be the optimal atom reservoir for LEI.

82
However, the hybrid techniques appear to be the most promising
of those discussed, especially for dealing with the matrix
interferences from real samples. Further research on their
application to samples with complex matrices is warranted.

CHAPTER 5
EXPERIMENTAL
LEI
A block diagram of the experimental setup used for
graphite furnace-LEI is shown in Figure 6. An excimer laser
(Model LPX-240Í, Lambda Physik, Acton, MA) , operated with
XeCl, was used to pump two dye lasers (Model Scanmate 1,
Lambda Physik, Acton, MA). The output of the dye lasers was
directed into an air/acetylene flame for LEI spectrometry. A
repetition rate of 30 Hz was used for all LEI experiments.
Two different burners (both designed in this laboratory)
were examined in this work. The first burner design (Figure
7) consisted of a Teflon base and a 2 7/8 in. diameter brass
top. A flow of argon carried the vaporized sample up through
a central stainless-steel capillary (o.d.=l/8 in, i.d.=l/16
in) where the sample was injected up into the flame. Premixed
air and acetylene flowed up in stainless-steel capillaries
(o.d.=3/32 in, i.d.=l/32 in) surrounding the central capillary
and were ignited to form the flame.
83

Figure 6. Block diagram of the experimental setup for LEIS

00
OI

Figure 7.
First burner design used for LEIS

Flame
Sample
Side View
Top View

Additional argon flowed up through 1/32 in. holes in the brass
burner top to form an argon sheath around the flame.
A second burner was designed because of the large flame
noise associated with the first burner design. The second
burner was designed to try and minimize the flame noise by
miniaturizing the burner and, hence, the flame. This second
burner design (Figures 8 and 9) consisted of a bakelite body
and a stainless-steel rod (1/2 in. diameter) press fit into
the bakelite. A 1/8 in. hole was drilled into the stainless-
steel rod and a 0.095 inch stainless-steel central capillary
was inserted. A flow of argon carried the vaporized sample up
the central capillary where the sample was injected up into
the flame. Premixed air and acetylene flowed up in the space
between the central capillary and the stainless-steel rod and
was ignited to form the flame.
Graphite Furnace
A graphite furnace (System 3000, GBC Scientific
Instruments, Melbourne, Victoria, Australia) , modified by
Finnigan (Finnigan MAT/SOLA, Bremen, Germany) for use with an
ICP-MS, was used to vaporize all samples for flame LEI.
Pyrolytically coated graphite tubes (Part # 4090-73, CPI,
Santa Rosa, CA) were used for all graphite furnace-LEI
experiments. The temperature of the graphite furnace was
monitored by measuring the emission from the heated graphite

Figure 8. Diagram of new burner design with relative position
of the high voltage electrode and laser beams

90
Sample and Argon
from ETV

Figure 9. Detailed drawing of the new burner design

92
0.095" stainless steel capillary

93
tube with a photodiode mounted just outside the right window.
The vaporized sample was transferred from the graphite furnace
to the burner by a flow of argon through a PTFE transfer tube
(Cat. # AP-06375-02, Cole-Parmer Instrument Company, Niles,
IL) approximately 1.7 m long (i.d.=5/32 in, o.d.=l/4 in),
which was connected to another piece of PTFE tubing (i.d.=3/32
in, o.d.=l/8 in) by a Teflon reducing union (1/4 in to 1/8 in,
Swagelock Co., Solon, OH). The 1/8 in. tubing was fitted onto
the central capillary of the burner with the newer burner
design.
The furnace modifications made by Finnigan included
separating the argon flow through the center of the graphite
tube from the flow around the tube and replacing the left
window of the furnace with a sample transfer interface. The
argon flow through the graphite tube was adjusted with a mass
flow controller. The sample transfer interface consisted of
a tantalum tube placed about 5 mm from the graphite tube, a
stainless-steel holder that fitted into the original window
housing, and a PTFE adapter to connect the holder to the
transfer tubing.
This interface was further modified in our laboratory.
The tantalum tube was extended to within 2 mm of the graphite
tube and the PTFE adapter was eliminated by threading the
transfer tubing such that the tubing could be directly
attached to the stainless-steel holder. A sharpened 1/8 in.

94
diameter graphite electrode (Part # L4236, Spectrographic
Services, Sussex, NJ) was used to plug the injection hole of
the graphite tube during heating of the graphite furnace.
Figure 10 shows a cross-sectional view of the graphite
furnacewith the tantalum extraction probe on the left.
A water-cooled stainless-steel electrode (o.d.=3 mm) was
immersed in the flame and a negative high voltage (-800 V)
applied. The high voltage was optimized by adjusting the
voltage until the signal-to-noise ratio was a maximum. An
aluminum box was used to partially shield the system from
extraneous radio frequency (rf) noise. The laser-induced
charge was collected through an electrical connection to the
central burner capillary and transferred to a circuit
consisting of a 10 kfl resistor to ground and a 1 nF capacitor
in series. The signal then passed through a transimpedance
amplifier (Model Al, THORN EMI Gencom, Fairfield, NJ), low
noise amplifier (Model SR560, Stanford Research Systems,
Sunnyvale, CA), and a boxcar averager (Model SR250, Stanford
Research Systems, Sunnyvale, CA) . The signal from the boxcar
was collected on a notebook personal computer (Model
Highscreen 386SX/33, Highscreen Computer, Würselen, Germany)
through a computer interface (Model SR245, Stanford Research
Systems, Sunnyvale, CA) using the Stanford data collection
program (SR 265 v. 3.lx, Stanford Research Systems, Sunnyvale,

Figure 10. Cut-away view of the graphite furnace showing the tantalum sample extraction
interface

Modified Window
Housing
PTFE Tubing
To
Burner
Dosing Graphite Plug
Hole . y¡Cooling Argon
Sample
Transfer
Argon
Window
Tantalum ^ X.
Tube Graphite Graphite Tube
Electrode
Window
Holder

97
CA). A gate width of ~800 ns was used on the boxcar.
Procedure and Conditions
For graphite furnace-LEI of magnesium, Coumarin 153 and
Coumarin 120 (Lambda Physik, Acton, MA) were the laser dyes
used. Both dyes were dissolved in high purity methanol
(Optima grade, Fisher Scientific, Pittsburgh, PA) to the
concentrations specified by the manufacturer. The output of
the Coumarin 153 at 570.4 nm was frequency doubled with a BBO
III crystal (Lambda Physik, Acton, MA) to provide optical
excitation at 285.2 nm. The output of the Coumarin 120
provided optical excitation at 435.2 nm. Laser pulse
durations were approximately 40 ns (full width at half
maximum), rather than the LPX-240Í nominal duration of 14 ns.
Pulse energies were typically 150 mj for the excimer, 3 mj for
the Coumarin 153, 50 nJ for (Coumarin 153 doubled by BBO
III), and 4 mj for X2 (Coumarin 120). Conversion efficiencies
were typically 8% for Coumarin 120, 6% for Coumarin 153, and
2% for the BBO III crystal. [285.213 nm: 3s2 1S„ - 3s (2S)
3p 1P1°, A21 = 4.95 X 108 s'1] was focused into a 5 times beam
expander (Model 15600, Oriel Corporation, Stratford, CT) by a
quartz cylindrical lens [focal length (fl) = 3.5 in]. A,2
[435.191 nm: 3s(2S)3p ^ - 3s(2S)6d 3D2, A32 = 2.1 X 107 s'1]
was focused with a quartz lens (fl = 41 in) into the flame.

98
At the flame, is a square beam approximately 4 mm wide and
X2 is a round beam approximately 6 mm in diameter encompassing
X.j. Both laser beams were positioned immediately below the
high voltage electrode for efficient charge collection.
The vaporization temperature for Mg was optimized by
increasing the furnace temperature until no cleaning peak was
observed. The graphite furnace temperature program used for
magnesium is shown in Table 2. Aqueous Mg standards were
prepared by diluting a stock solution (1000 pg/mL) with ultra
pure 2% nitric acid (Optima grade, Fisher Scientific,
Pittsburgh, PA). Sample volumes of 10 pL were used for all
injections. Injections were performed using a digital pipette
(Eppendorf Model 4810, Brinkman Instruments, Inc., Westbury,
NY). It was observed that at concentrations of Mg below 100
ppb, the precision was worse than that observed for the higher
concentrations. Therefore, the feasibility of using a matrix
modifier to act as a carrier for the lower concentration
samples was evaluated. The main criterion for choosing a
matrix modifier was that it should not contribute a
substantial Mg signal (less than our blank contamination of
about 3 pg/mL). Ultra pure methanol (Optima grade, Fisher
Scientific, Pittsburgh, PA) was used as a carrier for the
lower concentrations of Mg. Analysis of the methanol

Table 2. Graphite furnace temperature program for magnesium
Step
Temperature (°C)
Ramp time (s)
Ramp hold (s)
1.
Drying
90
10
60
2 .
Cool Down
(add carrier)
20
10
90
3.
Vaporization
2000
3
6
4 .
Cool Down
20
1
20
5.
Cleaning
2500
3
6
6.
Cool Down
20
1
10

100
indicated no detectable Mg was present. However, a blank LEI
signal (not due to Mg) was observed with the methanol carrier.
This signal was reproducible and was therefore subtracted from
the sample peaks. An optimal injection volume of 6 nL of
methanol was determined by optimizing the Mg LEI signal while
minimizing the blank contribution. The methanol was injected
after the drying step before the vaporization step.
All gas flows were optimized using the signal-to-noise
for the Mg LEI signal. The gas flows were optimized by
adjusting each of the flows separately and then confirming
each setting after the initial optimization. Gas flow rates
of 622 cmVmin for air, 85 cmVmin for acetylene, and 357
cmVmin for the carrier argon were found to be optimal for Mg.
For graphite furnace-LEI of lead, Coumarin 153 and
Coumarin 307 (Lambda Physik, Acton, MA) were the laser dyes
used. Both dyes were dissolved in high purity methanol
(Optima grade, Fisher Scientific, Pittsburgh, PA) to the
concentrations specified by the manufacturer. The output of
the Coumarin 153 at 566.6 nm was frequency doubled with a BBO
III crystal (Lambda Physik, Acton, MA) to provide optical
excitation at 283.3 nm [A,1( 6p2 3P„ - 7s 3P1°] . A linewidth of
0.15 cm-1, typical pulse energy of 20 nJ, and approximate beam
diameter of 1 mm were measured for the first dye laser using
Coumarin 153. The Coumarin 307 was used with a different dye

101
laser (Model DL-14, Laser Photonics, Orlando, FL) and provided
optical excitation at 509.0 nm [X2i 7s 3pi° - 8p 3D2] . A
linewidth of 0.4 cm'1, typical pulse energy of 1.2 mj, and
approximate beam diameter of 5 mm were measured for the second
dye laser using Coumarin 307.
The graphite furnace temperature program used for lead in
blood is shown in Table 3. The vaporization temperature was
optimized using the same procedure as for Mg (see above). The
second ashing temperature was optimized by using the highest
temperature without loss of lead during the ashing. Aqueous
lead standards were prepared by diluting a stock solution
(1000 |ig/mL) with ultra pure 2% nitric acid (Optima grade,
Fisher Scientific, Pittsburgh, PA). A 100 mg/dL sodium
solution, used as a carrier, was prepared by dissolving high
purity NaCl (Alfa Aesar, Ward Hill, MA) in ultra pure water
(Millipore, Bedford, MA). The concentration of the NaCl was
optimized by optimizing the signal-to-noise ratio for the lead
LEI signal. Injection volumes of 10 pL were used for all
samples, standards and the carrier. Injections were performed
using a digital pipette (Eppendorf Model 4810, Brinkman
Instruments, Inc., Westbury, NY).
All blood samples and standards were prepared as follows.
Vials of blood were allowed to thaw completely at room
temperature. The vials were then rolled carefully between

Table 3. Graphite furnace temperature program for lead in blood
Step
Temperature (°C)
Ramp time (s)
Ramp hold (s)
1. Drying
90
5
40
2. Ashing 1
260
10
10
3. Ashing 2
450
5
15
4. Vaporization
1900
3
8
5. Cleaning
2500
3
6

103
hands and placed in a sonicator to ensure thorough mixing. A
volume of 50 |iL whole blood was pipetted into a vial
containing 1 mL of ultrapure water. The vials containing the
diluted blood were also rolled and sonicated for thorough
mixing. All blood samples and standards were refrozen
immediately after use. It should be noted that the dilution
of the whole blood with ultra pure water was needed only to
facilitate pipetting into the graphite furnace. If an
adequate pipet was available to inject whole blood into the
graphite furnace, this dilution should not be necessary.
Blood lead standards from the CDC and NIST were used. The
standards from the CDC's Blood Lead Laboratory Reference
System (BLLRS) program that were analyzed included pool ID#
694 (0.7 ng/dL) , 192 (3.9 ng/dL) , 1291 (10.6 ng/dL) , 0191
(19.3 ng/dL) , and 1092 (61.6 ng/dL) . The standards from NIST
included SRM 955a-l (5.01 ng/dL), -2 (13.53 ng/dL), -3 (30.63
(rg/dL) , and -4 (54.43 )ig/dL). One human blood sample of
unknown lead concentration was also analyzed.
Flame Gas Flows. Velocity, and Temperature
All gas flows were calibrated with a mass flowmeter
(Model ALK-50K, Teledyne Hastings-Raydist, Hampton, VA) at
experimental conditions by inserting the mass flowmeter

104
between the rotameter and burner for each gas.
Average flame gas velocity was measured by observing the
LEI signal at two different positions in the flame and
measuring the time differential between the two ion pulses
[144] . For this experiment, the burner head was grounded and
an iridium wire (diameter = 0.012 in) was inserted into the
flame above the laser beams (no potential applied). The Ir
wire was connected to the circuit and transimpedance amplifier
used for the regular LEI experiments described above. The
amplified signal was observed and recorded on a digital
storage oscilloscope (Model 620A, Tektronix, Beaverton, OR).
Ion pulses for 10 |1L samples of 100 ppb Mg solution were
observed on the oscilloscope. The Ir wire was moved a
distance, d, of 1 mm in the z-direction (up and down) and the
time differential, At, between the ion pulses was determined.
The flame gas velocity was calculated by the following
formula:
where v is the flame gas velocity (cm/s).
Flame temperature was measured using the two-line
emission method [145,146] with iron. This procedure has been
described in detail elsewhere [147], All iron solutions were

105
introduced into the miniature flame using a laboratory
constructed ultrasonic nebulizer (Figure 11). Iron emission
was collected with a quartz lens (fl = 3.5 in.) and focused
onto the monochromator (Model EU-700, GCA/McPherson
Instruments, Acton, MA) slit. The relative line intensities
of Fe emission at 382.043 nm and 371.994 nm were measured.
The absolute temperature, T, can be calculated from the
equation:
T =
0.6247 (Ej-E.,)
(43)
where Ex and E2 are the excitation energies (cm-1), gt and g2
are statistical weights of the states, At and A2 are transition
probabilities for the lines, v2 and v2 are frequencies of the
lines, and I2 and Ia are the relative intensities of the two
lines.
Noise study
For all of the noise studies, five different noises were
measured: the boxcar noise, the boxcar with transimpedance
amplifier (Al) noise, the "DC" background, the background with
rf noise, and dry run noise.
To measure the boxcar noise, the input of the boxcar was

Figure 11. Sketch of laboratory constructed ultrasonic nebulizer used

Drain
Argon
Analyte Solution
from Peristaltic Pump
-> Ar + Nebulized
Sample

108
electrically shorted while data was taken. For the boxcar
with A1 noise, the boxcar and A1 were connected as usual but
the A1 input was capped while data was taken. The previous
two noises should be the only noises associated with the
electronics and should also be the same for all experiments
with the different burners and flame gases.
The "DC" background noise or flame noise is the noise
associated with the flame and will vary with the flame gas
composition and the size of the flame. This noise was
recorded with the system ready for LEI (flame lit and high
voltage applied to the electrode) except that the lasers were
not firing. The background with rf noise is simply the "DC"
background noise with the lasers firing, so any rf noise from
the lasers should be included in this noise. The dry run
noise is the same as the background with rf noise except with
the graphite furnace firing (no sample injected) as during a
normal LEI experiment.
The noise of the first burner design was compared to the
new burner design. Also, the hydrogen/air flame was compared
to the acetylene/air flame for LEI of magnesium.
Fluorescence Dip and Fluorescence Profile of Flame
According to Omenetto et al. [148], when the second
excitation reaches a level close to the ionization continuum

109
from which collisional ionization occurs very rapidly, the
ionization yield can be determined by measuring what is known
as the fluorescence dip. This parameter describes the
decrease in resonance fluorescence from the first excited
level that occurs when adding a second pumping process. This
second excitation process depletes the atomic population of
the first excited level such that they do not return to the
ground state. The resonance fluorescence signal is always
proportional to the number density of excited atoms in the
first excited state [149] . As it has been shown from simple
theoretical considerations [20,48,62], the ionization yield
will approach unity when an optically saturating laser pulse
has a duration that significantly exceeds the reciprocal of
the effective ionization rate of the laser populated excited
state. However, before using the fluorescence measurement to
directly evaluate the ion yield, one must be sure that there
is no loss due to quenching collisions into a metastable
level. In the absence of collisional quenching, the
ionization yield, Yi( can be calculated as:
I21 (A^only) -I21 (padded)
IjjfXjOnly)
(44)
where I21 is the signal intensity for the resonance
fluorescence.
For both fluorescence experiments, the monochromator was

110
positioned at an angle of 90° from the laser beams (Figure
12) . For the fluorescence dip experiments, the resonance
fluorescence for Mg at 285.213 nm was collected using a quartz
lens (fl = 3.5 in.) and imaged onto the monochromator slit.
The LEI signal was monitored simultaneously as shown in Figure
12. First, the blank (ultra pure water) signal was measured
to determine the scatter (background) from the laser. Next,
a solution of 10 ppb Mg was injected in volumes of 10 pL. The
resonance fluorescence was first measured with only X, (285.213
nm) and then remeasured with X2 (435.191 nm) added. The
fluorescence dip was calculated from these measurements which
was used to estimate the ionization yield.
A profile of the relative atom concentrations in the
flame was obtained by observing the saturated resonance
fluorescence (285.213 nm) at various positions in the flame.
For any given spectral line of a metal vapor in a flame of
constant composition and temperature, the fluorescent
intensity, IF, is given by [150] :
Ir = C P° N„ (45)
where C is a constant term, P° is the incident radiant power,
and N0 is the ground state concentration of the absorbing
atoms. When full saturation is attained, the fluorescence
signal becomes insensitive to variations in the laser power

Figure 12. Block diagram of experimental setup for monitoring of both fluorescence and
LEI signals

Excimer
\
Laser

113
[5] . Therefore, the concentration of Mg atoms is directly
proportional to the saturated resonant fluorescent intensity.
The beam expander and cylindrical lens were removed from
(285.213 nm) for the fluorescence experiment. A
planoconvex quartz lens (fl = 4.5 in) was then inserted in
order to focus the beam down to a very small spot in the
flame. Resonance fluorescence was again collected at 90° with
a quartz collection lens (fl = 3.5 in) which was used to image
the flame onto the monochromator slit. Saturation of the
fluorescence was confirmed by insertion of neutral density
filters into the excitation beam. Saturation was confirmed at
several different positions within the flame. The burner was
translated in the x-, y-, and z-directions and a relative
spatial distribution of Mg atoms in the flame was obtained
from the relative fluorescence intensities.
Transport Efficiency
In order to determine the transport efficiency from the
furnace to the flame, a trapping technique modelled after that
described by Schmertmann et al. [151] was used. It should be
noted that the efficiency measured here includes the
vaporization efficiency of the furnace. It was assumed that
the vaporization efficiency of the furnace was 1.000 (+0.001)

114
since no observable cleaning peak was obtained with the
optimized vaporization temperature.
A sketch of the experimental setup is shown in Figure 13.
Cotton was used to trap the particulates transported from the
furnace by the flow of argon. Various types of cotton were
digested in 50% high-purity nitric acid (Optima grade, Fisher
Scientific, Acton, MA) and analyzed by flame-atomic absorption
to determine the cotton with the lowest Mg impurity. PADCO
cotton (Non-surgical bleached cotton, ACCO, Valley Park, MO),
purchased from Fisher Scientific was found to have the least
Mg impurity and was selected for use.
The trapping apparatus consisted of two glass bubblers
connected in tandem to the end of the transfer tubing leading
to the burner. The connection between the tubing and the
first bubbler included a one inch length of stainless-steel
capillary to make it as similar as possible to the connection
between the tubing and the mini-burner. The connection
between the two bubblers was a two-inch length of Tygon tubing
(i.d.= 0.375 in, o.d.= 0.625 in). A 1.60 g sample of the
cotton was placed into the bottom of each bubbler to trap the
particulates produced by the furnace.
Since the resulting trapping solution was to be analyzed
by atomic absorption, the final concentration had to be in the
range of 1-1000 ppb. This was achieved by the injection of

Figure 13. Sketch of experimental setup for transport efficiency measurement

Teflon
Tubing
^rC^V-1

117
100 samples of 100 ppm Mg solution (10 nL each). The cotton
was then digested in 60 mL of 50% high-purity HN03 in a 70“C
water bath for 30 minutes. The resulting solution was then
hot filtered into a 100 mL volumetric flask and diluted to the
mark with ultra pure water.
To confirm the validity of the trapping technique, the
LEI system was used to analyze Mg standards from 1 ppb to 100
ppm which indicated that the signal remained linear over this
range. The burner was connected to the end of the trapping
system for LEI analysis of any Mg that made it through the
trapping system. A >99.9% reduction in the LEI signal for a
100 ppm Mg sample confirms that the Mg is not escaping from
the trap.
In an effort to account for the analyte losses incurred
during transport, an analysis of the residue in the transfer
tubing was performed. The transfer tubing was rinsed with 50%
high-purity HN03 and the subsequent solution analyzed by
atomic absorption and LEI. To estimate the loss of Mg due to
diffusion through the graphite tube, the shroud gas flowing
around the outside of the graphite tube and out through the
dosing hole opening was diverted into the flame for LEI
analysis. The argon flow was adjusted to match the flow
through the inside of the graphite tube. A small piece of
graphite was used to plug the dosing hole. It should be noted

118
that this diffusion loss experiment may tend to overestimate
the losses due to diffusion through the graphite tube. This
is because the graphite plug was not held in place within the
dosing hole. Therefore, significant amounts of Mg may have
leaked out through the dosing hole. During the LEI
experiments, the graphite plug is securely held in place by
an aluminum holder.
Transimpedance Amplifier Calibration
The theoretical detection limit of the transimpedance
preamplifier was calculated by determining the equivalent
noise charge (ENC) of the preamplifier [152] . A pulse
generator (Model PG501, Tektronix, Beaverton, OR) was
connected to the input circuit consisting of a capacitor, Ca,
(1 pF) which was connected in series to the Thorne A1
preamplifier. Some parasitic or stray capacitance, Cp, in
parallel was also present in the circuit. The value of Cp is
typically around 15 to 30 pF. An input voltage, Uln, of 55.4
mV (50 ns pulse width) was delivered by the pulse generator.
If Cp»C„ then Q0=Uin-C,, where Q= is the charge. The ENC can be
calculated by the equation:
Qcsk _
Sp
ENC
S,
(46)

119
where SN is the RMS noise of the preamplifier and Sp is the
output of the preamplifier. To determine SN, the preamplifier
input was disconnected and the RMS noise of the preamplifier
measured using an oscilloscope.
The charge response of the detection system was measured
using the following scheme. The pulse generator described
above was connected to a calibration circuit consisting of a
50 fl resistor in parallel and a capacitance, Cc, in series.
A parasitic or stray capacitance, Cp, was again present in
parallel within this circuit. The calibration circuit was
then connected to the original input circuit and the A1
preamplifier. A voltage pulse, Uln, was applied and the output
of the preamplifier, Sp, was measured for each value of Cc.
If Cp»Cc, then Qc=Uln-Cc where Qc is the charge applied. Values
of 1 pF and 2 pF were used for C,.. A 55.4 mV pulse was used
for Uln (50 ns pulse duration) . The average signal output, Sp,
from the preamplifier corresponding to a 1 pF charge was
recorded. The amount of charge, Qc, was calculated and
divided by Sp to give the charge response of the detection
system.
Atomization Efficiency Measurement
The determination of atomization efficiencies by an
absorption measurement in the flame has been described by

120
others [5]. A block diagram of the experimental setup used
for the determination of the atomization efficiency for Mg is
shown in Figure 14. The emission from a Mg hollow cathode
lamp (Varian, San Fernando, CA) with power supply (Model EU-
703-62, Heath, Benton Harbor, MI) was modulated with a
mechanical chopper (Model SR540, Stanford Research Systems,
Sunnyvale, CA) and then imaged onto the flame with a
planoconvex quartz lens (fl = 4.5 in). The transmitted light
was then imaged onto the monochromator (described earlier)
slit with a biconvex quartz lens (fl = 3.5 in). The
photomultiplier tube (Model R928, Hamamatsu Photonics,
Bridgewater, NJ) output was sent to a current amplifier (Model
427, Keithley Instruments, Cleveland, OH) . The amplified
signal was then sent to a lock-in amplifier (Model 5207, EGSG
Princeton Applied Research, Princeton, NJ) and collected on a
personal computer (described earlier). The burner and
graphite furnace were the ones used for graphite furnace-LEI
experiments. The absorption at 285.213 nm for Mg was
measured.
The absorbance at the line center, A(v0) , is related to
the total number density, nT, of Mg atoms by [5] :
9o f"Tl
Z(T) Ava££
A (V0)
1.15 X 10-2
(47)

Figure 14. Block diagram of the experimental setup for the determination of the
atomization efficiency for Mg by atomic absorption

Lens
Monochromator
HCL
Chopper
Power
Supply

123
where g0 is the statistical weight of the ground state, Z(T)
is the electronic partition function, f is the absolute
oscillator strength, l is the absorption path length, and Aveff
is the effective line width of absorption.

CHAPTER 6
RESULTS AND DISCUSSION
After the system was assembled as described in Chapter 5
with the first burner design, the laser beams were aligned to
be coincident in space and time. The oscilloscope trace shown
in Figure 15 shows the coincidence of the laser beams in time
and was taken with a fast photodiode (Model ET2000, Electro-
Optics Technology, <200 ps risetime, Fremont, CA) and the
oscilloscope described earlier. From the trace, there
appeared to be good temporal overlap of the two laser beams
even though the maximum intensity for appears slightly
before the maximum for \2.
Magnesium as Analyte
Once the lasers were aligned, the ETV-FL-LEI system was
characterized with magnesium. Magnesium was chosen as the
analyte because Mg had been analyzed sucessfully by graphite
furnace LEI previously in our laboratory [4] and was being
analyzed by graphite furnace (electrothermal vaporization
124

Figure 15.
Oscilloscope trace of the laser beam timing

126

127
ETV-ICP-MS also in our laboratory [153] employing the same
graphite furnace design. However, it was found throughout the
course of our research with LEI that there are several
difficulties associated with Mg. Due to the relative
abundance of Mg in nature, sample contamination was
significant. Regular laboratory glassware could not be used
for sample preparation and storage. Instead, plastic or
Teflon volumetric flasks, pipettes, and storage bottles that
were carefully soaked and cleaned in high-purity nitric acid
and ultra pure water were used. These precautions appeared to
reduce sample contamination below the blank level.
There was also difficulty in finding a matrix
modifier/carrier for use with low Mg concentrations. The
inorganic matrix modifiers examined all contained significant
amounts of Mg impurity. Most of the organic matrix modifiers
examined produced laser-induced ionization signals most likely
from molecular species. Finally, the limit of detection for
Mg would ultimately be limited by the blank signal since the
stock nitric acid solution used to prepare standards and
samples contains -0.02 ppb Mg.
Ma LEI Signal
A typical LEI signal for Mg is shown in Figure 16. The
second trace is the signal from the photodiode monitoring the
furnace temperature. There is a ~0.4 s time differential

Figure 16. Typical LEI signal for magnesium with older burner

Signal (V)
129

130
between when the furnace reaches the vaporization temperature
and the appearance of the LEI signal. This is the sample
vapor transit time from the furnace to the flame. The Mg LEI
signal lasts ~1.5 s and ends well before the end of the
furnace vaporization cycle.
An important issue for applying LEI to standardless or
absolute analysis is the conversion of the collected signal,
in volts, to the induced charge, in coulombs, created in the
flame. For a successful conversion, the charge response of
the detection system must first be calibrated. The procedure
used to measure the charge response was outlined in the
previous chapter. The signal, Sp, corresponding to an input
potential of 55.4 mV and a capacitance, Cc, of 1 pF was found
to be 10.5 mV. The applied charge, Qc, of 5.5 x 10'14 C (3.44
X 105 electrons) was divided by the signal, Sp, to obtain the
charge response of 5.2 X 10"15 C/mV (3.3 X 104 electrons/mV).
The output of the boxcar (in mV) was divided by the gain
of the amplifier and then multiplied by the charge response of
the detection scheme (input circuit and A1 transimpedance
amplifier) which gives the charge (in C) produced by each
laser pulse. These were summed over the duration of the Mg
LEI peak to give the total charge collected that was produced
by the given Mg sample.
System Parameter Optimizations for Old Burner Design

131
Next, the system parameters were optimized for Mg. The
results of the transfer argon flow rate optimization are shown
in Figure 17. At low Ar flow rates (s0.6 L/min), the
precision of the signal was poor. This was due to broadening
of the sample peaks and the difficulty in determining where
peaks began and ended. At high Ar flow rates (al.5 L/min),
the sampling efficiency of the laser beams was decreased
because the analyte passed by too quickly. The optimal Ar
flow rate of 0.9 L/min was chosen because the best combination
of signal intensity and precision were obtained at this flow
rate.
The results of the optimization of the distance between
the burner and electrode are shown in Figure 18. When the
burner and electrode were close together (2.7 cm), the signal
was lower because the argon and sample vapor did not have
enough time to thoroughly mix with the flame gases. As a
result, the sample was not well atomized by the flame and
hence the lower signal. When the electrode and burner were
far apart (4.7 cm), the Mg atoms began to dissipate. Lower
field strength may also have contributed to the rapid
reduction in signal. An optimal burner-to-electrode distance
of 3.2 cm was chosen because the optimal combination of Mg-LEI
signal intensity and precision were obtained at this distance.
The results of the optimization of the voltage applied to
the LEI electrode are shown in Figure 19. The signal

Figure 17. Argon flow rate optimization for Mg with older burner

133

Figure 18. Burner-to-electrode distance optimization for Mg with older burner

Peak Area (Vs)
2.5
5.0

Figure 19. Applied voltage optimization for Mg with older burner

Signal (V)
137

138
increases as the applied voltage increases until a plateau is
reached around -600 V. The voltage applied to the electrode
was chosen to be -600 V. The best combination of signal
intensity and precision was obtained at this voltage.
Flame Profile with Old Burner
A horizontal profile of the flame (Figure 20) was
obtained by translating the burner horizontally along the axis
perpendicular to the laser beams. From the profile, it
appears that the flame is approximately 8 mm wide. Since the
laser beam for X1 is ~1 mm wide, a significant portion of the
signal was being missed.
Analytical Curve with Old...Burner
An analytical curve for Mg with the first burner design
was obtained and is shown in Figure 21. A slope of 0.856
(±0.009) C/mol was obtained although the linearity was not
very good as seen in the Log-Log plot of Figure 22 (slope =
0.85). A 3ct detection limit of only 12 ppb Mg was calculated.
The poor detection limit was partially due to the large flame
background noise with this flame. Therefore, a hydrogen/air
flame was examined as a possible alternative to the
acetylene/air flame that was being used.
The hydrogen/air flame should have a lower temperature
and thermal ionization associated with it. This should

Figure 20. Horizontal profile of the flame with the older burner

Signal (V)
Horizontal Position from Center (mm)
140

Figure 21. Analytical curve for Mg with the older burner

142

Figure 22. Log-Log plot of the analytical curve of Figure 21

-14 -13 -12 -11 -10
Log (Moles of Mg)
Log (Signal (V))

145
translate to a lower flame background noise. The noises of
the hydrogen/air flame and the acetylene/air flame were
measured and the results are shown graphically in Figure 23.
From the results, it appears that the noises for the two
flames are within about a factor of two of each other. The
noise during the dry furnace run appears to be lower than
without the furnace firing, however, within experimental error
they are the same. The hydrogen/airflame was not used for Mg
analysis since there was no significant lowering of the flame
background noise and a 10-fold loss in sensitivity was also
observed due to the lower temperature (and collisional
environment) of the hydrogen/air flame.
New Burner Design
Therefore, to alleviate the large flame background noise,
a new burner design was proposed and subsequently built by the
departmental machine shop. This new burner was shown in
Figures 8 and 9 and described in the previous chapter. An
essential feature of the new burner is the much smaller
dimensions of the burner and, hence, the flame. By reducing
the size of the flame, it was hoped that the flame background
noise would be reduced to a manageable level.
The noises of the new burner were measured as described
in the previous chapter. The noises for the new burner design
with an acetylene/air flame were then compared to the old

Figure 23. Noises (rms) for the hydrogen/air and acetylene/air flames with the older
burner design

2.5
2
£• 1.5
0
CO
“H
o
2
0
1
0.5
Hydrogen â–¡ Acetylene
Dry Run
DC+rf
147

148
burner design also with an acetylene/air flame. Figure 24
shows the Log of the noises (plus 15) for the new and old
burner designs. From the graph, it is evident that the noises
for the new burner design are about two orders of magnitude
lower than the old burner design. Therefore, a lowering of
the detection limit by two orders of magnitude would be
expected if the sensitivity remains the same.
System Parameter Optimizations With New Burner
The system parameters were optimized for the new burner
design. The results of the argon flow rate optimization are
shown in Figure 25. Once again, at the higher argon flows,
the sample passes through the flame too quickly to be probed
efficiently by the laser beams. At lower flow rates, the
sample peaks are broad and hard to define which results in
poor precision. An optimal argon flow rate of 0.3 L/min was
chosen since the best combination of LEI signal intensity and
precision were obtained at this flow.
The results of the flame gas optimizations are shown in
Figure 26 and Figure 27. The LEI signal for Mg appeared to be
fairly constant for acetylene flow rates between 80 and 100
mL/min. An optimal flow of 85 mL/min for acetylene was chosen
as the best precision was obtained at this flow rate. The Mg
LEI signal appeared to be constant for air flow rates between
0.55 and 0.65 L/min. An optimal flow rate of 0.62 L/min was

Figure 24. Comparison of the noises (rms) for the new and old burner designs

DC+rf Dry Run
o
15+Log(noise)
h-1 hO 00
--
- - - ' • -
^ÍÍPr|
â–¡
o
--
ld[
â–  91
U
z
CD
s:
051

Figure 25. Argon flow rate optimization with new burner

40
30
20
10
0
I
Argon Flow Rate (L/min)
152

Figure 26. Acetylene flow rate optimization with new burner

100
80
60
40
20
I 1
-I 1 1 . 1 1 1 . 1
60 70 80 90 100
Acetylene Flow Rate (mL/min)
T
no
154

Figure 27. Air flow rate optimization for new burner

60
50
40
30
20
10
0
i 1 r
I
i | i \ i | i | i | i |
4 0.5 0.6 0.7 0.8 0.9 1.0
Air Flow Rate (L/min)
156

157
chosen because good precision and signal intensity were
obtained at this flow.
The distance between the burner and the electrode was
optimized as before and the results are shown in Figure 28.
A burner-to-electrode distance of 14 mm was chosen since the
signal reached a maximum at this distance. The voltage
applied to the electrode was also optimized with the results
shown in Figure 29. At about -300 V, the LEI signal reaches
a plateau. A voltage of -400 V was chosen as optimal since
good precision was obtained at this voltage.
Flame Profile With New Burner
A horizontal profile of the flame was measured as
described previously and is shown in Figure 30. A Gaussian
profile was fit to the data. If the laser beam for is 4mm
wide, then the estimated spatial probing efficiency from the
data is 0.81 (±0.05).
A spatial profile of the relative fluorescence
intensities for Mg in the flame is shown in Figure 31. The
fluorescence intensities can be related to the relative Mg
atom concentrations in the flame. As a result, this profile
gives an indication of where the Mg atoms reside in the flame.
It appears that the laser beams should be centered about 10 mm
above the burner head. This is expected since it should take
a certain distance for the cold argon and sample to thoroughly

Figure 28. Burner-to-electrode distance optimization for new burner

5
30

Figure 29. Electrode voltage optimization with new burner

161

Figure 30. Horizontal profile of Mg atoms in flame with new burner

40
30
20
10
0
Horizontal Distance from Center (mm)
6
6
163

Figure 31. Fluorescence profile of Mg atoms in the flame with
the new burner

Distance From Center (mm) Y-direction
Distance Above Burner (mm)
K>
o
165

166
mix with the flame gases. The data shown in Figure 31 also
confirm that over 80% of the Mg atoms reside within a 4 mm
width. Over 99% of the Mg atoms reside within approximately
a 6 mm flame diameter.
Matrix Modifier/Carrier
Once all of the optimizations were performed, solutions
of different concentrations of magnesium were introduced to
check the linearity of the system response. It was observed
that at lower concentrations of Mg (<100 ppb), the precision
was worse than that observed for the higher concentrations.
Therefore, the feasibility of using a matrix modifier to act
as a carrier for the lower concentration samples was
evaluated.
The main criterion for choosing a matrix modifier was to
find one which contributed a negligible Mg LEI signal (less
than our blank signal from the HN03 of about 3 pg/mL). Ultra
pure methanol (Optima grade, Fisher Scientific, Acton, MA) and
xylene (Low trace metals grade, Mallinckrodt Chemicals, Paris,
KY) were chosen as possible modifiers. Analysis of the xylene
indicated a large blank LEI signal from the xylene alone. The
LEI signal from xylene was then measured while scanning the
dye laser for Xj.. The results shown in Figure 32 suggest that
the LEI signal is a result of molecular interferences from the

Figure 32. LEI signal for xylene while scanning dye laser for X,

70
1
284 286
Wavelength of X1
T
J L
(nm)
o L-
280
282
288
290
168

169
vaporized xylene. Therefore, xylene was not used for the
analysis of Mg by LEIS.
Subsequent analysis of the methanol indicated that no
detectable Mg was present, but a blank LEI signal (not due to
Mg) was observed with the methanol carrier. However, this
signal was reproducible and was therefore subtracted from the
sample signals. The effect of methanol on the Mg LEI signal
is shown in Figure 33. From the plot, it appears that
methanol does enhance the ionization signal for Mg, presumably
by enhancing the transport of the sample vapor from the
furnace to the flame. The optimal amount of methanol injected
was found to be 6 pL as the best precision and enhancement of
the Mg signal was observed with this volume.
Analytical Curve With New Burner
The analytical curve with methanol carrier (Figure 34)
was found to be linear for 2.5 to 100 ppb of Mg in 2% HN03.
A slope of 2.5 (±0.2) C/mol was obtained and the experimental
limit of detection (LOD, S/N=3) was estimated to be 2 ng/mL
(20 pg absolute). This LOD was limited by the blank signal
from the 2% HN03 and methanol carrier.
The limiting non-blank noise level was found to be 9.8 fC
rms. This limiting noise level was primarily due to rf noise
from extraneous sources. Using this limiting instrumental
noise, the LOD for Mg was calculated to be 29 pg/mL (290 fg

Figure 33. Effect of methanol on LEI signal for Mg

171

Figure 34. Analytical curve for Mg with new burner and methanol

120
100
80
60
40
20
0
173

174
absolute).
The extraneous rf noise may be reduced by the design and
construction of a better metal shielding box. If the rf noise
can be reduced to a level below that of the A1 preamplifier
noise, then the theoretical LOD for Mg was calculated to be
590 fg/mL (5.9 fg absolute). This compares favorably with
LODs recently determined for Mg with ETV-ICP-AES (O.lpg)
[154], ETV-ICP-MS and FANES (20 pg) [155], and ETV-AAS (0.4
pg) [156] .
Flame Temperature and Flame Gas Velocity With the New Burner
The flame temperature was measured using the two-line
method as indicated in the previous chapter. The values for
the parameters used to calculate the temperature are given in
Table 4 [157] . The ratio of the intensity of the two lines,
I2/11( was experimentally found to be 14.1. The value for the
flame temperature was calculated to be approximately 1973°C.
This is cooler than a normal air/acetylene flame (temperature
2267°C). However, this result was expected since our flame is
much smaller than a typical air/acetylene flame. A stream of
cold argon carrying the sample was injected up the center of
the flame which may have also lowered the flame temperature.
The flame gas velocity was measured as discussed in the
previous chapter. The time differential, At, was measured as
240 ps for a distance, d, of 1 mm. The flame gas velocity was

175
Table 4. Values used for flame temperature
calculation
Parameter
Value
k
6.95197 X 1CT1 cm^K'1
Vi
7.847 X 1014 Hz
Ei
33,096 cm'1
gi
9
Ax
6.68 X 107 s'1
v2
8.059 X 1014 Hz
e2
26, 875 cm'1
<32
11
a2
1.62 X 107 s'1
I2/I1
14.1 (measured)

176
then calculated to be 4.2 (±0.1) m/s.
Absolute Analysis
Once again, in order to consider graphite furnace-flame-
LEI for absolute or standardless analysis, all of the
efficiencies described in Chapter 3 must be measured. Here
the values for these efficiencies are reported.
Vaporization Efficiency
After the vaporization temperature was optimized, the
vaporization efficiency was checked by attempting to observe
the LEI signal associated with the cleaning step of the
furnace temperature program. However, no signal was observed
from the cleaning step even after increasing the preamplifier
gain ten times. Therefore, the vaporization efficiency was
assumed to be 1.000 (±0.001).
Transport Efficiency
As indicated in the previous chapter, three different
cotton samples were analyzed by Wendy Clevenger using flame-AA
to determine their Mg content for potential use in the
transport efficiency determination. All three cotton samples
were obtained from Fisher Scientific (Acton, MA) . The results
of the analysis are summarized in Table 5. As indicated

177
Table 5. Mg concentration in different cotton
samples
Sample
Concentration (ppb)
ACCO
12.5
PADCO
2.47
First Aid
21.8

178
before, the PADCO cotton (non-surgical bleached cotton, ACCO,
Valley Park, MO) had the lowest Mg content and was chosen for
use in the transport efficiency determination.
The results of the transport study indicated that the
transport efficiency was 0.17 (±0.05). It was determined that
about 8% of the analyte was being lost due to adhesion to the
tubing walls and about 24% due to diffusion through the
graphite walls of the furnace. However, the diffusion losses
may not be accurate because the dosing hole was not plugged
well as indicated in the previous chapter. The rest of the
analyte loss during transport was not accounted for. It is
suspected that the unaccounted analyte is escaping through the
2 mm gap between the graphite tube and the tantalum tube
inside the ETV.
Probing Efficiency
Once again, the probing efficiency of the laser beams, ep,
is the product of the spatial probing efficiency, e,, and the
temporal efficiency, st. The spatial probing efficiency was
estimated, from the fluorescence profile of the flame at a
distance of 10 mm above the burner head, to be 0.81 (±0.05).
This could be improved by expanding the laser beam for
However, expanding the beam may result in a loss of saturation
for the first step from the decreased power density of the

179
beam. The temporal probing efficiency can be calculated from
the diameter of the laser beam, the frequency of the pulsed
laser, and the velocity of the flame gases. The diameter of
the laser beam was 4 mm, the laser frequency used was 30 Hz,
and the velocity of the flame gases was measured to be 420
cm/s. The temporal efficiency was calculated to be 0.029
(±0.007) .
To increase the temporal probing efficiency of the
lasers, an increase in the repetition rate of the lasers was
proposed. Figure 35 shows the intensity of the LEI signal
with increasing repetition rate. It was expected that the
signal would increase with increasing repetition rate until a
plateau was reached when the temporal probing efficiency
reached unity. However, the results showed an increase in the
LEI signal until 200 Hz, where the signal dropped very
rapidly. To understand why this happened, the dye laser
conversion efficiencies were monitored with increasing
repetition rate as shown in Figure 36 (XJ and Figure 37 (X2) .
From Figure 36, it can be seen that the conversion efficiency
for ^ remained relatively stable with only a small reduction
in efficiency by 250 Hz. However, from Figure 37, it can be
seen that the conversion efficiency for \2 drops off very
quickly above 100 Hz. Unfortunately, as a result, repetition
rates of only up to 100 Hz can be used for Mg LEI measurements

Figure 35. Mg LEI signal with increasing laser repetition rate

500
400
300
200
100
0
T
"I 1 1 1 1
50 100 150
Laser Repetition Rate (Hz)
200
181

Figure 36. Dye laser conversion efficiency with increasing laser repetition rate for

Conversion Efficiency (%)
0.030
i 1 1 1 1 1 r
0.025-
0.020-
0.015-
0.010-
0.005-
0.000-
100 150 200
Laser Repetition Rate (Hz)
o
"T~
50
I
250
183

Figure 37. Dye laser conversion efficiency with increasing laser repetition rate for
X2

50 100 150 200 250
Laser Repetition Rate (Hz)
Conversion Efficiency (%)
581

186
with the present laser system. From these an overall laser
probing efficiency for the laser beams, eP, was determined to
be 0.023 (±0.006) .
Detection Efficiency
The detection efficiency, sd, is the product of the
ionization yield, Y±, and the efficiency of charge collection,
eD. From the fluorescence dip experiment, when the second step
was added, the result was a >99.9% dip in the fluorescence
signal. This indicated a high degreee of ionization within
the flame, as the only other source of fluorescence dip,
collisional redistribution to other excited levels,
contributes <2-3%. To confirm the absence of a fluorescence
signal, the gain on the boxcar was increased by a factor of
ten and the measurements repeated. To confirm the absence of
collisional quenching as a source of decreased fluorescence,
measurements were made at several incident laser powers,
confirming that near saturation was always achieved. From
these results, the ionization efficiency of Mg in the flame
was reported to be 0.98 (±0.03) .
The actual potential in the flame at the level of the
laser beams was determined by measuring the potential using an
iridium wire inserted directly into the flame. The actual
potential in the flame at the height of the laser beams, AV,

187
was found to be -540 V. The applied potential, V, was -800 V.
Therefore, the efficiency of charge collection, ed, was
calculated to be 0.69 (±0.03). From these the overall
detection efficiency, Ed, was calculated to be 0.68 (±0.03).
Atomization Efficiency
The atomization efficiency for Mg was measured as
described in the previous chapter. The values for the
parameters in Equation 45 were first determined. The
statistical weight of the ground state, g0, is equal to 1.
The electronic partition function, Z(T), was calculated from
the equation [158] :
Z(T) = 0.99101 + 0.013474 (T/103) (46)
-6.4659 X 10-3(T/103) 2 +9.7446 X 10-4 (T/103)3
where T is the temperature in K which was measured as ~2300 K.
The value for Z(T) was then calculated to be 0.99963. The
absolute oscillator strength, f, was estimated to be 1.81
[159]. The absorption path length, ! , was estimated to be
0.6 cm from the fluorescence profile of the flame. The
effective line width of absorption, Aveff, was calculated by
the equation [5]:
v
eff
f s„(v)
J SV(V0)
1
Sv(v0)
dv =
(47)

188
where Sv(v) is the spectral distribution and Sv(v0) is the
value of Sv(v) at the line center. The spectral distribution
(assuming a Voigt profile) at the line center is given by
[160]:
S.W =
f2(ln2)V2
kAvD(it)V2
(48)
where AvD is the doppler broadening half-width and 8(a,0) is
the Voigt integral (approximation). The doppler broadening
half-width is given by [161]:
Avd
f 2(ln2)kT 1
Lm/ (1000N,)I
Vo
c
(49)
where k is the Boltzmann constant (JK'1) , T is the temperature
(K), M is the atomic mass (g/mol) , NA is Avagadro' s number
(mol'1) , v0 is the line center frequency (Hz) , and c is the
speed of light (m/s). The Voigt integral may be approximated
by [161] :
5 (a, 0) = (1 + 1.2a)*1 for 0 8(a,0) = (0.56/a) for a>2
where a is the damping constant for Mg which is -0.60 for a
temperature of 2333 K [161].
From the above equations, it was calculated that the
total number density, nT, is related to the absorbance at line

189
center, A(v0) by:
nT = 1.08095 X 1012 A(v0) (51)
The total number of atoms passing through the flame, N, is
related to nT by:
N = F • /nTdt (52)
where F is the total gas flow (cmVs) . The total number of
atoms passing through the flame was calculated to be 7.77 X
10n atoms of Mg by estimating the total gas flow as the flow
of the argon which was 5.95 cmVs. The total number of Mg
atoms (as free atoms and molecules) reaching the flame was
calculated to be 2.1 X 1013 using the transport efficiency
calculated earlier. Therefore, the atomization efficiency, sa,
was estimated experimentally as 0.04 (±0.02).
An overall system efficiency was calculated as the
product of each of the individual efficiencies and found to be
1.0 X 10'5 (+0.7 X 10'5) . If we multiply the overall efficiency
by the Faraday constant (96,485 C/mol), we should obtain the
sensitivity of the method (slope of the calibration curve).
From the calculation, a sensitivity of 9 (±7) C/mol was
obtained. The slope of the calibration curve from the
experimental results for Mg was 2.5 (±0.2) C/mol which is
within experimental error of the calculated value. However,
the error in the calculated sensitivity was large. This was

190
due to the large uncertainties in the values used to calculate
the atomization efficiency/ sa, for Mg.
Although all of the system efficiencies were determined
experimentally, they do not appear to be useful for absolute
analysis. The transport, probing, detection, and atomization
efficiencies were all less than unity. Ideally, these should
all be equal to unity for absolute analysis. Since the
efficiencies are not unity, this could indicate that the
efficiencies vary from sample to sample and run to run.
Also, it appears that the matrix has a pronounced effect
on the transport efficiency as was evident from the need for
the addition of a carrier at low Mg concentrations. Since a
method must be free from matrix interferences to be considered
absolute, even if the system efficiencies were improved to
unity, the method would still not be considered absolute
because of this matrix dependence of the signal.
LEI of Lead
Excitation Scheme for Lead
Four different two-step excitation schemes were examined
for the analysis of lead by LEIS. All four schemes employed
the transition 6p2 3P0 - 7s 3P1° (283.3 nm) as the first step.
The four different second step transitions examined are shown

191
in Table 6 with the enhancement factor (two-step LEI signal
/one-step LEI signal) for each. The transition 7s 3P1° - 9p 3Pj
(509.0 nm) was chosen as the second step since the greatest
enhancement factor was obtained using this transition.
Carrier
The optimized system parameters were found to be the same
for lead as they were for Mg. Once all of the optimizations
were performed, aqueous solutions of different concentrations
of lead were introduced to determine the linearity of the
system response. It was observed that at lower concentrations
of lead (<100 (ig/dL) , the linearity of the analytical curve
was very poor. Therefore, the feasibility of using a matrix
modifier to act as a carrier for the lower concentration
samples was evaluated. High purity NaCl was chosen as a
possible carrier. A volume of 10 |iL sodium chloride solution
was injected with each sample. No blank signal was observed
for the NaCl. However, an enhancement of the lead ionization
signal was observed as well as excellent linearity for aqueous
lead concentrations s 10 |ig/dL, which can be seen in Figure
38. The Log-Log plot for the aqueous lead standards with NaCl
(Figure 39) yielded a slope of 0.98 (±0.03) and a standard
error of the estimates of 0.018. A slight curvature of the
lead signal towards the concentration axis was observed for

192
Table 6. Enhancement of LEI signal for different two-step
excitation schemes for lead
(run)
Transition
Enhancement Factor
498.1
7s 3P1° - 9p 3D2
111
500.5
7s 3Pa° - 9p 3Pi
37
509.0
7s 3P^° - 9p 3P^
1522
600.2
7s 3P1° - 8p 3D2
194

Figure 38. Analytical curve for aqueous lead standards with and without NaCl addition

Concentration of Lead (^g/dL)
o
~r
10
194

195
concentrations >10 ng/dL. It is believed that this signal
suppression may be due to chloride interference from the NaCl
[162]. Signal suppression due to space charge effects from
overloading of the miniature-flame may also contribute. The
pronounced nonlinearity of the aqueous lead solutions without
NaCl can also be seen from Figure 39.
Lower concentrations of blood lead standards were also
introduced to check the linearity of the system response. It
was observed that at all of the blood lead concentrations, the
analytical curve displayed good linearity as shown in Figure
40. A linear regression of the Log-Log plot for the blood
lead standards (Figure 39) yielded a slope of 1.00 (±0.03) and
a standard error of the estimates of 0.030. Therefore, a
carrier was not needed and NaCl was not used with the blood
samples and standards. It is believed, in this case, that the
blood matrix remaining after drying and ashing is vaporized
with the lead and acts as a carrier. There was no observable
ionization signal from the blood when one or more of the laser
beams was blocked. Therefore, there was no observable signal
resulting from the ionization of the blood matrix by the laser
beams.

Figure 39. Log-Log plot of analytical curves for aqueous lead and blood lead

197

Figure 40. Analytical curve for diluted blood lead standards

Peak Area (pC)
199

200
Calibration Behavior
The Log-Log plot of the analytical curve for aqueous lead
standards containing 100 |xg Na/dL is shown in Figure 39. The
slope of the analytical curve was found to be 0.522 ± 0.002
C/ (|xg/dL) with intercept 0 ± 1 X 10'9 C for 5 data points and
standard error of the estimates of 0.015. A linear dynamic
range of ~104 was found with the upper concentration limited
by overloading of the transimpedance amplifier. The lower
limiting noise was a result of extraneous rf noise and was
measured to be 9.0 X 10'17 C. A detection limit (3a) for
aqueous lead was calculated to be 5.2 X 10~4 |xg/dL (52 fg
absolute).
The Log-Log plot of the analytical curve for blood lead
is also shown in Figure 39. The slope of the analytical curve
was found to be 0.641 ± 0.004 C/(ng/dL) with intercept 0 ± 8
X 10'10 C for 5 data points and standard error of the estimates
of 0.011. A linear dynamic range of about 104 was found with
the upper concentration again limited by overloading of the
transimpedance amplifier. For diluted blood, a detection
limit (3a) of 4.2 X 10'4 (xg/dL (42 fg absolute) was calculated.
This corresponds to a detection limit of 8.9 X 10~3 |xg/dL (890
fg absolute) for lead in whole blood. This is well below the

201
CDC's 10 pg/dL level of concern, and would also be useful for
substantially lower blood lead concentrations. The higher
sensitivity of the blood standards compared to the aqueous
standards with NaCl added is most likely a result of the
better carrier characteristics of the blood matrix compared to
the NaCl.
One human blood sample of unknown lead concentration was
also analyzed by graphite furnace-flame-LEIS. A value of 8.7
pg/dL (± 0.3) was calculated from the analytical curve. The
human blood sample was also analyzed by Besteman et al. [163]
using capacitively coupled microwave plasma atomic emission
spectrometry (CCMP-AES) . A value of 8.7 |Xg/dL was also
obtained by that method. Therefore, the results obtained by
the two independent methods (one using ionization detection
and the other optical emission detection) were in good
agreement.

CHAPTER 7
CONCLUSIONS
Absolute Analysis
A new burner was designed and used for ETV-FL-LEI of Mg.
A two orders of magnitude reduction in the flame noise was
obtained with this new miniature burner design. The
vaporization, transport, probing, detection, and atomization
efficiencies for this new system were measured experimentally
for the possibility of absolute analysis. All of the
efficiencies, except the vaporization efficiency, were found
to be less than unity. For absolute analysis, ideally all of
these efficiencies should equal unity.
It was also found that the matrix has a pronounced effect
on the transport efficiency as was evident from the need for
a carrier (methanol) at low concentrations of Mg. Since a
method must be free from matrix interferences to be considered
absolute, even if the system efficiencies were improved to
unity, the method would still not be considered absolute
because of this matrix dependence of the transport efficiency.
Therefore, we must conclude that the combination of
202

203
electrothermal vaporizer with flame-laser enhanced ionization
should not be considered an absolute method.
Pb in Blood
The analysis of lead in a complex sample matrix, blood,
was attempted using ETV-flame-LEI. The results obtained show
that ETV-flame-LEIS can be readily used for the determination
of Pb concentrations in whole blood with minimal sample
preparation (21:1 dilution with ultrapure water). ETV
temperature programming allowed fro sufficient matrix removal
to free this method from ionization interferences usually
associated with LEIS of real samples. It was also found that
the blood matrix that did vaporize with the analyte acted as
a very good carrier. Therefore, the addition of a carrier at
low Pb concentrations was not necessary. A detection limit of
8.9 X 10'3 pg/dL (890 fg absolute) for lead in whole blood was
obtained. This compares favorably with other methods
currently used to determine blood lead concentrations [12,13].
Future Work
Although ETV-flame-LEI was found to not be suitable for
absolute analysis, there still exists the possibility for
standardless analysis. If this were to be considered, first

204
the transport, probing, and detection efficiencies must all be
increased to unity. In order to increase the transport
efficiency, the ETV-to-burner interface must be redesigned.
Possibly, the ETV and burner could be designed as a single
unit where there is no tubing between the burner and ETV. To
increase the probing efficiency to unity, the laser beam for
must be expanded to encompass the flame and a higher
repetition rate laser (a 1 kHz) must be used. To increase the
detection efficiency to unity, the laser beams must be
positioned as close as possible to the high voltage electrode.
The atomization efficiency may be less than unity as long as
it remains constant over time. A different flame gas mixture,
such as N20/air, may be examined in order to improve the
atomization efficiency for Mg. After these changes are made
and if the efficiencies were improved, the stability of the
calibration over time would still need to be established for
the possibility for standardless analysis.
Perhaps the most promising area of future work lies in
the application of ETV-flame-LEIS to trace element analysis of
samples with complex matrices. The analysis of other trace
elements in blood could be attempted. Also, the analysis of
trace elements in other complex matrices, such as biological
tissues, environmental samples, and seawater, could be
examined.

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Winefordner, J. Anal. At. Spectrom., 11, 479 (1996).

BIOGRAPHICAL SKETCH
Ken Riter was born on May 14, 1970, in Sioux Falls, SD.
He graduated from Washington Senior High School in Sioux
Falls, SD, in May, 1988. In May, 1992, he graduated from
Saint Olaf College in Northfield, MN, with a Bachelor of Arts
degree in chemistry. In August, 1992, he entered the Graduate
School at the University of Florida in Gainesville, FL.
216

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.
James D. Winefórdner, Chair
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 Philo@bphy.
Robert TSei
Associate ^rof§ssor 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.
William Weltner, Jr.
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.
Eric R. Allen
Professor of Environmental
Engineering Sciences
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,
a dissertation for the degree of Doctor of Philosophy.
Michael D. Annable
Assistant Professor of
Environmental Engineering
Sciences

LD
1780
1996
.RR*
UNIVERSITY OF FLORIDA
3 1262 08556 5934



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7DEOH &RQWLQXHG 0J ( $$ 0Q ( $$ 0Q < $$ 0Q ) $$ 0Q ( $$ 0Q ) $$ 0Q ( $$ 0Q ( $$ 0Q ( $$ 0Q 1 +$ 0Q 1 +$ 0R ) $1 0R ) $1 0R ) $1 0R ) $1 0R ) $1 0R ) $1 0R ) $1 0R ) $1 0R ) $1 0R ) $1 1D ( $$ 1D ( $$ 1D ( $$ 1D ) $$ 1D < 3%$ 1D < 3%$ 1D ( $$

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7DEOH &RQWLQXHG 1D < 3%$ 1D ( $$ 1D 1 +$ 1D < $$ 1D < 3%$ 1D 1 $$ 1D < 3%$ 1D .U FZ $$ 1D 1 +$ 1D ( $/ +$ 1D ( $$ 1D ) $$ 1D ) $$ 1D < +$ 1D < $$ 1D 1 $$ 1D 1 $$ 1D ( $$ URG 1D ) $$ 1L ( $$ 1L ( $$ 1L ( $$ 1L ) $$ 1L ) $$ 1L < $$ 1L < $$ 1L ( $$ 1L < $$

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7DEOH &RQWLQXHG 3E ( $$ 3E ) $$ 3E ( $$ 3E < $$ 3E ) $$ 3E < $$ 3E ( $$ 3E < $$ 3E ( $$ 3E ( $$ 3E ( $$ 5E .U FZ +$ 5E 1 3%$ 5E 1 $$ 5E +$ 5E 'LRGH +$ 6E ( $$ 6E ( $$ 6L ) $1 6Q ( $$ 6Q ( $$ 6Q ) $1 6Q < +$ 6Q < +$ 6Q ( $$ 6Q ) $1 6Q ) $$ 6Q ) $1

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7DEOH &RQWLQXHG 6Q ) $$ 6Q ( $$ 6Q ( $$ 6Q ) $1 6Q ) $1 6Q ) $1 6Q ) $1 6U ( $$ 6U ( $$ 6U $$ 6U < +$ 6U < $$ 6U ( $$ 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7L ) $1 7, ( $$ 7, ( $$ 7, ( $$ 7, ( $$

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7DEOH &RQWLQXHG 7 ( $$ 7 ) $$ 7 ( $/ +$ 7 < $$ 9 ) $1 9 ) $1 9 ) $1 9 ) $1 9 ) $1 9 ) $1 Z ( $$
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