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Fundamental studies of thermospray ionization processes

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Title:
Fundamental studies of thermospray ionization processes
Added title page title:
Thermospray ionization processes
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Lee, Mike S., 1960-
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English
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xiii, 236 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Acetates ( jstor )
Anions ( jstor )
Cations ( jstor )
Evaporation ( jstor )
Flow velocity ( jstor )
Ionization ( jstor )
Ions ( jstor )
Mass spectra ( jstor )
Plumes ( jstor )
Quaternary ammonium compounds ( jstor )
Ionization ( lcsh )
Liquid chromatography ( lcsh )
Mass spectrometry ( lcsh )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Includes bibliographical references (leaves 232-235).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Mike S. Lee.

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University of Florida
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University of Florida
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Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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AFG1047 ( NOTIS )
19045275 ( OCLC )

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FUNDAMENTAL STUDIES OF THERMOSPRAY IONIZATION PROCESSES


By

MIKE S. LEE























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


1987











ACKNOWLEDGEMENTS


I wish to express my sincere gratitude to Dr. Richard A. Yost for

his guidance, direction, and friendship during the completion of this

work. I wish to acknowledge the members of my committee, Dr. John

Eyler, Dr. James Winefordner, Dr. John Dorsey, and Dr. Joseph Delfino

for their various contributions to my thesis work and education at the

University of Florida.

I acknowledge the U.S. Army Chemical Research, Development, and

Engineering Center for their support of this work.

Many thanks go to the guys in the machine shop, Chester, Vern, and

Daly, for their help in the construction of the thermospray vacuum

chamber.

I thank special friends Mike Gehron, Ken Matuszak, and Kevin

McKenna who provided and shared the laughter, joy, and nervous energies

of a gradual student lifetime.

Mostly, I thank my number one family for their endless support and

enthusiasm in everything I do.
































A guitar, blanket, burrito, and you.













TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS................................................... ii

LIST OF TABLES.................................................... vi

LIST OF FIGURES ................................................... vii

ABSTRACT .......................................................... xii

CHAPTERS

I INTRODUCTION............................................. 1

Developments in Ionization Techniques for
Nonvolatile Molecules.................................. 2
Thermospray LC/MS Background............................. 12
Present Understanding of the Thermospray
Ionization Mechanism................................... 15
Overview of Thesis Organization.......................... 21

II EXPERIMENTAL............................................. 23

Vestec Thermospray LC/MS Interface....................... 23
HPLC Pumps ............................................... 27
Mass Spectrometer ........................................ 28
Vacuum Chamber Measurements.............................. 28

III PRELIMINARY CHARACTERIZATION OF THE VESTEC
THERMOSPRAY LC/MS INTERFACE............................ 30

Vaporization Studies..................................... 30
Characterization of thermospray
plume types....................................... 31
Variable flow with constant heat.................... 35
Constant flow with controlled heat.................. 38
Thermospray Mass Spectra................................. 42
Ammonium acetate .................................... 43
Ribavirin........................................... 48
Reproducibility of thermospray spectra
of ribavirin ...................................... 53
Purine and xanthine-type compounds.................. 56

IV THERMOSPRAY PLUME MEASUREMENTS IN VACUUM.................. 71

Plume Pressure Measurements .............................. 71
Plume Temperature Measurements ........................... 71
Plume Current Measurements............................... 82















CHAPTER PAGE

V INSTRUMENTAL PARAMETERS................................. 93

Tip Temperature......................................... 101
Source Temperature ...................................... 114
Flow Rate ............................................... 126
Source Pressure......................................... 137
Probe Position .......................................... 148
Summary of the Effects of Instrumental Parameters....... 158

VI SOLUTION CHARACTERISTICS ................................ 162

Mobile Phase and Sample Solvent......................... 162
Sample Concentration .................................... 174
Buffer Concentration .................................... 184
pH Study ................................................ 202
Thermospray Ionization Behavior in a Mixture............ 220
Summary of the Effects of Solution Characteristics...... 225

VII CONCLUSIONS AND FUTURE WORK............................. 228



LITERATURE CITED ................................................. 232

BIOGRAPHICAL SKETCH .............................................. 236













LIST OF TABLES


TABLE PAGE

I Summary of the major techniques used for ionizing
nonvolatile molecules..................................... 6

II Summary of the positive and negative ions observed
in the mass spectra of the purine and xanthine-type
compounds................................................. 62

III The effects of tip temperature on source temperature
and HPLC pump pressure .................................... 102

IV The effects of source temperature on source pressure...... 115

V The effects of flow rate on source pressure obtained
at tip temperatures corresponding to nearly dry plumes.... 127

VI Summary of the mobile phase-solvent studies............... 163

VII Ratios of various positive and negative ion intensities
of ribavirin .............................................. 185

VIII Positive and negative ion intensities of pure
ammonium acetate versus 100 ppm ribavirin................. 199

IX Forms of the ionizing groups of ammonium acetate
at various pH's........................................... 206

X Forms of the ionizing groups of histidine
at various pH's........................................... 207

XI Positive and negative RIC values of
0.1 M ammonium acetate .................................... 208

XII Positive and negative RIC values of 100 ppm histidine
in 0.1 M ammonium acetate................................. 214

XIII Summary of the two-component mixture study for
allantoin and 2-hydroxypurine............................. 222

XIV Summary of the two-component mixture study for
allantoin and 2,6-diamino-8-purinol....................... 223

XV Summary of the two-component mixture study for
2-hydroxypurine and 2,6-diamino-8-purinol................. 224

XVI Summary of the three-component mixture study for
allantoin, 2-hydroxypurine, and 2,6-diamino-8-purinol..... 226

vi













LIST OF FIGURES


FIGURE PAGE

1.1 A simplified thermospray vaporization scheme
illustrating the four major processes which occur
during thermospray ionization ............................ 17

1.2 Direct ion evaporation and chemical ionization
mechanisms of thermospray ................................ 19

2.1 A schematic representation of the Vestec
thermospray LC/MS interface .............................. 25

3.1 A plot of tip temperature versus control
temperature for water at 1.0 mL/min...................... 33

3.2 A plot of probe temperature versus distance
along the probe for water at various flow rates.......... 37

3.3 A plot of probe temperature versus distance
along the probe for wet, nearly dry, and dry
water plumes............................................. 41

3.4 Positive and negative ion full scan thermospray
mass spectra of ammonium acetate......................... 45

3.5 MS/MS daughter spectra obtained for selected
positive and negative ions of ammonium acetate........... 47

3.6 Positive ion full scan mass spectrum of 1 jag ribavirin... 50

3.7 Negative ion full scan mass spectrum of 1 jAg ribavirin... 52

3.8 Positive ion full scan thermospray mass spectra
of ribavirin acquired several weeks apart................ 55

3.9 Negative ion full scan thermospray mass spectra
of ribavirin acquired several weeks apart................ 58

3.10 Model purine and xanthine-type compounds used for
determining the effects of various substituents
on thermospray ionization................................ 60

3.11 Positive ion intensity summation profiles of the
purine and xanthine compounds, ammonium acetate,
and RIC .................................................. 66








3.12 Negative ion intensity summation profiles of the
purine and xanthine compounds, ammonium acetate,
and RIC .................................................. 68

4.1 Plume temperatures measured along wet, nearly dry,
and dry plumes at 2.0 mL/min............................. 74

4.2 Plume temperatures measured along a nearly dry plume
at 2.0 mL/min for three manifold temperatures............ 77

4.3 Plume temperatures measured along nearly plumes
produced at various flow rates........................... 79

4.4 Plume temperatures measured along a nearly dry plume
produced at 2.0 mL/min at two manifold pressures......... 81

4.5 Plume currents measured along wet, nearly dry, and dry
plumes at 2.0 mL/min..................................... 84

4.6 Plume currents measured along a nearly dry plume at
2.0 mL/min for three manifold temperatures............... 87

4.7 Plume currents measured along nearly dry plumes
produced at various flow rates........................... 89

4.8 Plume currents measured along a nearly dry plume
produced at 2.0 mL/min at two different manifold
pressures ................................................ 91

5.1 A model showing the effects of temperature on
hypothetical thermospray plumes .......................... 95

5.2 A model showing the effects of pressure on
hypothetical thermospray plumes.......................... 98

5.3 A model showing the effects of probe position on
hypothetical thermospray plumes.......................... 100

5.4 A plot showing the effects of tip temperature on
the ion intensities of pure ammonium acetate............. 105

5.5 A plot showing the effects of tip temperature on
the ion intensities of ammonium acetate with 100 ppm
ribavirin present ........................................ 107

5.6 A plot showing the effects of tip temperature on
the positive ion intensities of 100 ppm ribavirin........ 109

5.7 A plot showing the effects of tip temperature on
the negative ion intensities of 100 ppm ribavirin........ ill

5.8 A plot showing the effects of source temperature
on the ion intensities of ammonium acetate............... 118


viii








5.9 A plot showing the effects of source temperature
on the ion intensities of ammonium acetate with
100 ppm ribavirin present ................................ 120

5.10 A plot showing the effects of source temperature
on the positive ion intensities of 100 ppm ribavirin..... 122

5.11 A plot showing the effects of source temperature
on the negative ion intensities of 100 ppm ribavirin..... 124

5.12 A plot showing the effects of flow rate on the
ion intensities of pure ammonium acetate................. 130

5.13 A plot showing the effects of flow rate on the
ion intensities of ammonium acetate with 100 ppm
ribavirin present ........................................ 132

5.14 A plot showing the effects of flow rate on the
positive ion intensities of 100 ppm ribavirin............ 134

5.15 A plot showing the effects of flow rate on the
negative ion intensities of 100 ppm ribavirin............ 136

5.16 A plot showing the effects of source pressure
on the ion intensities of pure ammonium acetate.......... 140

5.17 A plot showing the effects of source temperature
on the ion intensities of ammonium acetate with
100 ppm ribavirin present ................................ 142

5.18 A plot showing the effects of source pressure
on the positive ion intensities of 100 ppm ribavirin..... 144

5.19 A plot showing the effects of source pressure
on the negative ion intensities of 100 ppm ribavirin..... 147

5.20 A plot showing the effects of probe position on the
ion intensities of pure ammonium acetate................. 151

5.21 A plot showing the effects of probe position on the
ion intensities of ammonium acetate with 100 ppm
ribavirin present ........................................ 153

5.22 A plot showing the effects of probe position on the
positive ion intensities of 100 ppm ribavirin............ 155

5.23 A plot showing the effects of probe position on the
negative ion intensities of 100 ppm ribavirin............ 157

6.1 Positive ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the sample
solvent and mobile phases of methanol, water, and
ammonium acetate......................................... 166









6.2 Negative ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the sample
sample solvent and mobile phases of methanol, water, and
ammonium acetate......................................... 168

6.3 Positive ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the mobile phase
and sample solvents of methanol, water, and
ammonium acetate......................................... 171

6.4 Negative ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the mobile phase
and sample solvents of methanol, water, and
ammonium acetate......................................... 173

6.5 Positive ion thermospray mass spectra of various
amounts of ribavirin..................................... 176

6.6 Negative ion thermospray mass spectra of various
amounts of ribavirin..................................... 178

6.7 Calibration plots for the major positive ions of
ribavirin and the RIC .................................... 181

6.8 Calibration plots for the major negative ions of
ribavirin and the RIC .................................... 183

6.9 Positive ion intensities of the major ions of
ammonium acetate at various concentrations............... 187

6.10 Positive ion intensities of the major ions of
ammonium acetate obtained from 100 ppm ribavirin
standards prepared in various concentrations of
ammonium acetate......................................... 190

6.11 Positive ion intensities of 100 ppm ribavirin
prepared in various concentrations of ammonium acetate... 192

6.12 Negative ion intensities of the major ions of
ammonium acetate at various concentrations............... 194

6.13 Negative ion intensities of the major ions of
ammonium acetate obtained from 100 ppm ribavirin
standards prepared in various concentrations of
ammonium acetate......................................... 196

6.14 Negative ion intensities of 100 ppm ribavirin
prepared in various concentrations of ammonium acetate... 198

6.15 The structures of ammonium acetate and histidine
at pH 6 .0 ................................................ 204









6.16 Positive ion thermospray mass spectra of 0.1 M
ammonium acetate at various pH values.................... 210

6.17 Negative ion thermospray mass spectra of 0.1 M
ammonium acetate at various pH values.................... 212

6.18 Positive ion thermospray mass spectra of 100 ppm
histidine in 0.1 M ammonium acetate at various
pH values ................................................ 216

6.19 Negative ion thermospray mass spectra of 100 ppm
histidine in 0.1 M ammonium acetate at various
pH values ................................................ 218













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




FUNDAMENTAL STUDIES OF THERMOSPRAY IONIZATION PROCESSES


by

Mike S. Lee

December, 1987


Chairman: Richard A. Yost
Major Department: Chemistry

The effects of various parameters on thermospray ionization were

determined on a thermospray liquid chromatography/mass spectrometry

(LC/MS) interface. Particular changes in these parameters affect

specific processes of thermospray [vaporization, droplet evaporation,

droplet charging, and direct ion evaporation/chemical ionization (CI)]

and result in a change in the overall sensitivity and the relative

abundances in the mass spectra. The instrumental parameters such as

tip temperature, source temperature, flow rate, and source pressure

most directly affect the vaporization, droplet evaporation, and direct

ion evaporation/CI processes, while solution characteristics such as

mobile phase, sample solvent, sample concentration, buffer

concentration, pH, and mixture components most directly affect the

droplet charging and direct ion evaporation/CI processes. Each

parameter was varied systematically and the effects on a model sample

compound, ribavirin, was determined.


xii








Conditions of vaporization and droplet evaporation which produce

droplet-rich plumes favor the formation of fragment ions while

conditions which produce vapor-rich plumes favor the production of

pseudo-molecular ions. The role of a volatile electrolyte in

thermospray ionization was demonstrated with mobile phase-sample

solvent combinations of methanol, water, and ammonium acetate. The

distinction between the direct ion evaporation and CI mechanisms was

difficult to make. Although inconclusive, these results do indicate

that CI most likely plays a major role in sample molecule ionization by

thermospray. Recommendations for future work in thermospray

fundamentals are made and focus on methods which will help to

distinguish between the direct ion evaporation and CI mechanisms.


xiii














CHAPTER I
INTRODUCTION

This thesis describes fundamental studies of thermospray

ionization, a relatively new ionization technique used in liquid

chromatography/mass spectrometry (LC/MS). Although the thermospray

technique has experienced rapid growth and has proven to be a

successful method for the analysis of polar and labile compounds, the

actual mechanism of thermospray ionization is still unclear. The

objectives of our fundamental studies were to gain further insight into

the thermospray mechanism and to improve the reproducibility of the

thermospray ionization technique. This work was undertaken in hopes

that a better understanding of the various parameters associated with

the processes involved with thermospray would aid in the investigation

of much more complex and detailed conditions of thermospray ionization.

A better understanding of these processes should ultimately lead to a

more routine applicability of the thermospray LC/MS interface, improved

day-to-day reproducibility, and perhaps some potential instrumental

improvements.

In this introductory chapter, developments in ionization techniques

used for nonvolatile molecules are discussed. Background to the

thermospray technique along with the present understanding of the

thermospray ionization mechanism is presented, followed by an overview

of the thesis organization.











Developments in Ionization Techniques For Nonvolatile Molecules

Through the years mass spectrometry has undergone dramatic changes.

Exploration into new and novel ionization techniques has contributed

significantly to this change, resulting in the application of mass

spectrometry to an increasingly wide range of organic compounds.

Described below are the various ionization techniques used in mass

spectrometry and their capabilities and limitations.

In the late 1960s combined gas chromatography/mass spectrometry

(GC/MS) instruments with computer data systems became commercially

available so that routine quantitative analysis was possible. On these

systems electron ionization (EI) was used in essentially all analytical

applications. A "standard" electron energy of 70 eV was used for both

qualitative and quantitative analyses resulting in the production of an

intense, stable ion beam, and a highly reproducible fragmentation

pattern (eqn. 1.1).



M + e" M+ + 2e- (1.1)



Although the El mode remains the accepted preliminary ionization

mode for identification purposes, two factors exist which restrict El

to a limited range of organic compounds. First, the El mode requires

that the sample be vaporized prior to ionization which may lead to

decomposition of thermally unstable compounds. Therefore, useful

spectra of very polar and thermally labile compounds cannot be obtained

with EI. Second, the use of electron energies of 70 eV usually results

in extensive fragmentation and sometimes undetectable molecular ions.








In many cases chemical ionization (CI) techniques are capable of

providing complementary information to El spectra. CI achieves

ionization without transferring excessive energy to the sample

molecules resulting in the formation of adduct ions (often a protonated

molecule) which contain the intact molecular species of the analyte.

CI differs from El in that the sample molecules are ionized by

interaction with ions of a reagent gas rather than with electrons. The

site of protonation is most likely to occur on the heteroatom of

greatest proton affinity. CI is the subject of a book (1) and several

reviews in the literature (2,3) where processes of ion formation are

discussed in detail. Below is a brief description of ion formation by

CI.

Production of reagent ions is initiated by El of the reagent gas.

Even though the analyte and the reagent gas are present in the

ionization source, the reagent gas is present in great excess

(typically, the partial pressure of an analyte will be 10-5 torr,

whereas the reagent gas will be 1 torr); therefore, only reagent ions

result from El. The principle electron/molecule and ion/molecule

reactions that occur in CI for the case involving methane as reagent

gas are illustrated in eqns. 1.2-1.6. The methane molecules are

converted to molecular ions by EI, some of which decompose to fragment

ions of methane (CH3+, CH2+, CH+, etc.). The ions of methane react

with a methane molecule to produce the CH5+, C2H5+, and C3H5+ ions,

which do not react further with methane.



CH4 + e- & CH4+, CH3+, CH2+, (1.2)

CH+, H2+, H+ + 2e"








CH4 + + CH4 CH5+ + *CH3 (1.3)

CH3+ + CH4 C2H5+ + H2 (1.4)

SC2H4+ + H2
CH2+ + CH4 C2H3 + H2 + (1.5)
IL- C2H3+ + H2 + *H

C2H3 + CH4 C3H5+ + H2 (1.6)

There are many more reactions which occur between the methane ions

and the methane molecules; however, those shown above are the most

important because they lead to reagent ions which are unreactive with

molecular methane resulting in their accumulation in the ionization

source. Most other organic molecules have a sufficiently high proton

affinity to abstract a proton from these reagent ions resulting in a

reaction as shown in eqn. 1.7. An analyte molecule, M, will interact

with a reagent ion to produce a protonated molecule of the analyte,

(M+H)+. Similar reactions with C2H5+ and C3H5+ may occur resulting in

(M+H)+ as well as (M+C2H5)+ and (M+C3H5)+ adduct ions.



M + CH5+ (M+H)+ + CH4 (1.7)



Typical CI spectra contain abundant protonated molecular ion

species, adduct ion species (associated with the reagent gas ions) and

less fragmentation than observed in El spectra. Thus, molecular weight

information is usually more easily and reliably obtained. However, as

in the case with El, sample vaporization occurs prior to ionization and

thus, the sample must be thermally stable.

Recent research aimed at overcoming the thermal stability or

volatility barrier has resulted in a wide variety of ionization

techniques in which the direct production of pseudo-molecular ions from








nonvolatile molecules have been reported. These techniques produce

relatively stable even-electron species rather than the odd-electron

species produced initially by gas-phase El and can provide a

potentially attractive approach to applying mass spectrometry to large,

nonvolatile, or thermally labile compounds not amenable by conventional

ionization techniques such as El and CI. These techniques differ

primarily in the means employed for disrupting the solid or liquid

surface of the sample. However, a common feature of all these

techniques appears to be the "direct" production of molecular ions [M+,

(M+H)+, (M+Alkali)+] from a condensed phase without the formation of a

neutral gas-phase molecule as an intermediate.

There are several techniques which have been developed for

desorbing molecular ions from a condensed phase. All of the these

techniques have been reviewed in detail (4-6) or have been described

elsewhere in various publications (7-15). These techniques are listed

in Table I according to the method of energy application and the

physical state of the sample prior to energy deposition. Samples may

be either solid or liquid on the surface, depending on the technique

used, and an electrical field may be of primary importance to the ion

production mechanism or may only serve to focus the ions into the mass

spectrometer. Below is a brief description of these ionization

techniques.

Several of these desorption ionization techniques employ electrical

energy via a high electrical field and/or ohmic heating of the sample.

They generally involve ohmic heating of a metallic substrate in contact

with the sample or the production of charged droplets in the presence

or absence of an electrical field. These techniques include field















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desorption, electrohydrodynamic ionization, electrospray, and

thermospray.

With the field desorption (FD) technique, the sample is deposited

from solution onto the surface of a field anode, usually a 10 pm

tungsten wire covered with field-enhancing microneedles. Ionization

and desorption of molecules is achieved by applying a high voltage of

about 10 kV between the wire and a 2-3 mm distant counter-electrode

while resistively heating the emitter wire. The mass spectra produced

by FD exhibit molecular ions such as M+ and (M+H)+, and adduct ions

such as (M+Na)+ and (M+Cl)" depending on the sample solution. Three

distinctly different mechanisms contribute to the production of ions by

the FD technique. The ionization of molecules can occur by an electron

tunneling process, which takes place on the tips of the field enhancing

microneedles and requires a high threshold field strength (>109 V/m).

Ions can also be formed in condensed sample layers and extracted via a

field-induced desolvation mechanism. This mechanism requires lower

threshold field strengths for the production of gaseous ions. The

third mechanism is a thermal mechanism which is principally independent

of external fields. In general this mechanism does not contribute to

the emission of ions except in the cases of metal ions at emitter

temperatures > 800 K. Recent studies by Giessmann and Rollgen (16)

show that field emission of molecular ions depends on the temperature

of the emitter, the field strength, and the thickness and physical

state of the sample surface and that the "best anode temperature"

corresponds to melting of the solid sample on the field emitter.

The electrohydrodynamic (EHD) ionization technique developed

primarily by Evans and co-workers (17-19) is similar to FD in that a









high electrical field (6 kV) is applied to the sample. However, in

this case the sample is present in a liquid meniscus at the end of a

capillary tube. When liquids contained in a hypodermic syringe needle

or similar capillary are introduced into a vacuum, the interaction of

the electrical field and the liquid surface of the capillary causes a

distortion of the tip of the meniscus into a sharp cone. The liquid

surface is distorted further by the EHD process and charged droplets

are produced and accelerated away from the tip. This technique has

been shown to successfully generate pseudo-molecular ions

characteristic of any sample material which can be dissolved, together

with an electrolyte, in a nonvolatile organic solvent which can also

undergo appreciable interaction with cations and anions in solution.

In the electrospray (ES) technique developed by Dole and co-workers

(20,21) solutions are flowed through a hypodermic needle (5-20 pL/min)

biased at ca. 10 kV into a spray chamber at room temperature and

atmospheric pressure. Charged droplets are formed and rapidly

evaporate until the increase in surface charge density and the decrease

in radius of curvature result in electric fields strong enough to

desorb sample ions. The resulting dispersion of ions is passed through

a small orifice into an evacuated region to form a supersonic free jet.

Ions are sampled from the core of the jet and are introduced into the

mass spectrometer using a standard nozzle and skimmer.

The thermospray ionization technique produces ions from liquid

solutions without requiring strong electrical fields or an external

source of ionization. The technique developed by Vestal and co-workers

(22-25) produces ions directly from a flow of vaporized liquid (1-2

mL/min) containing a volatile electrolyte such as ammonium acetate.








When this vaporized liquid is introduced into the ion source of the

mass spectrometer, a supersonic jet of vapor containing a mist of fine

particles and droplets results. Molecular ions desorb or evaporate

directly from the evaporating droplets typically forming (M+H)+ and

adducts of ammonium acetate such as (M+NH4)+ and (M+CH3COO)>. A

detailed description of the basic principles and apparent ionization

mechanism of the thermospray ionization technique will be presented in

the next section.

Another group of ionization techniques employs particle impact on a

surface of the sample or a matrix containing the sample. In these

techniques the source of sample energization is a beam of ions or

neutrals with kinetic energies ranging from a few keV to more than 100

MeV. The compound of interest is desorbed and ionized directly from a

thin solid or liquid layer deposited on a metal substrate. These

techniques include secondary ion mass spectrometry, fast atom

bombardment, and 252Cf fission fragment desorption.

The secondary ion mass spectrometry (SIMS) technique involves the

bombardment of a sample, either solid (26) or dissolved in solvent

(27), with a beam of primary ions and analyzing the secondary ions that

emanate from the sample surface. In most cases an ion gun which

generates Xe+ or Ar+ ions is used to produce the primary bombarding ion

beam. The primary-ion beam is usually oriented perpendicular to the

mass analyzer with the sample mounted on a probe or target which can be

adjusted to provide for an angle of incidence ranging from 0 to 90.

The ionization mechanism of the SIMS technique appears to be identical

to the fast atom bombardment (FAB) technique (28,29).








In the FAB technique, a beam of atoms of Xe or Ar are directed

toward a sample contained in a polar, viscous, low-volatility solvent

(usually glycerol). Essentially the same orientation of atom beam,

sample target, and mass analyzer is used as in the SIMS technique. The

resulting energetic bombarding neutrals collide with the sample surface

producing many particles, both ions and neutrals. The bombardment of

the sample surface causes intense "hot spots" with the energy

deposition dependent on the angle on incidence. Angles of incidence

which approach 90 (grazing angle) result in the majority of the energy

deposition limited to the surface layers of the solution, resulting

primarily in the desorption of molecular ion species (30). Intact

molecular ions (M+H)+ and adduct ions (M+Na)+ which are desorbed are

characteristic of both SIMS and FAB spectra. Angles of incidence which

approach 0* result in a more intense mode of energy deposition which

causes intramolecular vibrations deeper within the matrix (31). This

orientation results in the degradation of the analyte and matrix and

consequently a low abundance of molecular ions. The presence or

absence of charge on the impacting primary particle has little effect

on the desorption process; however, the use of the neutral beam is

somewhat more convenient with mass spectrometers in which the ion

source is at high potential.

The 252Cf fission fragment desorption technique employs a beam of

fission fragments resulting from the radioactive decay of 252Cf which

penetrates the sample coated on a thin (<10 am) nickel foil (32,33).

The fission fragments pass through a 10 nm diameter region of the foil

depositing considerable amounts of energy and inducing electronic

excitation. Atoms and molecules within the foil perceive the swiftly








moving ions passing through the matrix as equivalent to a short burst

of photons (32). The electromagnetic radiation induces electronic

excitation, which quickly dissipates into a complex spectrum of atomic,

molecular, and matrix excitation, resulting in the desorption of intact

molecular ions, (M+H)+, and adduct ions, (M+Na)+, from the sample.

Finally, techniques which involve lasers with power densities

ranging from ca. 103 to 1010 W/cm2 and wavelengths from the UV to the

far-IR have been successfully employed to produce desorption of ions

from solid samples (34-36) or samples contained in a liquid matrix such

as glycerol (37). The laser (essentially a photon gun) is used to

bombard a specific area of the sample with photons in a manner

analogous to the SIMS and FAB techniques. Protonated molecules or

adduct ions with cations such as Na+ or K+ are typically produced, with

the extent of their fragmentation influenced by the power of the laser.

From a review of the recent literature on ionization of nonvolatile

molecules, it is evident that a wide variety of dissimilar techniques

produce mass spectra with similar features, even-electron pseudo-

molecular ions from nonvolatile molecules. The primary processes

involved in the first-step (energy deposition) are apparently quite

dependent on the kind of excitation used; furthermore, some of the

properties of the intermediate must depend on the nature of the primary

processes. It is not yet clear, however, whether the final step

leading to the production of a pseudo-molecular ion is dependent or

independent of the primary processes occurring in the condensed phase

or whether neutral gas-phase molecules play an integral role in pseudo-

molecular ion formation via CI processes.








Thermosprav LC/MS Background

For over a decade, several research groups have worked on the

development of a practical LC/MS interface. Because of the basic

incompatibilities between high performance liquid chromatography (HPLC)

and MS, the successful on-line coupling of these instrumental

techniques has been difficult to achieve (38-41). This is because the

normal operating conditions of a mass spectrometer (high vacuum, high

temperatures, gas-phase operation, and low flow rates) are opposed to

those used in HPLC (high pressures, low temperatures, liquid-phase

operation, and high flow rates).

The moving belt interface (42,43) and direct liquid introduction

(44-47) are two of the principal methods which have been used for LC/MS

interfacing other than thermospray. These two techniques are capable

of generating El (moving belt) and/or CI (either technique) spectra of

volatile and nonpolar type compounds which are also amenable to GC/MS

techniques. The moving belt interface provides one of the more

versatile approaches to coupling LC to MS in that it permits the

operation of the mass spectrometer in both the El and CI modes. In

this method the LC effluent is deposited on a polyimide or Kapton belt

where it is desolvated; it is then transported into the ion source

region of the mass spectrometer where the remaining sample residue is

flash vaporized and ionized. Direct introduction of the effluent from

a LC into the ion source was first described by McLafferty and co-

workers (48). In this technique, a fraction of the eluent (sample and

mobile phase) is passed through a capillary tube at flow rates of 10-60

pL/min. Microdroplets are formed through a pinhole (ca. 4 Am diam.)

and directed into the ions source where the neutral sample molecules








undergo CI processes with the mobile phase serving as the CI reagent

gas.

While it may be desirable and perhaps more convenient in certain

situations to have a LC/MS technique which is capable of providing El

and CI information identical to GC/MS, the moving belt and direct

liquid introduction techniques do not provide for a complementary

method; namely, the ability to provide useful mass spectral information

of chromatographically separated polar and thermally labile compounds

which are not amenable to GC/MS techniques. Although other recent

LC/MS approaches such as atmospheric pressure ionization (49), liquid

ion evaporation (50), electrospray (51,52), and the monodisperse

aerosol generator for introduction of liquid chromatographic effluents

(53) have attempted to provide for this capability, thermospray has by

far been the most successful, and as a result, has been the most widely

used. All of the LC/MS interfacing techniques listed above differ in

their principle of operation; however, all are capable of transport,

desolvation, and ionization of a liquid sample. Several reviews (54-

58) have appeared in the literature which describe in more detail the

principles of operation of these techniques. Thermospray is unique of

the LC/MS approaches in that it is the only technique which does not

require an external form of ionization or external electrical fields.

The thermospray technique, developed by Blakley and Vestal (22-25),

emerged from efforts to develop an LC/MS interface suitable for

efficiently analyzing samples dissolved in aqueous mobile phases at

typical HPLC flow rates (1-2 mL/min). The original approach involved

the production of a molecular beam of the vaporized effluent which

could be directed into an El or CI source until it was accidently









discovered that, under certain conditions of flow rate, temperature,

and mobile phase composition, ions were produced without the use of a

filament (22). Early attempts to vaporize the liquid eluent employed a

CO02 laser which was focused on droplets exiting the probe. However,

when the laser was accidently focused on the end of the vaporizer tube

instead, it was discovered that gas-phase ions could be produced

without the use of an external ionization source (23). The laser was

later replaced by an array of oxy-hydrogen torches (24,25). In the

present version, the power required to vaporize the liquid is supplied

by passing an electrical current through the capillary tube itself.

Thermospray LC/MS interfaces have been commercially available for

several years with a wide variety of applications reported in the

literature. The analysis of peptides and amino acids (59,60),

glucuronides (61), pesticides (62), dyes (63), and metabolites in

biological fluids (64) have been described recently. In general the

mass spectra obtained from these nonvolatile compounds contain

protonated molecular ions as the major peaks with few structurally

significant fragments. This lack of fragmentation is a significant

limitation in attempting to use thermospray LC/MS to determine the

structure of eluted compounds.

An instrumental development was recently reported which employs the

use of a repeller electrode in which increased fragmentation was

observed in thermospray mass spectra of caffeine, D 4030, D 2439,

bambuterol, and 16-hydroxyprednisolone (65). In most examples repeller

voltages of 120-180 V have been used to produce significant

fragmentation. The thermospray source which was used in our studies

was a relatively early model and was not equipped with a repeller








electrode; however, we have at our disposal collision-induced

dissociation and MS/MS to provide structural information.



Present Understanding of the Thermospray Ionization Mechanism

Thermospray begins with the spraying of aqueous mobile phases

containing a volatile electrolyte such as ammonium acetate through an

electrically heated stainless steel capillary into the source region of

the mass spectrometer at flow rates of 1 to 2 mL/min. A simplified

thermospray vaporization scheme is shown in Figure 1.1 and illustrates

the four major processes which occur during thermospray ionization

(vaporization, droplet evaporation, droplet charging, and direct ion

evaporation/CI). The vaporization of liquids in vacuum results in the

production of a supersonic jet of vapor containing a mist of fine

droplets and particles with estimated velocities of 105 cm/s (22). A

portion of the droplets produced are electrically charged, with an

equal distribution of positively and negatively charged droplets

(66,67). The actual size of the droplets is dependent on the vaporizer

tip diameter and the conditions affecting vaporization (temperature and

flow rate). As the droplets travel through the heated source region

they continue to vaporize because of the heat input from the

surrounding hot vapor. As the charged droplets shrink to a certain

diameter (1 pm) the electric field increases until it is strong enough

(107 V/m) to eject ions (direct ion evaporation) from the liquid

surface (68,69). Ions are then sampled into the mass analyzer region

through a conical exit aperture.

Two distinct stages are believed to be involved in the formation of

thermospray ions (Figure 1.2). The first stage is the formation of


































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gaseous ions out of the microdroplets produced by a direct ion

evaporation mechanism. These ions will be referred to as primary

thermospray ions. Primary ions produced in the thermospray process are

apparently identical to those present in solution. For example,

ammonium acetate produces (NH4)+ and (CH3COO)- ions with clusters of

these ions with water, ammonia, and acetic acid. The second stage

involves the primary thermospray ions in gas-phase ion/molecule

reactions with neutral gas phase sample molecules. These CI reactions

may remove the primary thermospray ions and produce new product ions.

This process has been recognized as of possible importance in the

thermospray ionization mechanism but has received much less attention

than the primary direct ion evaporation processes.

The difficulty in elucidating further details of the thermospray

mechanism lies in the complexity of the thermospray plume itself. The

thermospray plume consists of a dynamic collection of evaporating

droplets and particles moving at supersonic speeds, producing ions

without an external source of ionizing electrons. Studies performed by

Iribarne and Thomson (68,69) using an atomizer to generate charged

droplets have shown that small ions can separate or "evaporate" from

evaporating droplets carrying electrical charges. This model of ion

evaporation from liquid droplets appears to account satisfactorily for

the initial ionization (primary ions) observed in the thermospray

technique; however, it may not account for ionization produced in the

later stages of evaporation (secondary ions). In our studies we have

chosen to retain the integrity of the thermospray plume and perform

studies on actual thermospray plumes under actual thermospray








conditions of temperature and pressure in order to better understand

the processes occurring in these later stages of ion evaporation.

Our experiments were based on the fact that a particular

instrumental parameter or solution characteristic can greatly affect a

particular thermospray process. For example, source temperature would

be expected to have a major role in affecting the droplet evaporation

process. The approach used in our studies was to first obtain direct

physical measurements of actual thermospray plumes in vacuum and then

determine the effects of instrumental parameters and solution

characteristics on the ionization of a pure buffer (ammonium acetate)

and of a model compound (ribavirin) prepared in ammonium acetate.

Ribavirin was a particularly attractive choice as a model compound

because of its thermal liability which renders classical mass

spectrometric ionization techniques ineffective; thus, to date little

mass spectral data has been obtained on the compound. Also, ribavirin

is currently under investigation as a broad-based anti-viral compound

for which there is no sensitive analytical method currently available;

therefore, the characterization of its thermospray mass spectral

behavior would be desirable.



Overview of Thesis Organization

This work represents a systematic study of the effects of

instrumental parameters and solution characteristics on thermospray

ionization. The remaining chapters which follow this introductory

chapter will provide a detailed description of the experimental set-up,

and will discuss the results of our fundamental studies of thermospray

ionization. Chapter II will describe the various instruments used in








these studies which include a thermospray LC/MS interface, HPLC pumps,

mass spectrometer, and a specially designed vacuum chamber.

Preliminary studies dealing with the characterization of the Vestec

thermospray LC/MS interface are discussed in Chapter III. In Chapter

IV the results of direct physical measurements of actual thermospray

plumes in vacuum will be discussed. The effects of instrumental

parameters (Chapter V) and solution characteristics (Chapter VI) will

be discussed with respect to pure ammonium acetate and a ribavirin

standard prepared in ammonium acetate. The thesis ends with

conclusions and future work (Chapter VII), which will state our

conclusions from these fundamental studies and provide recommendations

for future work in this area of research based upon our results.











CHAPTER II
EXPERIMENTAL

Samples and Reagents

Analytical-grade ammonium acetate was obtained from Fisher

Scientific (Fair Lawn, New Jersey), and Mallinckrodt (St. Louis,

Missouri). Ribavirin was received as a gift from Dr. B.J. Gabrielson

of the U.S. Army Medical Research Institute of Infectious Diseases

(Fort Detrick, Frederick, Maryland). Purine, 2-aminopurine, adenine,

2-hydroxypurine, 2,6-diamino-8-purinol, 6-thioguanine, 6-

mercaptopurine, xanthine, hypoxanthine, 6-thioxanthine, uric acid,

allantoin, alloxan monohydrate, and histidine were obtained from Sigma

Chemical Co. (St. Louis, Missouri). All reagents were analytical

grade.



Vestec Thermospray LC/MS Interface

A Vestec thermospray LC/MS interface (Vestec Corp., Houston, TX)

was used. A schematic representation of the Vestec thermospray

interface is shown in Figure 2.1. The interface consists of a

vaporizer probe, thermospray ion source, equipped with a thoriated

iridium filament, discharge electrode, and pump out line (1 cm i.d.) to

a 300 L/min mechanical pump. Below is a description of the interface

components and a brief description of their primary functions.

The vaporizer probe consists of a stainless steel capillary tube

(0.015 mm i.d. X 1.5 mm o.d.) located inside a 6.5 mm o.d. probe which

is inserted into the mass spectrometer via a vacuum lock. Two

thermocouples are spotwelded to the capillary, one located near the

inlet and the other near the tip of the probe. The thermocouple





































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positioned at the inlet measures the temperature at which no

vaporization is occurring and is commonly referred to as T1 or the

control temperature. The thermocouple located at the tip measures T2

or the vaporizer tip temperature. The power required to vaporize the

liquid is supplied by passing an electrical current through the

capillary tube itself. A feedback controller is used to control the

power input to the vaporizer so that T1 is maintained constant. A

digital panel meter displays either Tj or T2.

The Vestec thermospray ion source is specially designed to

accommodate the excess vapor flows being delivered from the vaporizer

probe. Flow rates of up to 2.0 mL/min could be handled by the source

without shutting down the mass spectrometer. The source consists of a

4.11 cm X 1.91 cm X 1.27 cm block of stainless steel with openings for

the vaporizer probe (0.95 cm diam.), source heater (0.64 cm diam.),

pump out line (0.95 cm diam.), and an ion sampling cone (1.27 cm

diam.). A thermocouple is located downstream past the ion sampling

cone to measure the vapor temperature. The power input to a cartridge

heater is controlled by a temperature controller so that either the

vapor temperature or source block temperature is held constant. The

ion sampling cone measures 0.64 cm in height and the aperture is 0.050

mm in diameter. With the probe fully inserted into the source the

distance between the probe tip and the ion sampling cone aperture was

0.6 cm. We modified the source to accommodate a thermogauge so that

source pressures could be measured. (This line for source pressure

measurements was not a standard feature on the ion source.) Typical

indicated pressures inside the source ranged from 1.8 to 2.6 Torr. The

source pressure could be increased to a maximum of 8 Torr by partially








closing a valve on the pump out line. The source pressure line (1 mm

i.d.) was made to be interchangeable with the vapor thermocouple; thus,

only one reading (vapor temperature or source pressure) could be

obtained at any one time. In these studies the source was operated to

measure source pressure exclusively. Although the source was equipped

with external sources of ionization, a thoriated iridium filament and a

discharge electrode, they were not used in these studies.



HPLC Pumps

An ISCO LC-5000 precision syringe pump (ISCO, Inc., Lincoln, NE)

was used for all thermospray studies. The pump was capable of

producing a precise, pulse-free delivery of liquids (500 mL capacity)

at flow rates ranging from 1.0 pL/min to 6.7 mL/min with an output

pressure limit of 3,700 psi. The pump operates in a constant flow mode

and the pump pressure was digitally displayed in psi. The syringe

barrel and piston are made of 304 stainless steel and the pump seals

are graphite-filled Teflon. When changing solvents, the cylinder was

refilled and pumped three to four times with 50 mL volumes of the

desired solvent to ensure the thorough rinsing of the pump.

For experiments dealing with pump pulsing and plume measurements in

vacuum, an EM Science MACS 100 pump was used. This pump is a single

piston, constant stroke, reciprocating HPLC pump capable of producing

accurate flow rates ranging from 5 pL/min to 5.0 mL/min with a pressure

limit of 6000 psi. Along with flow rate, the refill speed (125 to 650

ms) could be controlled. Slow refill speeds were selected to yield a

pulsing flow while fast refill speeds were selected to yield a smoother

solvent flow.











Mass Spectrometer

A Finnigan triple stage quadrupole (TSQ45) mass spectrometer was

used. The El and CI spectra were obtained using a standard Finnigan

4500 EI/CI ion source with solids probe introduction of the samples.

Samples were heated and vaporized rapidly from 45 to 400*C in 3-4

minutes under temperature control. Electron energies of 70 and 100 eV

were used for El and CI, respectively. Source pressure for ammonia was

0.60 torr with the source temperature maintained at 140*C. Nitrogen

was used as the collision gas at pressures ranging from 1.2 to 2.5

mTorr. Collision energies of 25 to 30 eV were used. The electron

multiplier was typically operated at -1000 V for full scan mass spectra

and -1500 V to -1700 V for MS/MS spectra.

The mass spectrometer used was not dedicated to LC/MS resulting in

the frequent conversion back and forth between the standard EI/CI

source and the thermospray source. Thus, normal El tuning was

performed on a standard EI/CI source with the standard calibration

compound perfluorotributylamine (PCR Research Chemicals, Inc.,

Gainesville, FL) on the TSQ45 prior to installation of the thermospray

source. After installation of the thermospray source the lens voltages

were optimized with a 0.1 M ammonium acetate solution followed by a

solution of polyethylene glycol (PEG 200) (Sigma Chemical Co., St.

Louis, MO).



Vacuum Chamber Measurements

The vacuum chamber consisted of a modified Finnigan 1015 vacuum

console designed with entrance ports for the vaporizer probe,









temperature, pressure, and current probes, a six inch diameter

observation window, and a pump out line connected to a 300 L/min

mechanical pump. The pump-out line was situated 180* from the

vaporizer probe. Temperature and pressure measurements of the plumes

were obtained by placing the thermocouple (1 mm o.d.) and the pressure

sampling tube (2 mm i.d.) directly into the center of the plume,

perpendicular to the direction of the vapor and droplet flow from the

vaporizer probe. Current measurements were made using a Keithly 480

picoammeter (Cleveland, OH) obtained in the same manner as with the

temperature and pressure measurements with a 2" X 2" wire mesh screen

placed into the center of the plume. Sampling distances were varied by

adjusting the distance of the vaporizer probe from the sampling probes.












CHAPTER III
PRELIMINARY CHARACTERIZATION OF THE VESTEC THERMOSPRAY LC/MS INTERFACE

Preliminary characterization of the Vestec thermospray LC/MS

interface involved vaporization studies performed outside the mass

spectrometer where measurements of temperatures along the outside of

the vaporizer capillary were obtained. The resulting temperature

profiles were used to characterize the effects of tip temperature and

flow rate on vaporization and predict the state of the liquid inside

the probe under the various vaporization conditions typically

encountered with thermospray. These characterization studies also

involved the acquisition of thermospray mass spectra obtained under

"standard" thermospray conditions so that the features typical of

thermospray ionization mass spectra could be readily observed on our

LC/MS interface. These preliminary and general performance studies

were beneficial in providing results and information which were used

for the experiments discussed in Chapters IV and VI.



Vaporization Studies

The vaporization process of thermospray is responsible for the

production of a plume of droplets and vapor from a flow of liquid.

Flow rate and tip temperature are the necessary instrumental parameters

to achieve the partial or complete vaporization of a liquid mobile

phase. In these studies of vaporization, the effects of flow rate and

tip temperature on the temperature of the outside of the thermospray

vaporizer probe were determined. These studies involved experiments

which were performed outside the mass spectrometer on a vaporizer probe









with the outer (6.5 mm o.d.) metal sheath removed exposing the bare

(1.5 mm o.d.) capillary. From these studies information on the

processes occurring inside the probe during various conditions of

vaporization was obtained.



Characterization of Thermospray Plume Types

Two thermocouples are positioned along the capillary tube to allow

for temperature measurements at the point where electrical heating

starts (Tl), and at the tip of the probe (T2). These temperatures are

commonly referred to as the control temperature or T1, and the tip

temperature or T2. By measuring these temperatures on the vaporizer

probe as it is heated at a constant flow rate, a plot can be generated

which represents the resulting tip temperature (T2) versus the control

temperature (Tl), related to the power input to the vaporizer probe.

With these plots the various stages of vaporization can be illustrated.

We performed a "T2 versus TI" experiment with 1.0 mL/min of water to

illustrate these stages of vaporization and to help characterize the

thermospray plumes.

Figure 3.1 shows a typical T2 versus TI plot obtained for water

sprayed outside the mass spectrometer at a flow rate of 1.0 mL/min.

These plots are typically characterized by three slopes or regions

which represent the state of the liquid sample exiting the probe tip.

The first region (I) of the plot represents the steady heating of the

mobile phase. No vaporization occurs in this region as only liquid

exits the probe (tip temperature below the boiling point). The second

region (II) of the plot represents the partial vaporization of the

mobile phase. In this temperature region a fine mist of droplets and






























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vapor are observed to exit the probe. As the tip temperature is

increased, a greater fraction of the mobile phase is transformed into

the vapor phase (wet to nearly dry plumes). The boundaries of this

second region are marked by two points, point A and point B. Point A

corresponds to the temperature at which vaporization begins (T2 -

boiling point of the liquid) while point B corresponds to the

temperature at which 100% vaporization of the mobile phase is achieved.

The third region (III) of the plot represents the complete vaporization

of the mobile phase. At these tip temperatures only vapor exits the

probe tip (dry plume) resulting in the rapid increase in tip

temperature.

From this study three distinct plume types can be defined with

respect to their apparent droplet content and physical characteristics

(see Figure 3.1). Wet plumes consist of relatively large droplets and

can be produced when operating at relatively low temperatures within

region II. These plumes appear as a dense cloud of droplets and vapor

accompanied by a "sizzling" sound as when water is sprayed on a hot

frying pan. When an object is placed within a wet plume at atmospheric

pressure, condensation occurs on the surface. Nearly dry plumes appear

to contain smaller droplets than wet plumes and can be produced when

operating at higher temperatures within region II. When an object is

placed within a nearly dry plume at atmospheric pressure, no

condensation occurs on the surface. Dry plumes consist of only vapor

(invisible plume) and can be produced when operating at temperatures

within region III. No condensation occurs on the surface of an object

when placed within a dry plume. These three plume types can be

reproduced for a given flow rate and will be referred to in Chapters








IV-VI. Further studies were performed which investigated the effects

of flow rate and temperature (power input) on vaporization and are

discussed below.



Variable Flow with Constant Heat

To determine the effects of flow rate on vaporization, temperature

was measured along the vaporizer capillary (1.5 mm o.d.) under

conditions of variable flow with fixed control temperature. A

thermocouple was placed directly on the side of the probe until a

stable temperature reading was attained (ca. 10 sec). A plot of probe

temperature versus distance along the probe across a 30 cm portion of

the probe at flow rates of 0.5 mL/min, 1.0 mL/min, and 4.0 mL/min is

shown in Figure 3.2. Increased flow rates result in higher

temperatures along the probe, higher pump back pressures, and a larger

plume of droplets and vapor (visually observed to be larger in diameter

and contain larger droplets).

We believe the increase in temperature along the probe with

increasing flow rate is due to compression of the vapor inside the

probe. As vaporization occurs at some point within the probe, the

volume increases enormously, resulting in the compression of this vapor

and an increase in pump back pressure. At higher flow rates the

compression is greater; therefore, the pressure increases. With the

increase in pressure, the average speed of the gas molecules increase

which results in an increase in temperature. This process is analogous

to the behavior of a gas contained in a cylinder with a moveable

piston. The pressure of the gas is the net effect of the collisions of

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When the piston is moved downward into the cylinder, the gas is

compressed, resulting in the increase in temperature and the increase

in the average speed of the molecules.

At each flow rate a temperature maximum was observed at various

distances along the probe. For example, temperature maxima occur at

ca. 8 cm for the flow rates of 0.5 and 1.0 mL/min. Temperatures along

the probe decrease from this maximum towards the vaporizer tip. We

believe this cooling effect is due to the evaporation of the droplets

produced within the probe and the temperature maximum represents the

position within the probe corresponding to the onset of vaporization.

Temperature along the probe is maximum at ca. 17 cm for water at 4.0

mL/min. This shift in temperature maxima can be explained by the fact

that the same amount of power is input to the probe at 1.0 and 4.0

mL/min. With a greater flow rate we predict that an increase in the

power input would be required with higher flow rates to produce

temperature maxima at corresponding distances along the probe.

However, with a constant power input the temperature maximum shifts

towards the probe tip which indicates that the onset of vaporization

occurs at distances closer to the probe tip with increasing flow rates.



Constant Flow with Controlled Heating

To determine the effects of control temperature (related to power

input) on vaporization, temperature was measured along the vaporizer

probe under conditions of constant flow with controlled heating. Three

TI temperature settings were selected at a 1.0 mL/min flow rate of

water to produce the three thermospray plume types, wet, nearly dry,

and dry, which correspond to visual and physical observations made with








the probe spraying into an open room at atmospheric pressure. A plot

of probe temperature versus distance along the probe at conditions of

constant flow and controlled heating is shown in Figure 3.3. Three

control temperatures were selected which produced the three described

plume types.

The increase in power to the vaporizer probe at a constant flow

rate resulted in higher temperatures along the probe, higher pump back

pressures, and a more complete vaporization of the mobile phase.

Temperatures toward the end of the probe decreased for the temperature

settings corresponding to wet and nearly dry plumes. However, at the

temperature setting which resulted in the production of a dry plume,

the probe temperature increased. The temperature increases here

because only the limited heat capacity of the vapor (dry plume) is

available to absorb the input heat. The gradual decrease in

temperature along the vaporizer probe observed at various portions of

the plot is probably due the evaporation of the resulting droplets

within the probe (similar to the flow rate study discussed above). The

increase in HPLC pump backpressure with increasing power input is

probably the result of more vapor created within the vaporizer probe.

The excess vapor cannot escape as rapidly as when it was liquid,

resulting in increased backpressures.

From the discussion of the effects of flow rate on temperature

measurements along the probe, it would seem likely that increasing the

power input to the vaporizer probe at a constant flow rate would result

in increased vaporization efficiency while shifting the vaporization

onset position (temperature maximum) further away from the probe tip.

Although increased vaporization was observed with an increase in power






































4f -.4 -.





4 -



ca


0ca* 1-4
'-4 01)


Ca2




04J 0:

(n4 0

4j



0)

4j 0


0)


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0 .



0 41 -

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41







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tro









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0 0 0 0 0
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VC) \ ^ .o



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o m q





aD -O
) OC


,,
00000000
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input to the probe, this shift in temperature maximum along the probe

was not clearly evident. A temperature maximum was observed at ca. 15

cm for a wet plume, while nearly dry and dry plumes each appear to

generate two temperature maxima which might be indicative of a sequence

of vaporization followed by condensation of the resulting vapor and

droplets followed by re-vaporization of the liquid. From these results

we cannot be certain whether this sequence is actually occurring or

whether this is just a reflection of the heating behavior of the probe.

A more detailed investigation into this heating process at various flow

rates might provide more conclusive results.



Thermospray Mass Spectra

The second portion of our preliminary characterization studies

involved the acquisition of positive and negative ion full scan

thermospray mass spectra of ammonium acetate, ribavirin, and several

purine and xanthine-type compounds. In cases where more fragmentation

was desired for structural information, daughter spectra were obtained

and the resulting fragmentations were interpreted. Spectral

characteristics of thermospray such as the observed ions of the

ammonium acetate buffer, formation of pseudo-molecular ions and

fragment ions are discussed. Experiments performed with ribavirin to

determine the reproducibility of positive and negative ion intensities

and relative abundances of the ions contained in the mass spectra are

described. Preliminary observations on the effect of various

functional groups on thermospray performance are discussed.









Ammonium Acetate

Positive ion and negative ion full scan thermospray mass spectra of

ammonium acetate obtained at 1.5 mL/min are shown in Figure 3.4.

Abundant positive ions at m/z 18, 36, 54, 60, 77, and 78 which

correspond to NH4+, NH4*-H20, NH4'-2H20, CH3CONH3+, CH3CONH2NH4+, and

CH3COOH*NH4+, respectively, were present in the positive ion mass

spectrum while abundant negative ions at m/z 59, 77, and 119 which

correspond to CH3COO-, CH3COO'H20, and CH3COOH-CH3COO", respectively,

were present in the negative ion mass spectrum. A negative ion was

observed at m/z 155 in the spectrum of ammonium acetate at a very low

relative abundance and corresponds to CH3COOH*CH3COO'-2H20.

Figure 3.5 shows the MS/MS daughter spectra obtained for selected

thermospray ions of ammonium acetate. The positive daughter spectrum

of m/z 36 shows the loss of 18 (H20) resulting in the formation of NH4+

at m/z 18. The positive daughter spectrum of m/z 60 shows the loss of

17 (NH3) resulting in the formation of the fragment ion at m/z 43 which

corresponds to CH3CO+. Here it is assumed that the positive ion at m/z

60 is not an acetic acid ion (CH3COOH)+ but rather (M+H)+ of an

impurity of acetamide, CH3CONH2. Likewise, the positive ion at m/z 77

is presumably not a M+ of ammonium acetate (CH3COO-'NH4+) but rather an

ammonia adduct ion formed from the acetamide impurity, CH3CONH2NH4+.

The positive daughter spectrum of m/z 77 shows two successive losses of

17 (NH3) to yield the fragment ions at m/z 60 amidee impurity) and m/z

43 (CH3CO)+. Another fragmentation observed was the loss of 59

(CH3CONH2) resulting in the formation of NH4+ at m/z 18. The negative

daughter spectrum of m/z 59 shows that this ion is very stable to

fragmentation. We presume this behavior to correspond the acetate ion



































cd


4 -

ra

U)



Uc,
















o
0 e
o-o
4-1


ca
U)

-4




-4
44








4103













Owi 0



























+
NX


0
0


0
a?


,_


0

+3c'


X
Z4


eouepunqv GA!We/IG %


1-


ej

0
0

0


0



0
0
0

0 -~


0

Ic'
0





s--


-w








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N


. i I i I


eouepunqy eAulely1 %
























Figure 3.5

MS/MS daughter spectra obtained for selected positive and negative ions
of ammonium acetate. A collision energy of 25 eV was used with 1.2
mTorr collision gas pressure (N2).












NH4- H20



180.8





.0


120 140


12 149


I/E 28 40


- NH;


180.0














50.
T9.


NH

4 43
I


CH3CONH 2NH 4

77


S I I I I I
28 40 68 t o1o 120 148
















:O 40 E6 88 1Be 120 14








CH3COOH CH3COO

CH3COOH 119


20 408 88 1

CH3CONH;


W E .B


49 68 is


148








(CH3COO)-. The negative daughter spectrum of m/z 119 shows the loss of

60 (CH3COOH) to yield the acetate fragment ion at m/z 59.

All positive ions of ammonium acetate produced by thermospray at

these conditions contain an ammonium ion in their structure, while the

negative ions contain an acetate ion. Under these conditions the

positive and negative reconstructed ion currents (RIC) were essentially

equal. The origin of the acetamide impurity is not known. This

compound may be the result of a side reaction which occurs during the

commercial synthesis of ammonium acetate. Buffer solutions were

prepared from ammonium acetate obtained from two sources, Fisher

Scientific and Mallinkrodt. The resulting spectra were virtually

identical, each containing abundant positive ions at m/z 60 and 77

corresponding to the (M+H)+ and (M+NH4)+ of the amide impurity.



Ribavirin

The positive ion full scan thermospray mass spectrum of 1 pg of

ribavirin injected is shown in Figure 3.6. The spectrum contains ions

at m/z 113, 130, 245, 262, and 489 which correspond to the protonated

triazole-carboxamide ring of ribavirin, the NH4+ adduct of the

triazole-carboxamide molecule, (M+H)+, (M+NH4)+, and (2M+H)+,

respectively. The negative ion full scan thermospray mass spectrum of

ribavirin is shown in Figure 3.7 and contains ions at m/z 111, 171,

243, 303, and 355 which correspond to the loss of a proton from the

triazole-carboxamide ring, the acetate adduct of the triazole-

carboxamide molecule, (M-H)', (M+CH3COO)', and (M+lll)', respectively.

Although not observed in this particular mass spectrum, a (2M-H)" ion

at m/z 487 was frequently observed. In both the positive ion and







































444



a)



4- 4-





CO,



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eouepunqV eA!J lel:G %









negative ion spectra the ammonium acetate ions are observed in the

background. Under the particular conditions at which these spectra

were obtained, the summed negative ion intensity of ribavirin was about

ten times greater than the summed positive ion intensity. The (M+H)+

and (M+CH3COO)" ions of ribavirin were present as the base peaks in

their respective spectra. In both ion modes abundant fragment ions

were formed. Especially interesting in the negative ion mode was the

addition of the triazole-carboxamide ring fragment (m/z 111) to a

molecule of ribavirin which suggests the occurance of gas-phase

ion/molecule reactions.



Reproducibility of Thermospray Spectra of Ribavirin

Because a mass spectrometer dedicated for thermospray was not

available in our laboratory, the standard EI/CI source was replaced

with the thermospray source prior to its use. The thermospray source

remained on the mass spectrometer for two to four days at a time and

yielded reproducible ion intensities and mass spectra during this time.

However, from week-to-week the relative abundances of ions observed in

the mass spectra would sometimes change as well as the relative

positive and negative ion intensities. These week-to-week fluctuations

are illustrated below.

The positive ion full scan thermospray mass spectrum of ribavirin

shown in Figure 3.8a was acquired three weeks prior to the spectrum

shown in Figure 3.8b. Although these spectra were obtained under

identical conditions of thermospray (instrumental parameters and

solution characteristics), the resulting spectra are significantly

different. The low relative abundances of the ions at m/z 113 and 489




































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4-



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U ,4 (-4
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(observed in Figure 3.8a) are the major differences between the two

spectra. However, the relative abundances of the ions at m/z 130, 245,

and 262 remain relatively constant and the pseudo-molecular ion at m/z

245 remains the base peak. The negative ion full scan thermospray mass

spectra of ribavirin (Figure 3.9), also acquired three weeks apart,

showed similar differences as observed with the positive ions. The

decreased relative abundances of the ions at m/z 111, 243, 355, and 487

(observed in Figure 3.9a) are the major differences between the two

spectra while the pseudo-molecular ion at m/z 303 remains the base

peak.



Purine and Xanthine-type Compounds

As part of our preliminary characterization of the LC/MS interface

we investigated the effects of various substituents on thermospray

ionization. The compounds which were selected for this study have

structures similar to purine with most differing by only one or two

functional groups. These compounds can be divided into two groups:

one group consisting of substituted purines and the other consisting of

substituted xanthine or substructures of xanthine (Figure 3.10).

Purine along with pyrimidine are the parent compounds of the two

classes of nitrogenous bases found in nucleotides and consist of a

pyrimidine ring and an imidazole ring fused together. Xanthine is a

degradation product of purines and has a core structure similar to

purine.

Both positive ion and negative ion full scan thermospray mass

spectra were obtained from standard solutions (50-100 ppm) of each of

the purine and xanthine compounds. Most of the compounds showed



































4 WW0CN U 0
> 01 W (U ~ -0 -9



o o

4 ',4 .J4 0e
44 41 44 '


0, r-4 $
SW (1)

Ca 41J j"j
W E1--Wca cN i

(U ca J a) w
3: 0 a




cn 0W W'L
~ca cn U 04 c4 a)



ca W W.-
(D- 4) (UN Q.

En 41 U) 04.)


ca cd cz I -4 En(1
Zj (UU






























,w


In


In
0in


01


eOUBpunqV eAIlBelf %


| | i | | = = = |


I I I I i I I I I


-*- _._ i__mi_ i III-


lilli


eouepunqy eAII81e %

























Figure 3.10

Model compounds used for determining the effects of various
substituents on thermospray ionization. Note that most of the
compounds contain the purine core structure.







H


'N:-N
Purine NW 20
Purine (MW 120)


2-Aminopurine
(MW 135)


NH2 H
N NY',, ,


Adenine
(MW 135)


2-Hydroxypurine
(MW 136)


2,6- Diamino-8 -purinol
(MW 166)


6-Thioguanine
(MW 167)


6-Mercaptopurine
(MW 152)


0 H
HN N"


H
Xanthine (MW 152)


0 H
N



Hypoxanthine
(MW 136)


SH H



H
6-Thioxanthine
(MW 168)


0 H

HNN

H
Uric Acid
(MW 168)


0
II H
H2NCNH N0
HO
0


Alloxan Monohydrate
(MW 160)


Allantoin
(MW 158)









characteristics in the mass spectrum typical of thermospray ionization;

abundant pseudo-molecular ions with very little fragmentation.

Molecular ions appeared as the (M+H)+ and/or (M+NH4)+ in the positive

ion mass spectra and as the (M-H)- and/or (M+CH3COO)" in the negative

ion mass spectra. The instrumental parameters and the results of the

positive ion and negative ion thermospray mass spectra are summarized

in Table II. These ions were present in the mass spectrum with ion

intensities greater than three times the background ("chemical noise").

Thermospray of purine yielded the (M+H)+ ion in the positive ion

spectrum and (M-H)', (M+CH3COO)', and (2M-H)" ions in the negative ion

spectrum. All of the substituted purine compounds show the same

behavior as purine in the positive ion mode and yield a (M+H)+

molecular ion. Two of the substituted purines, 2-hydroxypurine and 6-

mercaptopurine, also produced (M+NH4)+ ions. 2-hydroxypurine was the

only purine-type compound which contained a hydroxy group in its

structure. 6-thioguanine has a structure very similar to 6-

mercaptopurine but did not form an ammonium adduct. The only

difference between these two structures is the presence of an amine at

the two-position on 6-thioguanine.

All of the substituted purine compounds yield (M-H)- ions in their

negative ion mass spectra. In addition to (M-H)" ions, some compounds

(2-aminopurine, adenine, and 2,6-diaminopurinol) formed (M+CH3COO)"

adduct ions. The structure of these compounds differ from purine by

the addition of an amine group (two amine groups in the case for 2,6-

diaminopurinol) at various positions on their structures. Purine,

hydroxypurine, 6-mercaptopurine, and 6-thioguanine do not show the

























I 0





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0rt Y 014 <410 0

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r-,
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t- +




0- 0 0 0
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unr- 'C '4 ri -n
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addition of acetate in their negative ion spectra nor do they contain a

primary amine group on their structures.

Xanthine, 6-thioxanthine, uric acid, and alloxan did not produce

any positive thermospray ions. A common feature to these compounds is

that their structures contain more than one doubly-bonded

electronegative group such as oxygen or sulfur. Hypoxanthine, with a

similar in structure to xanthine with the doubly-bonded oxygen at the

2-position removed, yields (M+H)+ and (M+NH4)+ ions. Similarly,

allantoin (an oxidation product of uric acid) also yields abundant

(M+H)+ and (M+NH4)+ ions. Xanthine, hypoxanthine, 6-thioxanthine, and

allantoin yield (M-H)- negative ion species. Alloxan monohydrate

produced negative ions at m/z 113, 115, and 175. The ions at m/z 113

and 115 correspond to fragment ions for the loss of HCOOH and CO2,

respectively. The ion at m/z 175 corresponds to an acetate adduct of

the 115- fragment ion. Both hypoxanthine (which contains no primary

amine functionality in its structure) and allantoin yield (M-CH3COO)"

ions while uric acid did not produce any negative ions.

Figures 3.11 and 3.12 show the resulting ion intensity summation

profiles of the abundant positive and negative ions of these purine and

xanthine-type compounds and ammonium acetate. These chromatograms

represent a single 25 AL flow-injection of each compound. Several

sharp downward spikes are present both profiles. These spikes

correspond to the switching of the sample injection valve from load to

inject or vice-versa. These spikes occur since the switching of the

injection valve causes a momentary interruption of the liquid flow and

thus, the signal decreases abruptly. A rapid switching technique will

eliminate these spikes while a slower, more deliberate switching will





























) (U 0) Q0.) 0 -s 4-4 )
r -. 4" ) .4 U -Y r-4 (L) O' CM 0 .4
-r4 1 c4 Ci uJ C o 0 I
4 4-1 Z Q) r. -4-



0 u H p P O
41 ,c 0) 0Ca)- C )





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0 0 0
o g a: -, o O.. o






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4-4 4. o 0


0 C 0 u C OO


0 .4 0










S. 0 4 0
Q) 0 0 CN '0' 4-1 Z
W 4-4










0 M E*
2 4 0)- 4 <








U) C )X ,)GJUC -H cn
O Ci C) w O S CV) -4
o < i 0 ^ 0c Co 0 10




i 4 0 4.1 U '-' C4 C1






























i
-3



co -







co o


C03


04


juaJJno uol


luejjno uol


Old



































4)o U ) P. ()
Z -4 414 .,-q C.
-r 4 CU 4-J -J

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~-0 4) a


-4 -4 9 :

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w 0U

oz u, a)a
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P.- -4 E. 0 -

.,- V-4 Z

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b C'-,4 C*


0 CU4iO )


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C-4 .4 0
a -4

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CC CC




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5-4 P.0 0

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r4-4 44
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0 (L.) a.


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zn


-'a)



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ca)4.) o

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result in these spikes. These profiles illustrate the rapid nature of

thermospray with a flow injection technique where 13 samples were run

in approximately eight minutes, which corresponds to 100 samples per

hour. The RIC appears significantly elevated above the zero and does

not increase significantly for any of these compounds. This is because

the scanning interval of the mass spectrometer included the masses

corresponding to the ammonium acetate buffer ions which were present

throughout the acquisition.

It is interesting to note that the ion intensity profile of the

ammonium acetate ions decrease during the ion production of the purine

and xanthine-type compounds. Thus, it seems that the sample ions are

produced at the expense of the ammonium acetate ions. Whether this

ionization behavior is the result of direct competition between the

sample and ammonium acetate in the direct ion evaporation mechanism, or

a depletion process involving sample molecules and ammonium acetate

ions (reagent ions) in a CI mechanism, or a combination of both, cannot

be concluded here.

These preliminary investigations show the effects of various

substituent groups on the thermospray ionization of purine and

xanthine-type compounds. These results indicate that particular

functional groups present in the sample compound structure can

influence the thermospray mass spectra. Structures which contain at

least two thiol or doubly-bonded oxygen groups generally do not produce

positive ions by thermospray ionization. Allantoin is an exception to

this observation. We believe that the presence of a primary amine

group in the allantoin structure helps to overcome this effect of the

doubly-bonded oxygens and promoted the formation of both (M+H)+ and





70



(M+NH4)+ ions. The presence of amine groups combined with the absence

of thiol or doubly-bonded oxygen groups seem to promote the formation

of (M+CH3COO)" ions in the negative ion thermospray ionization mode.

Uric acid, which contains three doubly-bonded oxygens on its structure,

does not produce any positive or negative ions by thermospray

ionization.










CHAPTER IV
THERMOSPRAY PLUME MEASUREMENTS IN VACUUM

Direct physical measurements of thermospray plumes were made in

vacuum to obtain information on the droplet evaporation and droplet

charging processes of thermospray. The measurements were made in a

specially designed vacuum chamber which permitted visual observation of

the plume, as well as sampling of pressure, temperature, and current

within the plume. Because this chamber was considerably larger (8.5 cm

i.d.) than the Vestec thermospray source (1 cm i.d. with an ion

sampling cone and other structures protruding into the source), the

effect of the source geometry may change the thermospray plume

behavior. Thermospray plumes were generated with the Vestec vaporizer

probe as measurements were obtained at positions along the center of

the plume in the manner described in Chapter II. The results of these

plume measurements are discussed below.



Plume Pressure Measurements

Plume pressures remained constant for all distances measured within

the plume at constant flow rate. Pressures of 2.4, 2.6, and 2.8 Torr

were obtained with flow rates of 1.5, 1.75, and 2.0 mL/min water,

respectively. Other instrumental parameters had no effect on pressure

measurements.



Plume Temperature Measurements

Plume temperature measurements were obtained at various tip

temperatures, manifold temperatures, flow rates, and manifold

pressures. (The adjustment of manifold temperatures and manifold









pressures approximated the effects of source temperature and source

pressure.) Much condensation (in the form of ice) was observed to

occur on the thermocouple at distances close to the probe tip (<1 cm)

while no condensation was observed to occur on the thermocouple at

distances further from the probe tip. The formation of ice on the

thermocouple would likely result in low temperature readings until the

ice either falls off or melts away at which point the temperature

readings would be expected to increase to temperatures more

representative of the surrounding vapor. Therefore, low and perhaps

fluctuating temperature readings may be indicative of a droplet-rich

plume, while stable or constant temperature readings may be indicative

of a vapor-rich plume. For these measurements no specially designed

temperature sensing probes were used. Therefore, these measurements

would be expected to provide only an approximation of the actual plume

temperature. Essentially, these measurements provide a means of

monitoring the production of droplets within the plume.

A plot of plume temperatures measured at various distances along

the plume for the three plume types at a flow rate of 2.0 mL/min is

shown in Figure 4.1. All three plume types behave similarly with the

greatest temperature fluctuations occurring at distances less than 1.5

cm. Beyond this distance the temperature readings become more stable.

Wet plumes produce the lowest temperature readings while dry plumes

produce the highest temperature readings. Thus, the vapor-rich

portions of wet, nearly dry, and dry plumes are attained at virtually

the same distances away from the probe tip. Also, higher tip

temperatures result in higher temperatures within the vapor region of

the plumes.






























4-J


C/)




'-4





















r-4
co



a-)

ca




'4J

ca


CO 4



4j4




4-

























































0 0
CN c(


0
C"j





0-
6



0
CM


0
U-n


0
0


I lilt II jilt I I liii


0
U~)


(0) eJneJedwjei.


L6












lo
C?
-0n
0





Mq
-^










l0
- r












-N





-



o6


0

a.




0
I-.
0


CL
E
0
a-
U.





U)









Figure 4.2 shows the plume temperatures measured at various

distances along a nearly dry plume produced at 2.0 mL/min for three

manifold temperatures. With increasing distances from the probe tip

the resulting plume temperature readings surpass the manifold

temperature. It appears that higher manifold temperatures result in a

more rapid attainment of stable plume temperatures. Therefore,

increased manifold temperatures are likely to result in a more rapid

droplet evaporation process and the attainment of droplet-rich plumes

at distances closer to the probe tip.

Figure 4.3 shows a plot of plume temperatures measured at various

distances along the plume for nearly dry plume types produced at

various flow rates. Plumes produced at flow rates of 1.5 and 1.75

mL/min show the greatest fluctuation in temperature at distances less

than 3 cm from the probe tip. At distances beyond 3 cm the measured

temperatures were more stable. Plumes produced at 2.0 mL/min show the

greatest fluctuation in temperature at distances less than 1 cm from

the probe tip. At distances beyond 1 cm the measured temperatures were

more stable. Thus, it appears that higher flow rates may yield vapor-

rich plumes at closer distances to the probe tip than plumes produced

at lower flow rates. This faster evaporation of droplets with

increasing flow rates may be due to the increase in tip temperatures

with higher flow rates.

Figure 4.4 shows the plume temperatures measured at various

distances along a nearly dry plume produced at 2.0 mL/min for manifold

pressures of 2.8 and 760 Torr and a manifold temperature of 24C.

Plume temperatures increase with increasing distances from the probe at

2.8 Torr, however, at 760 Torr plume temperatures decrease with






























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increasing distances along the probe. One reason which may account for

this behavior at high pressures may be the fact that at 760 Torr much

condensation occurs at all distances measured along the plume. This

may be the case since at 2.8 Torr, the probe temperature is enough to

maintain a "warm" spray with little condensation occurring. At 760

Torr, however, higher pressures combined with a manifold temperature of

only 24*C results in a "cool" spray with much condensation occurring

within the plume.



Plume Current Measurements

Plume current measurements were obtained at various tip

temperatures, manifold temperatures, flow rates, and manifold

pressures. As with the plume temperature measurements, much

condensation was observed to occur on the wire mesh used as an

electrode at distances close to the probe tip (<1 cm), while at

distances further from the probe tip no condensation was observed to

occur. Condensation on the wire sampling mesh would affect the current

measurements by changing the fraction of the droplets/vapor which are

intercepted by the mesh (nominally 80% transparent). We believe that

the plume currents measured here are most representative of the droplet

charging process and that it is unlikely that ion formation (if any) in

the gas phase contributes significantly to these current measurements

of water plumes.

Plume currents measured at various distances along the plume for

the three plume types at a flow rate of 2.0 mL/min of water are shown

in Figure 4.5. Wet and nearly dry plumes show similar plume current

behavior with distance sampled and produce negative currents of




























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approximately -500 nA at 0.5 cm. The magnitude of these plume currents

decrease with increasing distances from the probe tip (as droplets

evaporate to produce vapor) and gradually approach zero. Dry plumes

generate little current (<20 nA) and shows virtually no fluctuation

with distance sampled.

Figure 4.6 shows the plume currents measured at various distances

along a nearly dry plume produced at 2.0 mL/min for three manifold

temperatures. These plume currents generally show similar behavior

with distance sampled. The largest negative currents were observed at

0.5 cm from the probe tip and decrease to current values approaching

zero. Lower manifold temperatures have less effects on the plume

current readings. This is probably due to the increased condensation

which occurs at lower manifold temperatures.

Figure 4.7 shows a plot of plume currents measured at various

distances along the plume for nearly dry plume types produced at

various flow rates. Plume currents were greatest at distances close to

the probe tip. As the sampling distance was increased, the resulting

plume current gradually approached zero. Higher flow rates produce

more droplets and thus, yield higher plume currents.

Figure 4.8 shows the plume currents measured at various distances

along a nearly dry plume produced at 2.0 mL/min for manifold pressures

of 2.8 and 10 Torr. Plume currents are initially negative at distances

close to the probe tip and change to positive values at distances

further from the probe. At 10 Torr plume currents change from negative

to positive at ca. 1 cm while at 2.8 Torr plume currents change from

negative to positive at ca. 2 cm.

The results from these plume current studies suggest that water
































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Full Text
FUNDAMENTAL STUDIES OF THERMOSPRAY IONIZATION PROCESSES
By
MIKE S. LEE
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
1987

ACKNOWLEDGEMENTS
I wish to express my sincere gratitude to Dr. Richard A. Yost for
his guidance, direction, and friendship during the completion of this
work. I wish to acknowledge the members of my committee, Dr. John
Eyler, Dr. James Winefordner, Dr. John Dorsey, and Dr. Joseph Delfino
for their various contributions to my thesis work and education at the
University of Florida.
I acknowledge the U.S. Army Chemical Research, Development, and
Engineering Center for their support of this work.
Many thanks go to the guys in the machine shop, Chester, Vern, and
Daly, for their help in the construction of the thermospray vacuum
chamber.
I thank special friends Mike Gehron, Ken Matuszak, and Kevin
McKenna who provided and shared the laughter, joy, and nervous energies
of a gradual student lifetime.
Mostly, I thank my number one family for their endless support and
enthusiasm in everything I do.
ii

A guitar, blanket, burrito, and you.
iii

TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS Ü
LIST OF TABLES vi
LIST OF FIGURES vii
ABSTRACT xii
CHAPTERS
I INTRODUCTION 1
Developments in Ionization Techniques for
Nonvolatile Molecules 2
Thermospray LC/MS Background 12
Present Understanding of the Thermospray
Ionization Mechanism 15
Overview of Thesis Organization 21
II EXPERIMENTAL 23
Vestec Thermospray LC/MS Interface 23
HPLC Pumps 27
Mass Spectrometer 28
Vacuum Chamber Measurements 28
III PRELIMINARY CHARACTERIZATION OF THE VESTEC
THERMOSPRAY LC/MS INTERFACE 30
Vaporization Studies 30
Characterization of thermospray
plume types 31
Variable flow with constant heat 35
Constant flow with controlled heat 38
Thermospray Mass Spectra 42
Ammonium acetate 43
Ribavirin 48
Reproducibility of thermospray spectra
of ribavirin 53
Purine and xanthine-type compounds 56
IV THERMOSPRAY PLUME MEASUREMENTS IN VACUUM 71
Plume Pressure Measurements 71
Plume Temperature Measurements 71
Plume Current Measurements 82
iv

CHAPTER PAGE
V INSTRUMENTAL PARAMETERS 93
Tip Temperature 101
Source Temperature 114
Flow Rate 126
Source Pressure 137
Probe Position 148
Summary of the Effects of Instrumental Parameters 158
VI SOLUTION CHARACTERISTICS 162
Mobile Phase and Sample Solvent 162
Sample Concentration 174
Buffer Concentration 184
pH Study 202
Thermospray Ionization Behavior in a Mixture 220
Summary of the Effects of Solution Characteristics 225
VII CONCLUSIONS AND FUTURE WORK 228
LITERATURE CITED 232
BIOGRAPHICAL SKETCH 236
v

LIST OF TABLES
TABLE PAGE
ISummary of the major techniques used for ionizing
nonvolatile molecules 6
IISummary of the positive and negative ions observed
in the mass spectra of the purine and xanthine-type
compounds 62
IIIThe effects of tip temperature on source temperature
and HPLC pump pressure 102
IV The effects of source temperature on source pressure 115
V The effects of flow rate on source pressure obtained
at tip temperatures corresponding to nearly dry plumes.... 127
VI Summary of the mobile phase-solvent studies 163
VIIRatios of various positive and negative ion intensities
of ribavirin 185
VIIIPositive and negative ion intensities of pure
ammonium acetate versus 100 ppm ribavirin 199
IXForms of the ionizing groups of ammonium acetate
at various pH's 206
XForms of the ionizing groups of histidine
at various pH's 207
XIPositive and negative RIC values of
0.1 M ammonium acetate 208
XIIPositive and negative RIC values of 100 ppm histidine
in 0.1 M ammonium acetate 214
XIIISummary of the two-component mixture study for
allantoin and 2-hydroxypurine 222
XIVSummary of the two-component mixture study for
allantoin and 2,6-diamino-8-purinol 223
XVSummary of the two-component mixture study for
2-hydroxypurine and 2,6-diamino-8-purinol 224
XVISummary of the three-component mixture study for
allantoin, 2-hydroxypurine, and 2,6-diamino-8-purinol 226
vi

LIST OF FIGURES
FIGURE PAGE
1.1 A simplified thermospray vaporization scheme
illustrating the four major processes which occur
during thermospray ionization 17
1.2 Direct ion evaporation and chemical ionization
mechanisms of thermospray 19
2.1 A schematic representation of the Vestec
thermospray LC/MS interface 25
3.1 A plot of tip temperature versus control
temperature for water at 1.0 mL/min 33
3.2 A plot of probe temperature versus distance
along the probe for water at various flow rates 37
3.3 A plot of probe temperature versus distance
along the probe for wet, nearly dry, and dry
water plumes 41
3.4 Positive and negative ion full scan thermospray
mass spectra of ammonium acetate 45
3.5 MS/MS daughter spectra obtained for selected
positive and negative ions of ammonium acetate 47
3.6 Positive ion full scan mass spectrum of 1 fig ribavirin... 50
3.7 Negative ion full scan mass spectrum of 1 fig ribavirin... 52
3.8 Positive ion full scan thermospray mass spectra
of ribavirin acquired several weeks apart 55
3.9 Negative ion full scan thermospray mass spectra
of ribavirin acquired several weeks apart 58
3.10 Model purine and xanthine - type compounds used for
determining the effects of various substituents
on thermospray ionization 60
3.11 Positive ion intensity summation profiles of the
purine and xanthine compounds, ammonium acetate,
and RIC 66
vii

3.12 Negative ion intensity summation profiles of the
purine and xanthine compounds, ammonium acetate,
and RIC 68
4.1 Plume temperatures measured along wet, nearly dry,
and dry plumes at 2.0 mL/min 74
4.2 Plume temperatures measured along a nearly dry plume
at 2.0 mL/min for three manifold temperatures 77
4.3 Plume temperatures measured along nearly plumes
produced at various flow rates 79
4.4 Plume temperatures measured along a nearly dry plume
produced at 2.0 mL/min at two manifold pressures 81
4.5 Plume currents measured along wet, nearly dry, and dry
plumes at 2.0 mL/min 84
4.6 Plume currents measured along a nearly dry plume at
2.0 mL/min for three manifold temperatures 87
4.7 Plume currents measured along nearly dry plumes
produced at various flow rates 89
4.8 Plume currents measured along a nearly dry plume
produced at 2.0 mL/min at two different manifold
pressures 91
5.1 A model showing the effects of temperature on
hypothetical thermospray plumes 95
5.2 A model showing the effects of pressure on
hypothetical thermospray plumes 98
5.3 A model showing the effects of probe position on
hypothetical thermospray plumes 100
5.4 A plot showing the effects of tip temperature on
the ion intensities of pure ammonium acetate 105
5.5 A plot showing the effects of tip temperature on
the ion intensities of ammonium acetate with 100 ppm
ribavirin present 107
5.6 A plot showing the effects of tip temperature on
the positive ion intensities of 100 ppm ribavirin 109
5.7 A plot showing the effects of tip temperature on
the negative ion intensities of 100 ppm ribavirin Ill
5.8 A plot showing the effects of source temperature
on the ion intensities of ammonium acetate 118
viii

5.9A plot showing the effects of source temperature
on the ion intensities of ammonium acetate with
100 ppm ribavirin present 120
5.10 A plot showing the effects of source temperature
on the positive ion intensities of 100 ppm ribavirin 122
5.11 A plot showing the effects of source temperature
on the negative ion intensities of 100 ppm ribavirin 124
5.12 A plot showing the effects of flow rate on the
ion intensities of pure ammonium acetate 130
5.13 A plot showing the effects of flow rate on the
ion intensities of ammonium acetate with 100 ppm
ribavirin present 132
5.14 A plot showing the effects of flow rate on the
positive ion intensities of 100 ppm ribavirin 134
5.15 A plot showing the effects of flow rate on the
negative ion intensities of 100 ppm ribavirin 136
5.16 A plot showing the effects of source pressure
on the ion intensities of pure ammonium acetate 140
5.17 A plot showing the effects of source temperature
on the ion intensities of ammonium acetate with
100 ppm ribavirin present 142
5.18 A plot showing the effects of source pressure
on the positive ion intensities of 100 ppm ribavirin 144
5.19 A plot showing the effects of source pressure
on the negative ion intensities of 100 ppm ribavirin 147
5.20 A plot showing the effects of probe position on the
ion intensities of pure ammonium acetate 151
5.21 A plot showing the effects of probe position on the
ion intensities of ammonium acetate with 100 ppm
ribavirin present 153
5.22 A plot showing the effects of probe position on the
positive ion intensities of 100 ppm ribavirin 155
5.23 A plot showing the effects of probe position on the
negative ion intensities of 100 ppm ribavirin 157
6.1 Positive ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the sample
solvent and mobile phases of methanol, water, and
ammonium acetate 166
ix

6.2 Negative ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the sample
sample solvent and mobile phases of methanol, water, and
ammonium acetate 168
6.3 Positive ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the mobile phase
and sample solvents of methanol, water, and
ammonium acetate 171
6.4 Negative ion thermospray mass spectra of ribavirin
with 0.1 M ammonium acetate buffer as the mobile phase
and sample solvents of methanol, water, and
ammonium acetate 173
6.5 Positive ion thermospray mass spectra of various
amounts of ribavirin 176
6.6 Negative ion thermospray mass spectra of various
amounts of ribavirin 178
6.7 Calibration plots for the major positive ions of
ribavirin and the RIC 181
6.8 Calibration plots for the major negative ions of
ribavirin and the RIC 183
6.9 Positive ion intensities of the major ions of
ammonium acetate at various concentrations 187
6.10 Positive ion intensities of the major ions of
ammonium acetate obtained from 100 ppm ribavirin
standards prepared in various concentrations of
ammonium acetate 190
6.11 Positive ion intensities of 100 ppm ribavirin
prepared in various concentrations of ammonium acetate... 192
6.12 Negative ion intensities of the major ions of
ammonium acetate at various concentrations 194
6.13 Negative ion intensities of the major ions of
ammonium acetate obtained from 100 ppm ribavirin
standards prepared in various concentrations of
ammonium acetate 196
6.14 Negative ion intensities of 100 ppm ribavirin
prepared in various concentrations of ammonium acetate... 198
6.15 The structures of ammonium acetate and histidine
at pH 6.0 204
x

6.16 Positive ion thermospray mass spectra of 0.1 M
ammonium acetate at various pH values 210
6.17 Negative ion thermospray mass spectra of 0.1 M
ammonium acetate at various pH values 212
6.18 Positive ion thermospray mass spectra of 100 ppm
histidine in 0.1 M ammonium acetate at various
pH values 216
6.19 Negative ion thermospray mass spectra of 100 ppm
histidine in 0.1 M ammonium acetate at various
pH values 218
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
FUNDAMENTAL STUDIES OF THERMOSPRAY IONIZATION PROCESSES
by
Mike S. Lee
December, 1987
Chairman: Richard A. Yost
Major Department: Chemistry
The effects of various parameters on thermospray ionization were
determined on a thermospray liquid chromatography/mass spectrometry
(LC/MS) interface. Particular changes in these parameters affect
specific processes of thermospray [vaporization, droplet evaporation,
droplet charging, and direct ion evaporation/chemical ionization (Cl)]
and result in a change in the overall sensitivity and the relative
abundances in the mass spectra. The instrumental parameters such as
tip temperature, source temperature, flow rate, and source pressure
most directly affect the vaporization, droplet evaporation, and direct
ion evaporation/CI processes, while solution characteristics such as
mobile phase, sample solvent, sample concentration, buffer
concentration, pH, and mixture components most directly affect the
droplet charging and direct ion evaporation/CI processes. Each
parameter was varied systematically and the effects on a model sample
compound, ribavirin, was determined.
xii

Conditions of vaporization and droplet evaporation which produce
droplet-rich plumes favor the formation of fragment ions while
conditions which produce vapor-rich plumes favor the production of
pseudo-molecular ions. The role of a volatile electrolyte in
thermospray ionization was demonstrated with mobile phase-sample
solvent combinations of methanol, water, and ammonium acetate. The
distinction between the direct ion evaporation and Cl mechanisms was
difficult to make. Although inconclusive, these results do indicate
that Cl most likely plays a major role in sample molecule ionization by
thermospray. Recommendations for future work in thermospray
fundamentals are made and focus on methods which will help to
distinguish between the direct ion evaporation and Cl mechanisms.
xiix

CHAPTER I
INTRODUCTION
This thesis describes fundamental studies of thermospray
ionization, a relatively new ionization technique used in liquid
chromatography/mass spectrometry (LC/MS). Although the thermospray
technique has experienced rapid growth and has proven to be a
successful method for the analysis of polar and labile compounds, the
actual mechanism of thermospray ionization is still unclear. The
objectives of our fundamental studies were to gain further insight into
the thermospray mechanism and to improve the reproducibility of the
thermospray ionization technique. This work was undertaken in hopes
that a better understanding of the various parameters associated with
the processes involved with thermospray would aid in the investigation
of much more complex and detailed conditions of thermospray ionization.
A better understanding of these processes should ultimately lead to a
more routine applicability of the thermospray LC/MS interface, improved
day-to-day reproducibility, and perhaps some potential instrumental
improvements.
In this introductory chapter, developments in ionization techniques
used for nonvolatile molecules are discussed. Background to the
thermospray technique along with the present understanding of the
thermospray ionization mechanism is presented, followed by an overview
of the thesis organization.
1

2
Developments in Ionization Techniques For Nonvolatile Molecules
Through the years mass spectrometry has undergone dramatic changes.
Exploration into new and novel ionization techniques has contributed
significantly to this change, resulting in the application of mass
spectrometry to an increasingly wide range of organic compounds.
Described below are the various ionization techniques used in mass
spectrometry and their capabilities and limitations.
In the late 1960s combined gas chromatography/mass spectrometry
(GC/MS) instruments with computer data systems became commercially
available so that routine quantitative analysis was possible. On these
systems electron ionization (El) was used in essentially all analytical
applications. A "standard" electron energy of 70 eV was used for both
qualitative and quantitative analyses resulting in the production of an
intense, stable ion beam, and a highly reproducible fragmentation
pattern (eqn. 1.1).
M + e* » M+ + 2e' (1.1)
Although the El mode remains the accepted preliminary ionization
mode for identification purposes, two factors exist which restrict El
to a limited range of organic compounds. First, the El mode requires
that the sample be vaporized prior to ionization which may lead to
decomposition of thermally unstable compounds. Therefore, useful
spectra of very polar and thermally labile compounds cannot be obtained
with El. Second, the use of electron energies of 70 eV usually results
in extensive fragmentation and sometimes undetectable molecular ions.

3
In many cases chemical ionization (Cl) techniques are capable of
providing complementary information to El spectra. Cl achieves
ionization without transferring excessive energy to the sample
molecules resulting in the formation of adduct ions (often a protonated
molecule) which contain the intact molecular species of the analyte.
Cl differs from El in that the sample molecules are ionized by
interaction with ions of a reagent gas rather than with electrons. The
site of protonation is most likely to occur on the heteroatom of
greatest proton affinity. Cl is the subject of a book (1) and several
reviews in the literature (2,3) where processes of ion formation are
discussed in detail. Below is a brief description of ion formation by
Cl.
Production of reagent ions is initiated by El of the reagent gas.
Even though the analyte and the reagent gas are present in the
ionization source, the reagent gas is present in great excess
(typically, the partial pressure of an analyte will be 10'^ torr,
whereas the reagent gas will be 1 torr); therefore, only reagent ions
result from El. The principle electron/molecule and ion/molecule
reactions that occur in Cl for the case involving methane as reagent
gas are illustrated in eqns. 1.2-1.6. The methane molecules are
converted to molecular ions by El, some of which decompose to fragment
ions of methane (CH3+, CH2+, CH+, etc.). The ions of methane react
with a methane molecule to produce the CH5+, C2H5+, and C3H5+ ions,
which do not react further with methane.
CH4 + 6
ch4+, ch3+, ch2+,
CH+, H2+, H+ + 2e*
(1.2)

4
ch4+- + ch4 —
* ch5+ + -ch3
(1.3)
ch3+ + ch4 —
♦ c2h5+ + h2
c2h4+ + h2
(1.4)
ch2+ + ch4 -
* C2H3+ + H2 + -H
(1.5)
C9Hi+ + CH4 —
* c3h5+ + h2
(1.6)
There are many more reactions which occur between the methane ions
and the methane molecules; however, those shown above are the most
important because they lead to reagent ions which are unreactive with
molecular methane resulting in their accumulation in the ionization
source. Most other organic molecules have a sufficiently high proton
affinity to abstract a proton from these reagent ions resulting in a
reaction as shown in eqn. 1.7. An analyte molecule, M, will interact
with a reagent ion to produce a protonated molecule of the analyte,
(M+H)+. Similar reactions with C2H5+ and C3H5+ may occur resulting in
(M+H)+ as well as (M+C2H5)+ and (M+C3H5)+ adduct ions.
M + CH5+ ♦ (M+H)+ + CH4 (1.7)
Typical Cl spectra contain abundant protonated molecular ion
species, adduct ion species (associated with the reagent gas ions) and
less fragmentation than observed in El spectra. Thus, molecular weight
information is usually more easily and reliably obtained. However, as
in the case with El, sample vaporization occurs prior to ionization and
thus, the sample must be thermally stable.
Recent research aimed at overcoming the thermal stability or
volatility barrier has resulted in a wide variety of ionization
techniques in which the direct production of pseudo-molecular ions from

5
nonvolatile molecules have been reported. These techniques produce
relatively stable even-electron species rather than the odd-electron
species produced initially by gas-phase El and can provide a
potentially attractive approach to applying mass spectrometry to large,
nonvolatile, or thermally labile compounds not amenable by conventional
ionization techniques such as El and Cl. These techniques differ
primarily in the means employed for disrupting the solid or liquid
surface of the sample. However, a common feature of all these
techniques appears to be the "direct" production of molecular ions [M+,
(M+H)+, (M+Alkali)+] from a condensed phase without the formation of a
neutral gas-phase molecule as an intermediate.
There are several techniques which have been developed for
desorbing molecular ions from a condensed phase. All of the these
techniques have been reviewed in detail (4-6) or have been described
elsewhere in various publications (7-15). These techniques are listed
in Table I according to the method of energy application and the
physical state of the sample prior to energy deposition. Samples may
be either solid or liquid on the surface, depending on the technique
used, and an electrical field may be of primary importance to the ion
production mechanism or may only serve to focus the ions into the mass
spectrometer. Below is a brief description of these ionization
techniques.
Several of these desorption ionization techniques employ electrical
energy via a high electrical field and/or ohmic heating of the sample.
They generally involve ohmic heating of a metallic substrate in contact
with the sample or the production of charged droplets in the presence
or absence of an electrical field.
These techniques include field

Table I. Summary of the major techniques used for ionizing nonvolatile molecules.
Technique
Form of Eneren/ Application
Field Desorption
Electrical field (108 V/m)
Electrohydrodynamic
Ionization
Electrical field (6 kV)
Electrospray
Electrical field (10 kV)
Thermospray
Resistive heating
252Cf Fission
High energy ions (MeV)
Secondary Ion Mass
Spectrometry
low energy ions (keV)
Fast Atan Bombardment
low energy neutrals (keV)
Laser Desorption
Power densities ranging
from 103-1010 W/cm2 (ps
to min)
Sample
Description
Solid
Application of electric field to the
surface of the field emitter in
vacuum.
Liquid
Application of electric field to a
liquid meniscus at the end of a
capillary tube in vacuum.
Liquid
Application of electric field to a
spray (uL/min) at atmospheric
pressure.
Liquid
Introduction of a spray (mL/min) of
fine droplets and particles into
vacuum.
Solid
Bombardment of the sample (placed on
foil) with fission fragments.
Solid
Bombardment of the sample with low
energy ions.
Liquid
Bombardment of the sample (contained
in a suspension of glycerol) with
low energy neutrals.
Solid
Irradiation of solid samples with a
laser.
ON

7
desorption, electrohydrodynamic ionization, electrospray, and
thermospray.
With the field desorption (FD) technique, the sample is deposited
from solution onto the surface of a field anode, usually a 10 /an
tungsten wire covered with field-enhancing microneedles. Ionization
and desorption of molecules is achieved by applying a high voltage of
about 10 kV between the wire and a 2-3 mm distant counter-electrode
while resistively heating the emitter wire. The mass spectra produced
by FD exhibit molecular ions such as IT1" and (M+H) + , and adduct ions
such as (M+Na)+ and (M+Cl)' depending on the sample solution. Three
distinctly different mechanisms contribute to the production of ions by
the FD technique. The ionization of molecules can occur by an electron
tunneling process, which takes place on the tips of the field enhancing
raicroneedles and requires a high threshold field strength (>10^ V/m) .
Ions can also be formed in condensed sample layers and extracted via a
field-induced desolvation mechanism. This mechanism requires lower
threshold field strengths for the production of gaseous ions. The
third mechanism is a thermal mechanism which is principally independent
of external fields. In general this mechanism does not contribute to
the emission of ions except in the cases of metal ions at emitter
temperatures > 800 K. Recent studies by Giessmann and Rollgen (16)
show that field emission of molecular ions depends on the temperature
of the emitter, the field strength, and the thickness and physical
state of the sample surface and that the "best anode temperature"
corresponds to melting of the solid sample on the field emitter.
The electrohydrodynamic (EHD) ionization technique developed
primarily by Evans and co-workers (17-19) is similar to FD in that a

8
high electrical field (6 kV) is applied to the sample. However, in
this case the sample is present in a liquid meniscus at the end of a
capillary tube. When liquids contained in a hypodermic syringe needle
or similar capillary are introduced into a vacuum, the interaction of
the electrical field and the liquid surface of the capillary causes a
distortion of the tip of the meniscus into a sharp cone. The liquid
surface is distorted further by the EHD process and charged droplets
are produced and accelerated away from the tip. This technique has
been shown to successfully generate pseudo-molecular ions
characteristic of any sample material which can be dissolved, together
with an electrolyte, in a nonvolatile organic solvent which can also
undergo appreciable interaction with cations and anions in solution.
In the electrospray (ES) technique developed by Dole and co-workers
(20,21) solutions are flowed through a hypodermic needle (5-20 ¿uL/min)
biased at ca. 10 kV into a spray chamber at room temperature and
atmospheric pressure. Charged droplets are formed and rapidly
evaporate until the increase in surface charge density and the decrease
in radius of curvature result in electric fields strong enough to
desorb sample ions. The resulting dispersion of ions is passed through
a small orifice into an evacuated region to form a supersonic free jet.
Ions are sampled from the core of the jet and are introduced into the
mass spectrometer using a standard nozzle and skimmer.
The thermospray ionization technique produces ions from liquid
solutions without requiring strong electrical fields or an external
source of ionization. The technique developed by Vestal and co-workers
(22-25) produces ions directly from a flow of vaporized liquid (1-2
mL/min) containing a volatile electrolyte such as ammonium acetate.

9
When this vaporized liquid is introduced into the ion source of the
mass spectrometer, a supersonic jet of vapor containing a mist of fine
particles and droplets results. Molecular ions desorb or evaporate
directly from the evaporating droplets typically forming (M+H)+ and
adducts of ammonium acetate such as (M+NH^.)"1" and (M+CH3COO)". A
detailed description of the basic principles and apparent ionization
mechanism of the thermospray ionization technique will be presented in
the next section.
Another group of ionization techniques employs particle impact on a
surface of the sample or a matrix containing the sample. In these
techniques the source of sample energization is a beam of ions or
neutrals with kinetic energies ranging from a few keV to more than 100
MeV. The compound of interest is desorbed and ionized directly from a
thin solid or liquid layer deposited on a metal substrate. These
techniques include secondary ion mass spectrometry, fast atom
bombardment, and ^^Cf fission fragment desorption.
The secondary ion mass spectrometry (SIMS) technique involves the
bombardment of a sample, either solid (26) or dissolved in solvent
(27), with a beam of primary ions and analyzing the secondary ions that
emanate from the sample surface. In most cases an ion gun which
generates Xe+ or Ar+ ions is used to produce the primary bombarding ion
beam. The primary-ion beam is usually oriented perpendicular to the
mass analyzer with the sample mounted on a probe or target which can be
adjusted to provide for an angle of incidence ranging from 0° to 90°.
The ionization mechanism of the SIMS technique appears to be identical
to the fast atom bombardment (FAB) technique (28,29).

10
In the FAB technique, a beam of atoms of Xe or Ar are directed
toward a sample contained in a polar, viscous, low-volatility solvent
(usually glycerol). Essentially the same orientation of atom beam,
sample target, and mass analyzer is used as in the SIMS technique. The
resulting energetic bombarding neutrals collide with the sample surface
producing many particles, both ions and neutrals. The bombardment of
the sample surface causes intense "hot spots" with the energy
deposition dependent on the angle on incidence. Angles of incidence
which approach 90° (grazing angle) result in the majority of the energy
deposition limited to the surface layers of the solution, resulting
primarily in the desorption of molecular ion species (30). Intact
molecular ions (M+H)+ and adduct ions (M+Na)+ which are desorbed are
characteristic of both SIMS and FAB spectra. Angles of incidence which
approach 0° result in a more intense mode of energy deposition which
causes intramolecular vibrations deeper within the matrix (31). This
orientation results in the degradation of the analyte and matrix and
consequently a low abundance of molecular ions. The presence or
absence of charge on the impacting primary particle has little effect
on the desorption process; however, the use of the neutral beam is
somewhat more convenient with mass spectrometers in which the ion
source is at high potential.
The fission fragment desorption technique employs a beam of
fission fragments resulting from the radioactive decay of ^52Cf which
penetrates the sample coated on a thin (<10 nm) nickel foil (32,33).
The fission fragments pass through a 10 nm diameter region of the foil
depositing considerable amounts of energy and inducing electronic
excitation. Atoms and molecules within the foil perceive the swiftly

11
moving ions passing through the matrix as equivalent to a short burst
of photons (32). The electromagnetic radiation induces electronic
excitation, which quickly dissipates into a complex spectrum of atomic,
molecular, and matrix excitation, resulting in the desorption of intact
molecular ions, (M+H)+, and adduct ions, (M+Na)+, from the sample.
Finally, techniques which involve lasers with power densities
ranging from ca. 10^ to 10^® w/cm^ and wavelengths from the UV to the
far-IR have been successfully employed to produce desorption of ions
from solid samples (34-36) or samples contained in a liquid matrix such
as glycerol (37) . The laser (essentially a photon gun) is used to
bombard a specific area of the sample with photons in a manner
analogous to the SIMS and FAB techniques. Protonated molecules or
adduct ions with cations such as Na+ or K+ are typically produced, with
the extent of their fragmentation influenced by the power of the laser.
From a review of the recent literature on ionization of nonvolatile
molecules, it is evident that a wide variety of dissimilar techniques
produce mass spectra with similar features, even-electron pseudo-
molecular ions from nonvolatile molecules. The primary processes
involved in the first-step (energy deposition) are apparently quite
dependent on the kind of excitation used; furthermore, some of the
properties of the intermediate must depend on the nature of the primary
processes. It is not yet clear, however, whether the final step
leading to the production of a pseudo-molecular ion is dependent or
independent of the primary processes occurring in the condensed phase
or whether neutral gas-phase molecules play an integral role in pseudo-
molecular ion formation via Cl processes.

12
Thermosnrav LC/MS Background
For over a decade, several research groups have worked on the
development of a practical LC/MS interface. Because of the basic
incompatibilities between high performance liquid chromatography (HPLC)
and MS, the successful on-line coupling of these instrumental
techniques has been difficult to achieve (38-41). This is because the
normal operating conditions of a mass spectrometer (high vacuum, high
temperatures, gas-phase operation, and low flow rates) are opposed to
those used in HPLC (high pressures, low temperatures, liquid-phase
operation, and high flow rates).
The moving belt interface (42,43) and direct liquid introduction
(44-47) are two of the principal methods which have been used for LC/MS
interfacing other than thermospray. These two techniques are capable
of generating El (moving belt) and/or Cl (either technique) spectra of
volatile and nonpolar type compounds which are also amenable to GC/MS
techniques. The moving belt interface provides one of the more
versatile approaches to coupling LC to MS in that it permits the
operation of the mass spectrometer in both the El and Cl modes. In
this method the LC effluent is deposited on a polyimide or Kapton belt
where it is desolvated; it is then transported into the ion source
region of the mass spectrometer where the remaining sample residue is
flash vaporized and ionized. Direct introduction of the effluent from
a LC into the ion source was first described by McLafferty and co¬
workers (48). In this technique, a fraction of the eluent (sample and
mobile phase) is passed through a capillary tube at flow rates of 10-60
¿iL/min. Microdroplets are formed through a pinhole (ca. 4 ¿¿m diam.)
and directed into the ions source where the neutral sample molecules

13
undergo Cl processes with the mobile phase serving as the Cl reagent
gas.
While it may be desirable and perhaps more convenient in certain
situations to have a LC/MS technique which is capable of providing El
and Cl information identical to GC/MS, the moving belt and direct
liquid introduction techniques do not provide for a complementary
method; namely, the ability to provide useful mass spectral information
of chromatographically separated polar and thermally labile compounds
which are not amenable to GC/MS techniques. Although other recent
LC/MS approaches such as atmospheric pressure ionization (49), liquid
ion evaporation (50), electrospray (51,52), and the monodisperse
aerosol generator for introduction of liquid chromatographic effluents
(53) have attempted to provide for this capability, thermospray has by
far been the most successful, and as a result, has been the most widely
used. All of the LC/MS interfacing techniques listed above differ in
their principle of operation; however, all are capable of transport,
desolvation, and ionization of a liquid sample. Several reviews (54-
58) have appeared in the literature which describe in more detail the
principles of operation of these techniques. Thermospray is unique of
the LC/MS approaches in that it is the only technique which does not
require an external form of ionization or external electrical fields.
The thermospray technique, developed by Blakley and Vestal (22-25),
emerged from efforts to develop an LC/MS interface suitable for
efficiently analyzing samples dissolved in aqueous mobile phases at
typical HPLC flow rates (1-2 mL/min). The original approach involved
the production of a molecular beam of the vaporized effluent which
could be directed into an El or Cl source until it was accidently

14
discovered that, under certain conditions of flow rate, temperature,
and mobile phase composition, ions were produced without the use of a
filament (22). Early attempts to vaporize the liquid eluent employed a
CO2 laser which was focused on droplets exiting the probe. However,
when the laser was accidently focused on the end of the vaporizer tube
instead, it was discovered that gas-phase ions could be produced
without the use of an external ionization source (23). The laser was
later replaced by an array of oxy-hydrogen torches (24,25). In the
present version, the power required to vaporize the liquid is supplied
by passing an electrical current through the capillary tube itself.
Thermospray LC/MS interfaces have been commercially available for
several years with a wide variety of applications reported in the
literature. The analysis of peptides and amino acids (59,60),
glucuronides (61), pesticides (62), dyes (63), and metabolites in
biological fluids (64) have been described recently. In general the
mass spectra obtained from these nonvolatile compounds contain
protonated molecular ions as the major peaks with few structurally
significant fragments. This lack of fragmentation is a significant
limitation in attempting to use thermospray LC/MS to determine the
structure of eluted compounds.
An instrumental development was recently reported which employs the
use of a repeller electrode in which increased fragmentation was
observed in thermospray mass spectra of caffeine, D 4030, D 2439,
bambuterol, and 16-hydroxyprednisolone (65). In most examples repeller
voltages of 120-180 V have been used to produce significant
fragmentation. The thermospray source which was used in our studies
was a relatively early model and was not equipped with a repeller

15
electrode; however, we have at our disposal collision-induced
dissociation and MS/MS to provide structural information.
Present Understanding of the Thermosnrav Ionization Mechanism
Thermospray begins with the spraying of aqueous mobile phases
containing a volatile electrolyte such as ammonium acetate through an
electrically heated stainless steel capillary into the source region of
the mass spectrometer at flow rates of 1 to 2 mL/min. A simplified
thermospray vaporization scheme is shown in Figure 1.1 and illustrates
the four major processes which occur during thermospray ionization
(vaporization, droplet evaporation, droplet charging, and direct ion
evaporation/CI). The vaporization of liquids in vacuum results in the
production of a supersonic jet of vapor containing a mist of fine
droplets and particles with estimated velocities of 10^ cm/s (22). A
portion of the droplets produced are electrically charged, with an
equal distribution of positively and negatively charged droplets
(66,67). The actual size of the droplets is dependent on the vaporizer
tip diameter and the conditions affecting vaporization (temperature and
flow rate). As the droplets travel through the heated source region
they continue to vaporize because of the heat input from the
surrounding hot vapor. As the charged droplets shrink to a certain
diameter (1 /¿m) the electric field increases until it is strong enough
(10^ V/m) to eject ions (direct ion evaporation) from the liquid
surface (68,69). Ions are then sampled into the mass analyzer region
through a conical exit aperture.
Two distinct stages are believed to be involved in the formation of
thermospray ions (Figure 1.2). The first stage is the formation of

Figure 1.1
A simplified thermospray vaporization scheme illustrating the
four major processes which occur during thermospray
ionization: vaporization, droplet evaporation, droplet
charging, and direct ion evaporation/CI.

VAPORIZATION
r
DROPLET EVAPORATION
DROPLET CHARGING
O o
0 0 00Ooo o o .o .
0 0°o 0 °° o • °
DIRECT ION
EVAPOR ATION/CI

Figure 1.2
Possible mechanisms for the formation of thermospray i
from evaporating droplets by direct ion evaporation or
chemical ionization.

Direct Ion Evaporation
4 T
I
NH3 + MH +
vs. Chemical Ionization

20
gaseous ions out of the microdroplets produced by a direct ion
evaporation mechanism. These ions will be referred to as primary
thermospray ions. Primary ions produced in the thermospray process are
apparently identical to those present in solution. For example,
ammonium acetate produces (NH4)+ and (CH3COO)' ions with clusters of
these ions with water, ammonia, and acetic acid. The second stage
involves the primary thermospray ions in gas-phase ion/molecule
reactions with neutral gas phase sample molecules. These Cl reactions
may remove the primary thermospray ions and produce new product ions.
This process has been recognized as of possible importance in the
thermospray ionization mechanism but has received much less attention
than the primary direct ion evaporation processes.
The difficulty in elucidating further details of the thermospray
mechanism lies in the complexity of the thermospray plume itself. The
thermospray plume consists of a dynamic collection of evaporating
droplets and particles moving at supersonic speeds, producing ions
without an external source of ionizing electrons. Studies performed by
Iribarne and Thomson (68,69) using an atomizer to generate charged
droplets have shown that small ions can separate or "evaporate" from
evaporating droplets carrying electrical charges. This model of ion
evaporation from liquid droplets appears to account satisfactorily for
the initial ionization (primary ions) observed in the thermospray
technique; however, it may not account for ionization produced in the
later stages of evaporation (secondary ions). In our studies we have
chosen to retain the integrity of the thermospray plume and perform
studies on actual thermospray plumes under actual thermospray

21
conditions of temperature and pressure in order to better understand
the processes occurring in these later stages of ion evaporation.
Our experiments were based on the fact that a particular
instrumental parameter or solution characteristic can greatly affect a
particular thermospray process. For example, source temperature would
be expected to have a major role in affecting the droplet evaporation
process. The approach used in our studies was to first obtain direct
physical measurements of actual thermospray plumes in vacuum and then
determine the effects of instrumental parameters and solution
characteristics on the ionization of a pure buffer (ammonium acetate)
and of a model compound (ribavirin) prepared in ammonium acetate.
Ribavirin was a particularly attractive choice as a model compound
because of its thermal lability which renders classical mass
spectrometric ionization techniques ineffective; thus, to date little
mass spectral data has been obtained on the compound. Also, ribavirin
is currently under investigation as a broad-based anti-viral compound
for which there is no sensitive analytical method currently available;
therefore, the characterization of its thermospray mass spectral
behavior would be desirable.
Overview of Thesis Organization
This work represents a systematic study of the effects of
instrumental parameters and solution characteristics on thermospray
ionization. The remaining chapters which follow this introductory
chapter will provide a detailed description of the experimental set-up,
and will discuss the results of our fundamental studies of thermospray
ionization. Chapter II will describe the various instruments used in

22
these studies which include a thermospray LC/MS interface, HPLC pumps,
mass spectrometer, and a specially designed vacuum chamber.
Preliminary studies dealing with the characterization of the Vestec
thermospray LC/MS interface are discussed in Chapter III. In Chapter
IV the results of direct physical measurements of actual thermospray
plumes in vacuum will be discussed. The effects of instrumental
parameters (Chapter V) and solution characteristics (Chapter VI) will
be discussed with respect to pure ammonium acetate and a ribavirin
standard prepared in ammonium acetate. The thesis ends with
conclusions and future work (Chapter VII), which will state our
conclusions from these fundamental studies and provide recommendations
for future work in this area of research based upon our results.

CHAPTER II
EXPERIMENTAL
Samples and Reagents
Analytical-grade ammonium acetate was obtained from Fisher
Scientific (Fair Lawn, New Jersey), and Mallinckrodt (St. Louis,
Missouri). Ribavirin was received as a gift from Dr. B.J. Gabrielson
of the U.S. Army Medical Research Institute of Infectious Diseases
(Fort Detrick, Frederick, Maryland). Purine, 2-aminopurine, adenine,
2-hydroxypurine, 2,6 - diamino - 8 - purino1, 6 - thioguanine , 6-
mercaptopurine, xanthine, hypoxanthine, 6-thioxanthine, uric acid,
allantoin, alloxan monohydrate, and histidine were obtained from Sigma
Chemical Co. (St. Louis, Missouri). All reagents were analytical
grade.
Vestec Thermosprav LC/MS Interface
A Vestec thermospray LC/MS interface (Vestec Corp., Houston, TX)
was used. A schematic representation of the Vestec thermospray
interface is shown in Figure 2.1. The interface consists of a
vaporizer probe, thermospray ion source, equipped with a thoriated
iridium filament, discharge electrode, and pump out line (1 cm i.d.) to
a 300 L/min mechanical pump. Below is a description of the interface
components and a brief description of their primary functions.
The vaporizer probe consists of a stainless steel capillary tube
(0.015 mm i.d. X 1.5 mm o.d.) located inside a 6.5 mm o.d. probe which
is inserted into the mass spectrometer via a vacuum lock. Two
thermocouples are spotwelded to the capillary, one located near the
inlet and the other near the tip of the probe. The thermocouple
23

Figure 2.1
A schematic representation of the Vestec thermospray LC/MS
interface. In our source the vapor temperature thermocouple
was replaced with a 1 mm i.d. tube which was connected to a
Granville-Phillips thermogauge for pressure measurement.

ION SAMPLING
TRAP
a
MECHANICAL PUMP

26
positioned at the inlet measures the temperature at which no
vaporization is occurring and is commonly referred to as Tq or the
control temperature. The thermocouple located at the tip measures T2
or the vaporizer tip temperature. The power required to vaporize the
liquid is supplied by passing an electrical current through the
capillary tube itself. A feedback controller is used to control the
power input to the vaporizer so that Tq is maintained constant. A
digital panel meter displays either Tq or T2.
The Vestec thermospray ion source is specially designed to
accommodate the excess vapor flows being delivered from the vaporizer
probe. Flow rates of up to 2.0 mL/min could be handled by the source
without shutting down the mass spectrometer. The source consists of a
4.11 cm X 1.91 cm X 1.27 cm block of stainless steel with openings for
the vaporizer probe (0.95 cm diam.), source heater (0.64 cm diam.),
pump out line (0.95 cm diam.), and an ion sampling cone (1.27 cm
diam.). A thermocouple is located downstream past the ion sampling
cone to measure the vapor temperature. The power input to a cartridge
heater is controlled by a temperature controller so that either the
vapor temperature or source block temperature is held constant. The
ion sampling cone measures 0.64 cm in height and the aperture is 0.050
mm in diameter. With the probe fully inserted into the source the
distance between the probe tip and the ion sampling cone aperture was
0.6 cm. We modified the source to accommodate a thermogauge so that
source pressures could be measured. (This line for source pressure
measurements was not a standard feature on the ion source.) Typical
indicated pressures inside the source ranged from 1.8 to 2.6 Torr. The
source pressure could be increased to a maximum of 8 Torr by partially

27
closing a valve on the pump out line. The source pressure line (1 mm
i.d.) was made to be interchangeable with the vapor thermocouple; thus,
only one reading (vapor temperature or source pressure) could be
obtained at any one time. In these studies the source was operated to
measure source pressure exclusively. Although the source was equipped
with external sources of ionization, a thoriated iridium filament and a
discharge electrode, they were not used in these studies.
HPLC Pumps
An ISCO LC-5000 precision syringe pump (ISCO, Inc., Lincoln, NE)
was used for all thermospray studies. The pump was capable of
producing a precise, pulse-free delivery of liquids (500 mL capacity)
at flow rates ranging from 1.0 /xL/min to 6.7 mL/min with an output
pressure limit of 3,700 psi. The pump operates in a constant flow mode
and the pump pressure was digitally displayed in psi. The syringe
barrel and piston are made of 304 stainless steel and the pump seals
are graphite-filled Teflon. When changing solvents, the cylinder was
refilled and pumped three to four times with 50 mL volumes of the
desired solvent to ensure the thorough rinsing of the pump.
For experiments dealing with pump pulsing and plume measurements in
vacuum, an EM Science MACS 100 pump was used. This pump is a single
piston, constant stroke, reciprocating HPLC pump capable of producing
accurate flow rates ranging from 5 ¿xL/min to 5.0 mL/min with a pressure
limit of 6000 psi. Along with flow rate, the refill speed (125 to 650
ms) could be controlled. Slow refill speeds were selected to yield a
pulsing flow while fast refill speeds were selected to yield a smoother
solvent flow.

28
Mass Spectrometer
A Finnigan triple stage quadrupole (TSQ45) mass spectrometer was
used. The El and Cl spectra were obtained using a standard Finnigan
4500 EI/CI ion source with solids probe introduction of the samples.
Samples were heated and vaporized rapidly from 45 to 400°C in 3-4
minutes under temperature control. Electron energies of 70 and 100 eV
were used for El and Cl, respectively. Source pressure for ammonia was
0.60 torr with the source temperature maintained at 140°C. Nitrogen
was used as the collision gas at pressures ranging from 1.2 to 2.5
mTorr. Collision energies of 25 to 30 eV were used. The electron
multiplier was typically operated at -1000 V for full scan mass spectra
and -1500 V to -1700 V for MS/MS spectra.
The mass spectrometer used was not dedicated to LC/MS resulting in
the frequent conversion back and forth between the standard EI/CI
source and the thermospray source. Thus, normal El tuning was
performed on a standard EI/CI source with the standard calibration
compound perfluorotributylamine (PCR Research Chemicals, Inc.,
Gainesville, FL) on the TSQ45 prior to installation of the thermospray
source. After installation of the thermospray source the lens voltages
were optimized with a 0.1 M ammonium acetate solution followed by a
solution of polyethylene glycol (PEG 200) (Sigma Chemical Co., St.
Louis, MO).
Vacuum Chamber Measurements
The vacuum chamber consisted of a modified Finnigan 1015 vacuum
console designed with entrance ports for the vaporizer probe,

29
temperature, pressure, and current probes, a six inch diameter
observation window, and a pump out line connected to a 300 L/min
mechanical pump. The pump-out line was situated 180° from the
vaporizer probe. Temperature and pressure measurements of the plumes
were obtained by placing the thermocouple (1 mm o.d.) and the pressure
sampling tube (2 mm i.d.) directly into the center of the plume,
perpendicular to the direction of the vapor and droplet flow from the
vaporizer probe. Current measurements were made using a Keithly 480
picoammeter (Cleveland, OH) obtained in the same manner as with the
temperature and pressure measurements with a 2" X 2" wire mesh screen
placed into the center of the plume. Sampling distances were varied by
adjusting the distance of the vaporizer probe from the sampling probes.

CHAPTER III
PRELIMINARY CHARACTERIZATION OF THE VESTEC THERMOSPRAY LC/MS INTERFACE
Preliminary characterization of the Vestec thermospray LC/MS
interface involved vaporization studies performed outside the mass
spectrometer where measurements of temperatures along the outside of
the vaporizer capillary were obtained. The resulting temperature
profiles were used to characterize the effects of tip temperature and
flow rate on vaporization and predict the state of the liquid inside
the probe under the various vaporization conditions typically
encountered with thermospray. These characterization studies also
involved the acquisition of thermospray mass spectra obtained under
"standard" thermospray conditions so that the features typical of
thermospray ionization mass spectra could be readily observed on our
LC/MS interface. These preliminary and general performance studies
were beneficial in providing results and information which were used
for the experiments discussed in Chapters IV and VI.
Vaporization Studies
The vaporization process of thermospray is responsible for the
production of a plume of droplets and vapor from a flow of liquid.
Flow rate and tip temperature are the necessary instrumental parameters
to achieve the partial or complete vaporization of a liquid mobile
phase. In these studies of vaporization, the effects of flow rate and
tip temperature on the temperature of the outside of the thermospray
vaporizer probe were determined. These studies involved experiments
which were performed outside the mass spectrometer on a vaporizer probe
30

31
with the outer (6.5 mm o.d.) metal sheath removed exposing the bare
(1.5 mm o.d.) capillary. From these studies information on the
processes occurring inside the probe during various conditions of
vaporization was obtained.
Characterization of Thermosprav Plume Types
Two thermocouples are positioned along the capillary tube to allow
for temperature measurements at the point where electrical heating
starts (Ti), and at the tip of the probe (T2)• These temperatures are
commonly referred to as the control temperature or T]_, and the tip
temperature or T2 • By measuring these temperatures on the vaporizer
probe as it is heated at a constant flow rate, a plot can be generated
which represents the resulting tip temperature (T2) versus the control
temperature (T^), related to the power input to the vaporizer probe.
With these plots the various stages of vaporization can be illustrated.
We performed a "T2 versus T]_" experiment with 1.0 mL/min of water to
illustrate these stages of vaporization and to help characterize the
thermospray plumes.
Figure 3.1 shows a typical T2 versus T^ plot obtained for water
sprayed outside the mass spectrometer at a flow rate of 1.0 mL/min.
These plots are typically characterized by three slopes or regions
which represent the state of the liquid sample exiting the probe tip.
The first region (I) of the plot represents the steady heating of the
mobile phase. No vaporization occurs in this region as only liquid
exits the probe (tip temperature below the boiling point). The second
region (II) of the plot represents the partial vaporization of the
mobile phase. In this temperature region a fine mist of droplets and

Figure 3.1
A plot of tip temperature (T2) vs. control temperature (T;l)
for water at 1.0 mL/min. Region I represents the steady
heating of the liquid mobile phase with no vaporization
occurring. As more power is input to the probe vaporization
begins (A) as the tip temperature reaches the boiling point
of the liquid (100 C) . The tip temperature increases more
slowly in region II as much of the heat input is used to
transform the liquid into vapor. Point B represents the
temperature at which 100% vaporization is achieved as all of
the liquid is transformed in to vapor. The tip temperature
increases rapidly in region III as only vapor exits the probe
tip.

LO
LO

34
vapor are observed to exit the probe. As the tip temperature is
increased, a greater fraction of the mobile phase is transformed into
the vapor phase (wet to nearly dry plumes) . The boundaries of this
second region are marked by two points, point A and point B. Point A
corresponds to the temperature at which vaporization begins (T2 =
boiling point of the liquid) while point B corresponds to the
temperature at which 100% vaporization of the mobile phase is achieved.
The third region (III) of the plot represents the complete vaporization
of the mobile phase. At these tip temperatures only vapor exits the
probe tip (dry plume) resulting in the rapid increase in tip
temperature.
From this study three distinct plume types can be defined with
respect to their apparent droplet content and physical characteristics
(see Figure 3.1). Wet plumes consist of relatively large droplets and
can be produced when operating at relatively low temperatures within
region II. These plumes appear as a dense cloud of droplets and vapor
accompanied by a "sizzling" sound as when water is sprayed on a hot
frying pan. When an object is placed within a wet plume at atmospheric
pressure, condensation occurs on the surface. Nearly dry plumes appear
to contain smaller droplets than wet plumes and can be produced when
operating at higher temperatures within region II. When an object is
placed within a nearly dry plume at atmospheric pressure, no
condensation occurs on the surface. Dry plumes consist of only vapor
(invisible plume) and can be produced when operating at temperatures
within region III. No condensation occurs on the surface of an object
when placed within a dry pítame. These three plume types can be
reproduced for a given flow rate and will be referred to in Chapters

35
IV-VI. Further studies were performed which investigated the effects
of flow rate and temperature (power input) on vaporization and are
discussed below.
Variable Flow with Constant Heat
To determine the effects of flow rate on vaporization, temperature
was measured along the vaporizer capillary (1.5 mm o.d.) under
conditions of variable flow with fixed control temperature. A
thermocouple was placed directly on the side of the probe until a
stable temperature reading was attained (ca. 10 sec). A plot of probe
temperature versus distance along the probe across a 30 cm portion of
the probe at flow rates of 0.5 mL/min, 1.0 mL/min, and 4.0 mL/min is
shown in Figure 3.2. Increased flow rates result in higher
temperatures along the probe, higher pump back pressures, and a larger
plume of droplets and vapor (visually observed to be larger in diameter
and contain larger droplets).
We believe the increase in temperature along the probe with
increasing flow rate is due to compression of the vapor inside the
probe. As vaporization occurs at some point within the probe, the
volume increases enormously, resulting in the compression of this vapor
and an increase in pump back pressure. At higher flow rates the
compression is greater; therefore, the pressure increases. With the
increase in pressure, the average speed of the gas molecules increase
which results in an increase in temperature. This process is analogous
to the behavior of a gas contained in a cylinder with a moveable
piston. The pressure of the gas is the net effect of the collisions of
the moving molecules with the walls, tending to push the piston out.

Figure 3.2
A plot of probe temperature versus distance along the probe
for water at flow rates of 0.5 mL/min, 1.0 mL/min, and 4.0
mL/min with constant control temperature.

140
120
100
80
60
40
Nearly Dry H20 Plumes
o 0.5 mLIrnln
o 1.0 mLlmin
* 4.0 mLlmin
Tip = 124 C
Tip = 151 C
Tip = 215 C
II I I I II I I | I I I I! II II | I! II I'Tl I i ¡ I I II I I I I I | I I II I I I I r | II I
) 5 10 15 20 25
T I I I | I I I II l"l ITT]
30 35
Distance Along Probe (cm)
u>

38
When the piston is moved downward into the cylinder, the gas is
compressed, resulting in the increase in temperature and the increase
in the average speed of the molecules.
At each flow rate a temperature maximum was observed at various
distances along the probe. For example, temperature maxima occur at
ca. 8 cm for the flow rates of 0.5 and 1.0 mL/min. Temperatures along
the probe decrease from this maximum towards the vaporizer tip. We
believe this cooling effect is due to the evaporation of the droplets
produced within the probe and the temperature maximum represents the
position within the probe corresponding to the onset of vaporization.
Temperature along the probe is maximum at ca. 17 cm for water at 4.0
mL/min. This shift in temperature maxima can be explained by the fact
that the same amount of power is input to the probe at 1.0 and 4.0
mL/min. With a greater flow rate we predict that an increase in the
power input would be required with higher flow rates to produce
temperature maxima at corresponding distances along the probe.
However, with a constant power input the temperature maximum shifts
towards the probe tip which indicates that the onset of vaporization
occurs at distances closer to the probe tip with increasing flow rates.
Constant Flow with Controlled Heating
To determine the effects of control temperature (related to power
input) on vaporization, temperature was measured along the vaporizer
probe under conditions of constant flow with controlled heating. Three
T^ temperature settings were selected at a 1.0 mL/min flow rate of
water to produce the three thermospray plume types, wet, nearly dry,
and dry, which correspond to visual and physical observations made with

39
the probe spraying into an open room at atmospheric pressure. A plot
of probe temperature versus distance along the probe at conditions of
constant flow and controlled heating is shown in Figure 3.3. Three
control temperatures were selected which produced the three described
plume types.
The increase in power to the vaporizer probe at a constant flow
rate resulted in higher temperatures along the probe, higher pump back
pressures, and a more complete vaporization of the mobile phase.
Temperatures toward the end of the probe decreased for the temperature
settings corresponding to wet and nearly dry plumes. However, at the
temperature setting which resulted in the production of a dry plume,
the probe temperature increased. The temperature increases here
because only the limited heat capacity of the vapor (dry plume) is
available to absorb the input heat. The gradual decrease in
temperature along the vaporizer probe observed at various portions of
the plot is probably due the evaporation of the resulting droplets
within the probe (similar to the flow rate study discussed above). The
increase in HPLC pump backpressure with increasing power input is
probably the result of more vapor created within the vaporizer probe.
The excess vapor cannot escape as rapidly as when it was liquid,
resulting in increased backpressures.
From the discussion of the effects of flow rate on temperature
measurements along the probe, it would seem likely that increasing the
power input to the vaporizer probe at a constant flow rate would result
in increased vaporization efficiency while shifting the vaporization
onset position (temperature maximum) further away from the probe tip.
Although increased vaporization was observed with an increase in power

Figure 3.3
A plot of probe temperature versus distance along the probe
for water at 1.0 mL/min with controlled heating. This
resulted in the production of wet, nearly dry, and dry
plumes.

160
140
120
100
80
60
40
20
* Dry Tip = 263
i ii i ri ii i | n ii i i i ii | i i i ii ii rT'jT'nrrn m | i n mm | i i i i i i i i i | ii i i i i n i |
5 10 15 20 25 30 35
Distance Along Probe (cm)
4>-

42
input to the probe, this shift in temperature maximum along the probe
was not clearly evident. A temperature maximum was observed at ca. 15
cm for a wet plume, while nearly dry and dry plumes each appear to
generate two temperature maxima which might be indicative of a sequence
of vaporization followed by condensation of the resulting vapor and
droplets followed by re-vaporization of the liquid. From these results
we cannot be certain whether this sequence is actually occurring or
whether this is just a reflection of the heating behavior of the probe.
A more detailed investigation into this heating process at various flow
rates might provide more conclusive results.
Thermosprav Mass Spectra
The second portion of our preliminary characterization studies
involved the acquisition of positive and negative ion full scan
thermospray mass spectra of ammonium acetate, ribavirin, and several
purine and xanthine-type compounds. In cases where more fragmentation
was desired for structural information, daughter spectra were obtained
and the resulting fragmentations were interpreted. Spectral
characteristics of thermospray such as the observed ions of the
ammonium acetate buffer, formation of pseudo-molecular ions and
fragment ions are discussed. Experiments performed with ribavirin to
determine the reproducibility of positive and negative ion intensities
and relative abundances of the ions contained in the mass spectra are
described. Preliminary observations on the effect of various
functional groups on thermospray performance are discussed.

43
Ammonium Acetate
Positive ion and negative ion full scan thermospray mass spectra of
ammonium acetate obtained at 1.5 mL/min are shown in Figure 3.4.
Abundant positive ions at m/z 18, 36, 54, 60, 77, and 78 which
correspond to NH4+, NH4+-H20, NH4+-2H20, CH3CONH3+, CH3C0NH2NH4+, and
CH3COOHâ– NH4+, respectively, were present in the positive ion mass
spectrum while abundant negative ions at m/z 59, 77, and 119 which
correspond to CH3COO", CH3C00’-H20, and CH3C00H-CH3COO", respectively,
were present in the negative ion mass spectrum. A negative ion was
observed at m/z 155 in the spectrum of ammonium acetate at a very low
relative abundance and corresponds to CH3COOH•CH3COO*•2H2O.
Figure 3.5 shows the MS/MS daughter spectra obtained for selected
thermospray ions of ammonium acetate. The positive daughter spectrum
of m/z 36 shows the loss of 18 (H2O) resulting in the formation of NH4+
at m/z 18. The positive daughter spectrum of m/z 60 shows the loss of
17 (NH3) resulting in the formation of the fragment ion at m/z 43 which
corresponds to CH3C0+. Here it is assumed that the positive ion at m/z
60 is not an acetic acid ion (CH3C00H)+ but rather (M+H)+ of an
impurity of acetamide, CH3CONH2. Likewise, the positive ion at m/z 77
is presumably not a M+ of ammonium acetate (C^COO' • NH4+) but rather an
ammonia adduct ion formed from the acetamide impurity, CH3CONH2NH4+.
The positive daughter spectrum of m/z 77 shows two successive losses of
17 (NH3) to yield the fragment ions at m/z 60 (amide impurity) and m/z
43 (CH3CO). Another fragmentation observed was the loss of 59
(CH3CONH2) resulting in the formation of NH4+ at m/z 18. The negative
daughter spectrum of m/z 59 shows that this ion is very stable to
fragmentation. We presume this behavior to correspond the acetate ion

Figure 3.4
Positive and negative ion full scan thermospray mass spectra
of 0.1 M ammonium acetate obtained at 1.5 mL/min.

MIZ
% Relative Abundance
% Relative Abundance
cn
<£>
CD
S
N
cn
a:
-CO +
a:
o
S<7
160.0

Figure 3.5
MS/MS daughter spectra obtained for selected positive and negative ions
of ammonium acetate. A collision energy of 25 eV was used with 1.2
mTorr collision gas pressure (N2).

% Relative Abundance
% Relative Abundance
§
i
?
m
HN HNOO HO
% Relative Abundance
% Relative Abundance
% Relative Abundance
â– O
OTH-/HN

48
(CH3C00)'. The negative daughter spectrum of m/z 119 shows the loss of
60 (CH3COOH) to yield the acetate fragment ion at m/z 59.
All positive ions of ammonium acetate produced by thermospray at
these conditions contain an ammonium ion in their structure, while the
negative ions contain an acetate ion. Under these conditions the
positive and negative reconstructed ion currents (RIC) were essentially
equal. The origin of the acetamide impurity is not known. This
compound may be the result of a side reaction which occurs during the
commercial synthesis of ammonium acetate. Buffer solutions were
prepared from ammonium acetate obtained from two sources, Fisher
Scientific and Mallinkrodt. The resulting spectra were virtually
identical, each containing abundant positive ions at m/z 60 and 77
corresponding to the (M+H)+ and (M+NH4)+ of the amide impurity.
Ribavirin
The positive ion full scan thermospray mass spectrum of 1 /¿g of
ribavirin injected is shown in Figure 3.6. The spectrum contains ions
at m/z 113, 130, 245, 262, and 489 which correspond to the protonated
triazole-carboxamide ring of ribavirin, the NH4+ adduct of the
triazole-carboxamide molecule, (M+H) + , (M+NH4) + , and (2M+H) + ,
respectively. The negative ion full scan thermospray mass spectrum of
ribavirin is shown in Figure 3.7 and contains ions at m/z 111, 171,
243, 303, and 355 which correspond to the loss of a proton from the
triazole-carboxamide ring, the acetate adduct of the triazole-
carboxamide molecule, (M-H)‘, (M+CH3COO)', and (M+lll)*, respectively.
Although not observed in this particular mass spectrum, a (2M-H)' ion
at m/z 487 was frequently observed. In both the positive ion and

Figure 3.6
Positive ion full scan thermospray mass spectra
ribavirin obtained at 1.5 mL/min of 0.1 M ammonium
Other instrumental parmeters were as follows
temperature â– = 208 C; source temperature = 250 C
pressure =■ 2.2 Torr; probe position « 1.6 cm.
of 1 ug
acetate.
tip
; source

% Relative Abundance
cn Ci
o C.'
05
(M + H)

Figure 3.7
Negative ion full scan thermospray mass spectra of 1 ug
ribavirin obtained at 1.5 mL/min of 0.1 M ammonium acetate.
Other instrumental parameters were as follows: tip
temperature — 208 C; source temperature = 250 C; source
pressure = 2.2 Torr; probe position = 1.6 cm.

M/E 100 150 2Ó0 250 300 350 400 450 5Ó0
% Relative Abundance
zs
(M+CHr>COO)

53
negative ion spectra the ammonium acetate ions are observed in the
background. Under the particular conditions at which these spectra
were obtained, the summed negative ion intensity of ribavirin was about
ten times greater than the summed positive ion intensity. The (M+H)+
and (M+CH3COO)" ions of ribavirin were present as the base peaks in
their respective spectra. In both ion modes abundant fragment ions
were formed. Especially interesting in the negative ion mode was the
addition of the triazole-carboxamide ring fragment (m/z 111) to a
molecule of ribavirin which suggests the occurance of gas-phase
ion/molecule reactions.
Reproducibility of Thermosprav Spectra of Ribavirin
Because a mass spectrometer dedicated for thermospray was not
available in our laboratory, the standard EI/CI source was replaced
with the thermospray source prior to its use. The thermospray source
remained on the mass spectrometer for two to four days at a time and
yielded reproducible ion intensities and mass spectra during this time.
However, from week-to-week the relative abundances of ions observed in
the mass spectra would sometimes change as well as the relative
positive and negative ion intensities. These week-to-week fluctuations
are illustrated below.
The positive ion full scan thermospray mass spectrum of ribavirin
shown in Figure 3.8a was acquired three weeks prior to the spectrum
shown in Figure 3.8b. Although these spectra were obtained under
identical conditions of thermospray (instrumental parameters and
solution characteristics), the resulting spectra are significantly
different. The low relative abundances of the ions at m/z 113 and 489

Figure 3.8
Positive ion full scan thermospray mass spectra of ribavirin
acquired several weeks apart under identical thermospray
conditions and scanning parameters. Instrumental parameters
for the spectrum shown in a) were as follows: flow rate =
1.5 mL/min; tip temperature = 209 C; source temperature = 253
C; source pressure = 2.2 Torr; probe position = 1.6 cm.
Instrumental parameters for the spectrum shown in b) were as
follows: flow rate *= 1.5 mL/min; tip temperature = 209 C;
source temperature = 254 C; source pressure = 2.4 Torr; probe
position = 1.6 cm.

% Relative Abundance
% Relative Abundance
S
m
cn
G
3
cn
G
G
G
m
G
G
G
cn -
® J en
_”nJ
^-sl
G-
G
•
cn —
CD -
rO â– 
CD
G
ro
cn â– 
CD
co
CD-
CD -
CO
cn —
CD •
-
C£> -
cn —
g. â– 
cn
G-
CD
-CO
cn
ro
-cn
ho
-CD
CD
cn -
CD
CD—
CD -
cn —
CD
G-
G
ro
cn •
CD
CO
O-
CD
CO
cn —:
GD
G-
CD
U\ •
G - -
cn
G-
G)
-CO
cn
-co
G
ro
—u
cn
ro
-cn
ro
1
s>
X.
: j-c°
•;
sg
100.0

56
(observed in Figure 3.8a) are the major differences between the two
spectra. However, the relative abundances of the ions at m/z 130, 245,
and 262 remain relatively constant and the pseudo-molecular ion at m/z
245 remains the base peak. The negative ion full scan thermospray mass
spectra of ribavirin (Figure 3.9), also acquired three weeks apart,
showed similar differences as observed with the positive ions. The
decreased relative abundances of the ions at m/z 111, 243, 355, and 487
(observed in Figure 3.9a) are the major differences between the two
spectra while the pseudo-molecular ion at m/z 303 remains the base
peak.
Purine and Xanthine-type Compounds
As part of our preliminary characterization of the LC/MS interface
we investigated the effects of various substituents on thermospray
ionization. The compounds which were selected for this study have
structures similar to purine with most differing by only one or two
functional groups. These compounds can be divided into two groups:
one group consisting of substituted purines and the other consisting of
substituted xanthine or substructures of xanthine (Figure 3.10).
Purine along with pyrimidine are the parent compounds of the two
classes of nitrogenous bases found in nucleotides and consist of a
pyrimidine ring and an imidazole ring fused together. Xanthine is a
degradation product of purines and has a core structure similar to
purine.
Both positive ion and negative ion full scan thermospray mass
spectra were obtained from standard solutions (50-100 ppm) of each of
the purine and xanthine compounds. Most of the compounds showed

Figure 3.9
Negative ion full scan thermospray mass spectra of ribavirin
acquired several weeks apart under identical thermospray
conditions and scanning parameters. Instrumental parameters
for the spectrum shown in a) were as follows: flow rate =
1.5 mL/min; tip temperature = 209 C; source temperature - 253
C; source pressure - 2.2 Torr; probe position = 1.6 cm.
Instrumental parameters for the spectrum shown in b) were as
follows: flow rate - 1.5 mL/min; tip temperature = 209 C;
source temperature = 254 C; source pressure = 2.4 Torr; probe
position = 1.6 cm.

1 ■ 1 'T I ' | ■ i I ' I1'1! ■ I ■ I ' | ■ I ' f ■ I ' H [ ' 1 ' I ' I ■ I1- [ '■TH 'pr |1'T'TT' P^r+1*VH‘»,prrPT'>r’-r»T‘),t‘n'
(1/E 50 100 150 200 250 300 350 400 450
% Relative Abundance
% Relative Abundance
cn
©
3
cn
©
CD
©
©
©
©
©
m
©
©
(j\ .
©—
© •
CO
CO
-©
CO
CO
-cn
cn
i
K)
©
CD
X
-U
-CO
85

Figure 3.10
Model compounds used for determining the effects of
substituents on thermospray ionization. Note that most
compounds contain the purine core structure.
various
of the

60
H
Purine (MW 120)
2- Aminopurine
(MW 135)
Adenine 2-Hydroxypurine
(MW 135) (MW 136)
2,6-Diamino-8-purinol
(MW 166)
6-Thioguanine
(MW 167)
6-Mercaptopurine
(MW 152)
Xanthine (MW 152)
Hypoxanthine
(MW 136)
6-Thioxanthine
(MW 168)
Uric Acid
(MW 168)
H
H,NCN-
H
-UyC
Allantoin
(MW 158)
HO
H
HO
hr
NH
Alloxan Monohydrate
(MW 160)

61
characteristics in the mass spectrum typical of thermospray ionization;
abundant pseudo-molecular ions with very little fragmentation.
Molecular ions appeared as the (M+H)+ and/or (M+NH^)"1” in the positive
ion mass spectra and as the (M-H)" and/or (M+CH3COO)' in the negative
ion mass spectra. The instrumental parameters and the results of the
positive ion and negative ion thermospray mass spectra are summarized
in Table II. These ions were present in the mass spectrum with ion
intensities greater than three times the background ("chemical noise").
Thermospray of purine yielded the (M+H)+ ion in the positive ion
spectrum and (M-H)*, (M+CH3COO)*, and (2M-H)' ions in the negative ion
spectrum. All of the substituted purine compounds show the same
behavior as purine in the positive ion mode and yield a (M+H)+
molecular ion. Two of the substituted purines, 2-hydroxypurine and 6-
mercaptopurine, also produced (M+NH4.)+ ions. 2-hydroxypurine was the
only purine-type compound which contained a hydroxy group in its
structure. 6-thioguanine has a structure very similar to 6-
mercaptopurine but did not form an ammonium adduct. The only
difference between these two structures is the presence of an amine at
the two-position on 6-thioguanine.
All of the substituted purine compounds yield (M-H)‘ ions in their
negative ion mass spectra. In addition to (M-H)‘ ions, some compounds
(2-aminopurine, adenine, and 2,6-diaminopurinol) formed (M+CT^COO)"
adduct ions. The structure of these compounds differ from purine by
the addition of an amine group (two amine groups in the case for 2,6-
diaminopurinol) at various positions on their structures. Purine,
hydroxypurine, 6-mercaptopurine, and 6-thioguanine do not show the

Table II. Summary of the positive and negative ions observed3 in the
thermospray mass spectra of the purine and xanthine-type compounds.
Compound
Mol wt
Positive Ions (tRAi*3
Neqative Ions i%RA)^
Purine
120
121 (100) [MUi] +
119 (100)c [M-H]“
179 ( 239 (l)c [2M-H]“
2-aminopurine
135
136 (100) [MHI] +
134 (100) (M-H]“
194 (<1) (MKH3OOO]"
adenine
135
136 (100) [MtH] +
134 (51) [M-H]“
194 (100) [Mt 2-hydroxypurine
136
137 (2) [MUI] +
135 (100) [M-H]“
154 (100) [MINH4)+
6-mercaptopurine
152
153 (100) [M+H]+
151 (100) [M-H]“
170 (24) (MINH4)+
6-thioguanine
167
168 (100) [MHI] +
166 (100) [M-H]"
2,6-diaminopurinol
166
167 (100) [M+H] +
165 (38) [M-H]-
225 (100) [MKH3000]
Xanthine
152
d
151 (100) [M-H]-
Hypoxanthine
136
137 (100) [MIH]+
135 (67) [M-H]-
154 (6) [MINU4] +
195 (100) [MKH3OOO]
6-thioxanthine
168
d
167 (100) [M-H]-
Uric Acid
168
d
d
Allantoin
158
159 (47) [MtH]+
157 (100) [M-H]-
176 (100 [MfNH4]+
217 (1) [M*ai3000]-
Alloxan monohydrate
160
d
113® (11)
115c (69)
1759 (100)
M

aAll ion intensities with signal/noise > 3
kions normalized to the most abundant compound ion. Ammonium acetate typically dominate both the positive and
negative ion thermospray mass spectra.
cRelative abundances were normalized to m/z 119, the [M-H]- of purine and the C^GOOH-a^COO ion of ammonium
acetate.
c^No compound ions were present with intensities > 3 (signal/noise).
eFragment ion corresponding to the loss of HCOOH.
fFragment ion corresponding to the loss of 002 •
^Acetate adduct of the 115" fragment ion.
O'
LO

64
addition of acetate in their negative ion spectra nor do they contain a
primary amine group on their structures.
Xanthine, 6-thioxanthine, uric acid, and alloxan did not produce
any positive thermospray ions. A common feature to these compounds is
that their structures contain more than one doubly-bonded
electronegative group such as oxygen or sulfur. Hypoxanthine, with a
similar in structure to xanthine with the doubly-bonded oxygen at the
2-position removed, yields (M+H)+ and (M+NH4)+ ions. Similarly,
allantoin (an oxidation product of uric acid) also yields abundant
(M+H)+ and (M+NH4)+ ions. Xanthine, hypoxanthine, 6-thioxanthine, and
allantoin yield (M-H)' negative ion species. Alloxan monohydrate
produced negative ions at m/z 113, 115, and 175. The ions at m/z 113
and 115 correspond to fragment ions for the loss of HCOOH and CO2,
respectively. The ion at m/z 175 corresponds to an acetate adduct of
the 115" fragment ion. Both hypoxanthine (which contains no primary
amine functionality in its structure) and allantoin yield (H-CH3C00)"
ions while uric acid did not produce any negative ions.
Figures 3.11 and 3.12 show the resulting ion intensity summation
profiles of the abundant positive and negative ions of these purine and
xanthine-type compounds and ammonium acetate. These chromatograms
represent a single 25 /¿L flow-injection of each compound. Several
sharp downward spikes are present both profiles. These spikes
correspond to the switching of the sample injection valve from load to
inject or vice-versa. These spikes occur since the switching of the
injection valve causes a momentary interruption of the liquid flow and
thus, the signal decreases abruptly. A rapid switching technique will
eliminate these spikes while a slower, more deliberate switching will

Figure 3.11
Positive ion intensity summation profiles of a) the purine
and xanthine-type compounds, b) ammonium acetate, and c) RIC
trace obtained at 1.5 mL/min of 0.1 M ammonium acetate
buffer. Other instrumental parameters were as follows: tip
temperature ~ 210 C; source temperature = 250 C; source
pressure - 2.2 Torr; probe position - 1.6 cm. The peaks
correspond to (1) hypoxauthine, (2) 2,6-diamino-8-purinol,
(3) 2-aminopurine, (A) 6-mercaptopurine, (5) 2-hydroxypurine,
(6) purine, (7) 6-thioguanine, (8) allantoin, and (9)
adenine. The peak at 3' corresponds to carry-over from 2-
aminopurine (peak 3) . Note that the intensity profile of
ammonium acetate decreases during the ion production of the
sample compounds.

2:41 5:26 8:08 Time (min:sec)
RIC
Ion Current
Ion Current
co
co
99

Figure 3.12
Negative ion intensity summation profiles of a) the purine
and xanthine-type compounds, b) ammonium acetate, and c) RIC
trace obtained at 1.5 mL/min of 0.1 M ammonium acetate
buffer. Other instrumental parameters were as follows: tip
temperature - 210 C; source temperature = 250 C; source
pressure = 2.2 Torr; probe position = 1.6 cm. The peaks
correspond to (1) xanthine, (2) hypoxanthine, (3) 2,6-
diamino-8-purinol, (4) 2-aminopurine, (5) 6-mercaptopurine,
(6) 2-hydroxypurine, (7) purine, (8) 6-thioguanine, (9) 6-
thioxanthine, (10) allantoin, (11) adenine, and (12) alloxan
monohydrate. The peak at 4' corresponds to carry-over from
2-aminopurine (peak 4). Note that the intensity profile of
ammonium acetate decreases during the ion production of the
sample compounds.

2:41 5:26 8:08 Time (min:sec)
RIC Ion Current Ion Current
89

69
result in these spikes. These profiles illustrate the rapid nature of
thermospray with a flow injection technique where 13 samples were run
in approximately eight minutes, which corresponds to 100 samples per
hour. The RIC appears significantly elevated above the zero and does
not increase significantly for any of these compounds. This is because
the scanning interval of the mass spectrometer included the masses
corresponding to the ammonium acetate buffer ions which were present
throughout the acquisition.
It is interesting to note that the ion intensity profile of the
ammonium acetate ions decrease during the ion production of the purine
and xanthine-type compounds. Thus, it seems that the sample ions are
produced at the expense of the ammonium acetate ions. Whether this
ionization behavior is the result of direct competition between the
sample and ammonium acetate in the direct ion evaporation mechanism, or
a depletion process involving sample molecules and ammonium acetate
ions (reagent ions) in a Cl mechanism, or a combination of both, cannot
be concluded here.
These preliminary investigations show the effects of various
substituent groups on the thermospray ionization of purine and
xanthine-type compounds. These results indicate that particular
functional groups present in the sample compound structure can
influence the thermospray mass spectra. Structures which contain at
least two thiol or doubly-bonded oxygen groups generally do not produce
positive ions by thermospray ionization. Allantoin is an exception to
this observation. We believe that the presence of a primary amine
group in the allantoin structure helps to overcome this effect of the
doubly-bonded oxygens and promoted the formation of both (M+H)+ and

70
(M+NH4)+ ions. The presence of amine groups combined with the absence
of thiol or doubly-bonded oxygen groups seem to promote the formation
of (M+CH3COO)' ions in the negative ion thermospray ionization mode.
Uric acid, which contains three doubly-bonded oxygens on its structure,
does not produce any positive or negative ions by thermospray
ionization.

CHAPTER IV
THERMOSPRAY PLUME MEASUREMENTS IN VACUUM
Direct physical measurements of thermospray plumes were made in
vacuum to obtain information on the droplet evaporation and droplet
charging processes of thermospray. The measurements were made in a
specially designed vacuum chamber which permitted visual observation of
the plume, as well as sampling of pressure, temperature, and current
within the plume. Because this chamber was considerably larger (8.5 cm
i.d.) than the Vestec thermospray source (1 cm i.d. with an ion
sampling cone and other structures protruding into the source), the
effect of the source geometry may change the thermospray plume
behavior. Thermospray plumes were generated with the Vestec vaporizer
probe as measurements were obtained at positions along the center of
the plume in the manner described in Chapter II. The results of these
plume measurements are discussed below.
Plume Pressure Measurements
Plume pressures remained constant for all distances measured within
the plume at constant flow rate. Pressures of 2.4, 2.6, and 2.8 Torr
were obtained with flow rates of 1.5, 1.75, and 2.0 mL/min water,
respectively. Other instrumental parameters had no effect on pressure
measurements.
Plume Temperature Measurements
Plume temperature measurements were obtained at various tip
temperatures, manifold temperatures, flow rates, and manifold
pressures. (The adjustment of manifold temperatures and manifold
71

72
pressures approximated the effects of source temperature and source
pressure.) Much condensation (in the form of ice) was observed to
occur on the thermocouple at distances close to the probe tip (<1 cm)
while no condensation was observed to occur on the thermocouple at
distances further from the probe tip. The formation of ice on the
thermocouple would likely result in low temperature readings until the
ice either falls off or melts away at which point the temperature
readings would be expected to increase to temperatures more
representative of the surrounding vapor. Therefore, low and perhaps
fluctuating temperature readings may be indicative of a droplet-rich
plume, while stable or constant temperature readings may be indicative
of a vapor-rich plume. For these measurements no specially designed
temperature sensing probes were used. Therefore, these measurements
would be expected to provide only an approximation of the actual plume
temperature. Essentially, these measurements provide a means of
monitoring the production of droplets within the plume.
A plot of plume temperatures measured at various distances along
the plume for the three plume types at a flow rate of 2.0 mL/min is
shown in Figure 4.1. All three plume types behave similarly with the
greatest temperature fluctuations occurring at distances less than 1.5
cm. Beyond this distance the temperature readings become more stable.
Wet plumes produce the lowest temperature readings while dry plumes
produce the highest temperature readings. Thus, the vapor-rich
portions of wet, nearly dry, and dry plumes are attained at virtually
the same distances away from the probe tip. Also, higher tip
temperatures result in higher temperatures within the vapor region of
the plumes.

Figure 4.1
Plume temperatures measured along wet, nearly dry, and dry plumes at
2.0 mL/min of water.

Temperature (C)
Distance From Probe Tip (cm)

75
Figure 4.2 shows the plume temperatures measured at various
distances along a nearly dry plume produced at 2.0 mL/min for three
manifold temperatures. With increasing distances from the probe tip
the resulting plume temperature readings surpass the manifold
temperature. It appears that higher manifold temperatures result in a
more rapid attainment of stable plume temperatures. Therefore,
increased manifold temperatures are likely to result in a more rapid
droplet evaporation process and the attainment of droplet-rich plumes
at distances closer to the probe tip.
Figure 4.3 shows a plot of plume temperatures measured at various
distances along the plume for nearly dry plume types produced at
various flow rates. Plumes produced at flow rates of 1.5 and 1.75
mL/min show the greatest fluctuation in temperature at distances less
than 3 cm from the probe tip. At distances beyond 3 cm the measured
temperatures were more stable. Plumes produced at 2.0 mL/min show the
greatest fluctuation in temperature at distances less than 1 cm from
the probe tip. At distances beyond 1 cm the measured temperatures were
more stable. Thus, it appears that higher flow rates may yield vapor-
rich plumes at closer distances to the probe tip than plumes produced
at lower flow rates. This faster evaporation of droplets with
increasing flow rates may be due to the increase in tip temperatures
with higher flow rates.
Figure 4.4 shows the plume temperatures measured at various
distances along a nearly dry plume produced at 2.0 mL/min for manifold
pressures of 2.8 and 760 Torr and a manifold temperature of 24°C.
Plume temperatures increase with increasing distances from the probe at
2.8 Torr, however, at 760 Torr plume temperatures decrease with

Figure 4.2
Plume temperatures measured along a nearly dry plume at 2.0 mL/min of
water for three manifold temperatures.

Temperature (C)
200-1
150-
100-
50-
0 —
0.0
Nearly Dry Plumes
' i i i ' i i i i | i i i i i i i i | i i i i i i i i i | rrri i i i i~i | i i i i i i i i i | i i i i i i i i i |
1.0 2.0 3.0 4.0 5.0 6.0
Distance From Probe Tip (cm)

Figure 4.3
Plume temperatures measured along nearly dry plumes produced at various
flow rates of water.

200
180
160
140
120
100
80
60
40-
n i i i i i i i | i
> 1.0
TTTT
I I I I I I I
2.0
ni—| i i i
3.0
~1 i 1 1 • i i i i i i i i i i i i i
4.0 5.0
6.0
Distance From Probe Tip (cm),
vo

Figure 4.4
Plume temperatures measured along a nearly dry plume produced at 2.0
mL/min of water at two different manifold pressures.

80
70
60
50
40
30
20
10
111 M 1111111111111111111 n 11111111111111 rn 1111111111111111111111111111111111111 ni 111 m 11111 mi i] i rw m 111
) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Distance From Probe Tip (cm)

82
increasing distances along the probe. One reason which may account for
this behavior at high pressures may be the fact that at 760 Torr much
condensation occurs at all distances measured along the plume. This
may be the case since at 2.8 Torr, the probe temperature is enough to
maintain a "warm" spray with little condensation occurring. At 760
Torr, however, higher pressures combined with a manifold temperature of
only 24°C results in a "cool" spray with much condensation occurring
within the plume.
Plume Current Measurements
Plume current measurements were obtained at various tip
temperatures, manifold temperatures, flow rates, and manifold
pressures. As with the plume temperature measurements, much
condensation was observed to occur on the wire mesh used as an
electrode at distances close to the probe tip (<1 cm) , while at
distances further from the probe tip no condensation was observed to
occur. Condensation on the wire sampling mesh would affect the current
measurements by changing the fraction of the droplets/vapor which are
intercepted by the mesh (nominally 80% transparent). We believe that
the plume currents measured here are most representative of the droplet
charging process and that it is unlikely that ion formation (if any) in
the gas phase contributes significantly to these current measurements
of water plumes.
Plume currents measured at various distances along the plume for
the three plume types at a flow rate of 2.0 mL/min of water are shown
in Figure 4.5. Wet and nearly dry plumes show similar plume current
behavior with distance sampled and produce negative currents of

Figure 4.5
Plume currents measured along wet, nearly dry, and dry plumes at 2 0
mL/min of water.

Current (nA)
Distance From Probe Tip (cm)
00
-p"

85
approximately -500 nA at 0.5 cm. The magnitude of these plume currents
decrease with increasing distances from the probe tip (as droplets
evaporate to produce vapor) and gradually approach zero. Dry plumes
generate little current (<20 nA) and shows virtually no fluctuation
with distance sampled.
Figure 4.6 shows the plume currents measured at various distances
along a nearly dry plume produced at 2.0 mL/min for three manifold
temperatures. These plume currents generally show similar behavior
with distance sampled. The largest negative currents were observed at
0.5 cm from the probe tip and decrease to current values approaching
zero. Lower manifold temperatures have less effects on the plume
current readings. This is probably due to the increased condensation
which occurs at lower manifold temperatures.
Figure 4.7 shows a plot of plume currents measured at various
distances along the plume for nearly dry plume types produced at
various flow rates. Plume currents were greatest at distances close to
the probe tip. As the sampling distance was increased, the resulting
plume current gradually approached zero. Higher flow rates produce
more droplets and thus, yield higher plume currents.
Figure 4.8 shows the plume currents measured at various distances
along a nearly dry plume produced at 2.0 mL/min for manifold pressures
of 2.8 and 10 Torr. Plume currents are initially negative at distances
close to the probe tip and change to positive values at distances
further from the probe. At 10 Torr plume currents change from negative
to positive at ca. 1 cm while at 2.8 Torr plume currents change from
negative to positive at ca. 2 cm.
The results from these plume current studies suggest that water

Figure 4.6
Plume currents measured along a nearly dry plume at 2.0 mL/min of water
for three manifold temperatures.

Current (nA)
600 “} _r i i i i i i i i | i
0.0 1.0
I I | I I I I I I I I I | I 1 I f~l I I I I I IT I I I IT I I | I-| | 1 | | | | | |
2.0 3.0 4.0 5.0 6.0
Distance From Probe Tip (cm)
1
00

Figure 4.7
Plume currents measured along nearly dry plumes produced at various
flow rates of water.

Current (nA)
Distance From Probe Tip (cm)
CO
VD

Figure 4.8
Plume currents measured along a nearly dry plume produced at 2.0 mL/min
of water at two different martifold pressures.

Current (nA)
100-1
50-
0-
-50-
-100-
-150
i i i i i i i i i | i i i r
0.0 1.0
Nearly Dry Plumes, Tip = 220 C
Manifold Temp = 24 C
t i i i i i |—i i i i i r
2.0
i i i i i n i i-1 i—i—i i i i i i | i i i i i i i i i |
3.0 4.0 5.0 6.0
VO
Distance From Probe Tip (cm)

92
plumes produce primarily negatively charged droplets. This is in
disagreement with the droplet charging mechanism described by Dodd (67)
in which the charge distribution on the droplet is Gaussian with a zero
mean value. Perhaps more plume current studies performed with several
electrolytes may provide more insight into the actual charging
mechanism by thermosray. The magnitude of these plume currents
decrease with increasing distances from the probe tip. This suggests
that less droplets are present at distances further away from the probe
tip. Tip temperatures which result in the production of many droplets
in the vacuum chamber (wet and nearly dry) yield plumes with the
largest plume current values. Tip temperatures which result in very
little droplet production (dry) yield plumes with very low plume
current values. Higher manifold temperatures result in greater plume
current values. This suggests that higher manifold temperatures
provide the extra heat necessary to maintain a "warm" plume and prevent
the condensation of droplets resulting in larger current values. The
slight increase in positive plume current values observed with higher
manifold pressures may indeed indicate that pressure is involved with
the droplet charging mechanism. Perhaps further studies with
thermospray plumes in vacuum at various pressures will provide more
insight into this relationship.

CHAPTER V
INSTRUMENTAL PARAMETERS
This chapter describes the effects of various instrumental
parameters on the thermospray ionization behavior of a pure 0.1 M
ammonium acetate buffer and a 100 ppm (4 X 10M) ribavirin standard
prepared in 0.1 M ammonium acetate. In these studies tip temperature,
source temperature, flow rate, source pressure, and probe position were
varied systematically to determine their effect on thermospray
ionization. Each of these parameters differ in that they can directly
affect or sample a particular process of thermospray. The
relationships between the parameters which affect temperature,
pressure, and ion sampling position and the processes of thermospray
are discussed below.
The instrumental parameters which most directly affect temperature
are tip temperature and source temperature. These parameters are
expected to have the greatest effect on the vaporization and droplet
evaporation processes of thermospray. With increasing tip temperatures
(at constant flow rate) more efficient vaporization of the liquid will
occur, and with increasing source temperatures more efficient or rapid
evaporation of the liquid droplets will occur. From the models shown
in Figure 5.1, a large population of droplets will be expected to be
present near the probe tip while increasing amounts of vapor will be
expected to be present at distances further away from the probe tip.
Therefore, changes in temperature will effectively change the
thermospray plume characteristics (droplet/vapor population) at the ion
sampling orifice.
93

Figure 5.1
The effects of temperature (tip temperature or source temperature) on
thermospray plumes. With various amounts of heat a) low, b) medium,
and c) high, the droplet/vapor population can be altered.

95

96
The instrumental parameters which most directly affect pressure are
flow rate and source pressure. These parameters will be expected to
have the greatest effect on the direct ion evaporation and Cl processes
of thermospray. As shown in Figure 5.2, increasing flow rates and
increasing source pressures will most likely lead to a greater
accumulation of droplets and vapor within the source. The presence of
a greater number of droplets in the thermospray source will most likely
affect the direct ion evaporation process. Flow rate would be the
instrumental parameter which would be directly responsible for the
production of more droplets. The presence of more vapor will most
likely affect the Cl process. Source pressure would be the
instrumental parameter which would be directly related to the amount of
vapor present.
Although probe position is not directly related to a particular
thermospray process, a change in this instrumental parameter will
permit ion sampling along various portions of a thermospray plume,
independent of the effects of the other parameters. Figure 5.3 shows
the predicted effects of probe position on a thermospray plume. As the
probe is moved within the source, the effective ion sampling area of
the thermospray plume is altered. As in the case with temperature, a
large population of larger droplets will be expected to be present near
the probe tip, while increasing amounts of vapor (and fewer, smaller
droplets) will be expected to be present at distances further away from
the probe tip.
From these models it is apparent that the optimum conditions for
thermospray ionization would be those conditions of temperature,
pressure, and ion sampling position which result in maximum sample

Figure 5.2
The effects of pressure (flow rate or source pressure) on thermospray
plumes. With various pressures a) high, b) intermediate, and c) low,
the droplet/vapor population can be altered.

98
c)

Figure 5.3
The effects of probe position on thermospray plumes. Different
portions the plume (droplet-rich to droplet-lean) can be sampled at
various probe positions a) far, b) intermediate, and c) near.

100

101
ionization occurring at or near the ion sampling cone. Thus, a
systematic study of the instrumental parameters should result in the
determination of these optimum conditions and a better understanding of
the processes involved with thermospray ionization.
In these studies, the desired solution was filled into the syringe
pump and continuously pumped into the mass spectrometer. The scanning
parameters were the same for each experiment (15 to 500 amu in 0.40
seconds). When the desired instrumental parameter was adjusted, a full
scan mass spectrum acquisition was initiated and allowed to run for 20
seconds to several minutes. At the end of the acquisition, the
parameter under investigation was adjusted again and another full scan
mass spectrum acquisition was initiated. Since no injections were
made, the resulting ion intensity trace contained no peaks. Ion
intensity traces were characterized by constant, relatively non¬
fluctuating ion currents observed above baseline. Areas were obtained
by quantitating over a specified scan range, usually five scans (2
seconds) wide which corresponds to 77 ng of ammonium acetate and 1 /¿g
of ribavirin exiting the probe for a flow rate of 1.5 mL/min.
Repetitive measurements were obtained by quantitating a series of five
scan "peaks" within a single acquisition.
Tip Temperature
All instrumental parameters remained constant with a change in tip
temperature except for source temperature and HPLC pump pressure. The
effects of tip temperature on source temperature and HPLC pump pressure
are shown in Table III. The increase in source temperatures is the
result of the contributing heat input from the vaporizer probe. The

102
Table III. The effects of tip temperature on source temperature and
HPLC pump pressure.3
Tip Temp (C) Source Temp (O HPLC Pump Pressure (psi)
190
246
620
192
246
660
194
247
670
196
249
680
198
251
700
200
253
710
202
255
720
204
256
730
206
258
750
208
258
760
210
259
770
246
261
790
a) Obtained with flow rate - 1.5 mL/min (0.1 M ammonium acetate
buffer), source pressure - 2.2 Torr, and probe position = 1.6 cm.

103
increase in pump pressure is the result of increased vaporization of
the liquid (Chapter III). Note that tip temperatures could be adjusted
to produce the three plume types: wet (ca. 190 C), nearly dry (208 C) ,
and dry (243 C).
The effects of tip temperature on the ion intensities of pure
ammonium acetate are shown in Figure 5.4. Ion intensities of both the
positive and negative ions of ammonium acetate maximize at 208 C. The
effects of tip temperature on the ion intensities of ammonium acetate
with 100 ppm ribavirin present are shown in Figure 5.5. Ion
intensities of both the positive and negative ions of ammonium acetate
maximize at 194 C and decrease rapidly with increasing tip
temperatures. The effects of tip temperature on the positive ion
intensities of ribavirin are shown in Figure 5.6. The ion intensities
of the positive fragment ions at m/z 113 and 130 are optimum at 200 C
while the positive pseudo-molecular ions are optimum at higher tip
temperatures. The ion intensity of the (M+NH4.)+ adduct ion is maximum
at 204 C while the (M+H)+ and (2M+H)+ ions are maximum at 208 C (nearly
dry) . The negative ions of ribavirin show similar behavior as the
positive ions (Figure 5.7). Here, the ion intensity of the pseudo-
molecular ions (M-H)', (M+CH3COO)', (M+lll)*, and (2M-H)‘ ions are
optimum at 208 C while the negative fragment ions at m/z 111 and 171
are optimum at 200 C.
The presence of a sample compound, ribavirin, even at over 200
times lower concentration than ammonium acetate, clearly affects the
thermospray ionization behavior of ammonium acetate with changes in tip
temperature. Pure ammonium acetate shows maximum ion intensity at
nearly dry plume conditions (208 C); however, when present with

Figure 5.4
A plot showing the effects of tip temperature on the ion
intensities of pure ammonium acetate. Results of 4 replicate
measurements are shown.

Ion Intensity (counts)
3000000 -2
2500000 -
2000000 -
1500000 -
1000000 -
500000 -
0 I i i rr
180
o NH*
d nh/-h2o
* ch3conh2nh/
o ch3coo~
* CH3COOH-CH3COO

Figure 5.5
A plot showing the effects of tip temperature on the ion
intensities of ammonium acetate with 100 ppm ribavirin
present. Each plotted data point is the average of 4
replicate measurements.

Ion Intensity (counts)
107

Figure 5.6
A plot showing the effects of tip temperature on the positive
ion intensities of 100 ppm ribavirin. Each plotted data
point is the average of 4 replicate measurements.

Ion Intensity (counts)
109

Figure 5.7
A plot showing the effects of tip temperature on the negative
ion intensities of 100 ppm ribavirin. Each plotted data
point is the average of 4 replicate measurements.

Ion Intensity (counts)
in

112
ribavirin the ion intensity of ammonium acetate is maximum at wet plume
conditions (194 C). The rapid decrease in ion intensities of the ions
of ammonium acetate coinciding with the rapid increase in ion
intensities of ribavirin seems to suggest at least two possibilities
concerning the ionization mechanism of the sample compound, ribavirin.
One possibility which could account for this type of behavior is the
formation of ribavirin ions by a direct ion evaporation mechanism
similar to the formation of ammonium acetate ions. If this is the
case, then these results with tip temperature suggest a competition
between the direct ion evaporation of ions from ammonium acetate and
from ribavirin, where ammonium acetate ions are favored by relatively
low tip temperatures and ribavirin ions are favored by higher tip
temperatures. Another possibility may be the dominance of Cl processes
via the reagent ions of ammonium acetate and the neutral gas-phase
molecules of ribavirin. In this ionization scheme, the ammonium
acetate ion formation by thermospray would be the same whether pure in
solution or with ribavirin present and would follow the same behavior
with tip temperature as shown in Figure 5.4. However, the evaporation
of neutral gas-phase molecules of ribavirin would also maximize in the
same manner as the ions of ammonium acetate with changes in tip
temperature resulting in the depletion of the ammonium acetate ions
(reagent ions) and the formation of ribavirin ions. Of course a third
possibility may be that ribavirin ion formation is dependent on both
direct ion evaporation and Cl processes.
Another interesting result was the differences in dependence on tip
temperature of fragment and pseudo-molecular ion formation. Ion
intensities of the pseudo-molecular ions (both positive and negative)

113
of ribavirin maximize near temperatures corresponding to nearly dry
plumes (208 C) , while the ion intensities of the fragment ions (both
positive and negative) of ribavirin maximize near temperatures
corresponding to wet plume types (200 C) . This seems to indicate that
vaporization conditions affect the positive and negative ion formation
equally. From the earlier discussion of temperature effects on
thermospray plumes, low tip temperatures generally produce wet plume
types which contain larger droplets and to a certain extent slower
evaporating droplets. If this is the case, then these results indicate
that vaporization conditions which produce larger and, perhaps, more
slowly evaporating droplets at or near the ion sampling cone favor the
production of fragment ions, while vaporization conditions which
produce smaller and rapidly evaporating droplets favor the production
of pseudo-molecular ions. These droplet-rich plumes may contain an
abundance of large droplets, too large (>1 /im) to undergo direct ion
evaporation of pseudo-molecular ions. However, many of these large
droplets are able to come in contact with the heated source walls and
evaporate sample molecules. These sample molecules can become
pyrolyzed and produce a molecule (molecular weight 112 corresponding to
the triazole ring structure) where it can undergo Cl with the ammonium
acetate reagent ions. This possibility would account for the maximum
ion intensity of fragment ions at low tip temperatures. Likewise, the
vapor-rich plumes do not contain an abundance of large droplets but
rather an abundance of smaller droplets (<1 ¿¿m) and vapor. This
condition may be more favorable for the direct ion evaporation of
pseudo-molecular ions of ribavirin and/or the evaporation of a neutral
ribavirin molecule which can become ionized by Cl with the ammonium

114
acetate reagent ions. This possibility would account for the increase
in ion intensity of the pseudo-molecular ions with higher tip
temperatures.
Source Temperature
With changes in source temperature all instrumental parameters
remained constant except for tip temperature and source pressure.
Since the tip temperature has already been demonstrated to greatly
affect the thermospray ionization behavior, the tip temperature was
held constant to produce a nearly dry plume throughout these source
temperature experiments. With decreasing source temperatures the power
input to the vaporizer probe had to be increased (by increasing the
control temperature) in order to maintain a nearly dry plume.
Therefore, the heat provided by the source contributes to the vaporizer
probe in such a way that, at high source temperatures, less power is
required to the vaporizer probe to produce a nearly dry plume than at
low source temperatures. The effects of source temperature on source
pressure are shown in Table IV. Here, the indicated source pressure
was observed to decrease with decreasing source temperatures. (The
experiment was performed starting with high source temperatures and
ending with a low source temperature to minimize the effects of source
contamination.) We have two explanations as to why the source pressure
behaves in this way with a change in source temperature. Perhaps this
behavior is due to the continual "baking out" of the source at high
temperatures resulting in elevated source pressures. Another
possibility may be more complete evaporation of the droplets occurring

115
Table IV. The effects of source temperature on source pressure.3
Source Temo (C)
Source Pressure CTorr)*3
164
2.0
174
2.0
185
2.1
198
2.1
207
2.2
220
2.2
236
2.2
250
2.3
270
2.3
290
2.3
313
2.4
320
2.4
335
2.4
351
2.4
360
2.4
a) Obtained with flow rate -
buffer), tip temperature
- 1.5 mL/min (0.1 M ammonium acetate
- 209 C, and probe position - 1.6 cm.
b) Source pressures obtained from a Granville-Phillips thermogauge.

116
at higher source temperatures resulting in more vapor and higher source
pressures.
The effects of source temperature on the ion intensities of pure
ammonium acetate are shown in Figure 5.8. This plot is dominated by
the ion intensity of the (CH3COOH•CH3COO)' ion of ammonium acetate
which shows a gradual decrease in intensity with increasing source
temperature. The intensities of the other reagent ions remain
approximately constant. The effects of source temperature on the ion
intensities of ammonium acetate with 100 ppm ribavirin present are
shown in Figure 5.9. Here, the ion intensities of the ammonium acetate
ions show a more rapid decrease with source temperature than do the
ammonium acetate ions from pure buffer solution. Ion intensities are
maximum at 175 C and appear to stabilize at source temperatures greater
than 250 C. The effects of source temperature on the positive ion
intensities of ribavirin are shown in Figure 5.10. The fragment ions
at m/z 113 and 130 are more intense at low source temperatures (ISO¬
ZOO C) and at high source temperatures (335-360 C), while the pseudo-
molecular ions are more intense at intermediate source temperatures
(200-300 C) . The ion intensity of the (M+NH4)+ adduct ion is maximum
at 205 C while the (M+H)+ and (2M+H)+ ions are maximum at 250 C. The
negative ions of ribavirin show similar behavior with source
temperature (Figure 5.11). The ion intensity of the (M+CH3C00)' ion
maximizes at 250 C and dominates the plot. The ion intensities of the
fragment ion at m/z 111 is optimum at both high and low source
temperatures, while the fragment ion at m/z 171 is optimum at only low
source temperatures. The ion intensity of the (2M-H)~ ion shows
similar behavior to the (M+CH3COO)' ion and maximizes at 250 C. The

Figure 5.8
A plot showing the effects of source temperature on the ion
intensities of pure ammonium acetate. The results of 4
replicate measurements are shown.

Ion Intensity (counts)
800000 -i
600000 -
400000 -
200000 -
o NH*
a NH/-H20
0-
i i i i i i i i i I 1°i" i i i i°i ri°| i 19 rn
100
150
Source Temperature (C)
00

Figure 5.9
A plot showing the effects of source temperature on the ion
intensities of ammonium acetate with 100 ppm ribavirin
present. Each data point represents the average of 4
replicate measurements.

Ion Intensity (counts)
i i i i I
400
120

Figure 5.10
A plot showing the effects of source temperature on the
positive ion intensities of 100 ppm ribavirin. Each data
point represents the average of 4 replicate measurements.

Ion Intensity (counts)
Source Temperature (C)
122

Figure 5.11
A plot showing the effects of source temperature on the
negative ion intensities of 100 ppm ribavirin. Each data
point represents the average of A replicate measurements.

Ion Intensity (counts)
6000000 n
5000000 -
4000000 -
3000000 ~
2000000 ~
1000000 -
o-
o 111
a (112+CHjCOO)
* (M-H)~
i i ill nr
00
T-p-T
150
200 250 300
Source Temperature (C)
350
T 1 1 1
400
124

125
other pseudo-molecular ions do not exhibit this same behavior with
source temperature. The ion intensities of the (M-H)" and (M+lll)'
ions show an increase with increasing source temperatures.
As with the tip temperature experiments, the presence of a
relatively low concentration of a sample compound, ribavirin, affects
the thermospray ionization behavior of buffer with changes in source
temperature. The rapid decrease in ion intensities of the ammonium
acetate ions (with ribavirin present) coincides with the rapid increase
in ion intensities of the major pseudo-molecular ions of ribavirin,
(M+H)+, (M+NH4>+, (2M+H)+, and (M+CH3COO)'. This seems to suggest that
similar ionization schemes may exist for the ribavirin ionization
mechanism as discussed earlier with tip temperature.
From the earlier discussion of temperature effects on thermospray
plumes, high source temperatures would be expected to increase the
droplet evaporation process resulting in the production of smaller
droplets. If this is the case, then these results indicate that there
exists an optimum
source
temperature
(for
given conditions
of
vaporization) which
yields
droplets near
the
ion sampling cone
and
favors the production of particular ions. In this case it seems that
relatively high and/or low source temperatures favor the production of
fragment ions while intermediate source temperatures favor the
production of pseudo-molecular ions. Low source temperatures may
result in condensation on the source walls resulting in the evaporation
of pyrolysis products. High source temperatures may also result in
pyrolysis products in which the sample molecules decompose in the hot
droplets or in the hot gas-phase. Differences between the positive and
negative ion behavior of ribavirin may be indicative of the dependency

126
of the ion formation process (direct ion evaporation/CI) on droplet
size or may perhaps suggest different ionization mechanisms for the
production of these particular ions.
Flow Rate
The experiments dealing with the effects of flow rate on
thermospray ionization behavior were the most challenging since nearly
all instrumental parameters were affected by changes in flow rate,
making it difficult to determine the changes in ion intensities due to
changes in flow rate alone. With no obvious solution to this dilemma,
we chose to maintain tip temperatures in which nearly dry plume types
were produced at each flow rate with a constant source temperature. We
realize that the selection of this particular source temperature may
not be optimum for all of the flow rates studied and will probably
introduce some disparity in our measurements.
With increasing flow rates the resulting source pressure increases.
Tip temperatures increase and source temperatures increase with flow
rate; however, the tip temperatures reported here represent the final
adjusted temperature at which a stable nearly dry plume type was
produced while the source temperature was maintained constant at 258 C
for all flow rates. The effects of flow rate on the source pressure
are shown in Table V. Recall that the plume pressure measurements made
in the vacuum chamber at flow rates of 1.5, 1.75, and 2.0 mL/min of
water resulted in pressures of 2.4, 2.6, and 2.8 Torr, respectively.
In the previously discussed experiments, areas were quantitated
over an equal number of scans. However, this approach would not be
valid for these experiments since an equal number of scans would not

127
Table V. The effects of flow rate on source pressure obtained at tip
temperatures corresponding to nearly dry plumes.a
Flow Rate CmL/min)
Source Pressure CTorr')^>
Tíd Temp CO
0.75
1.7
178
1.00
1.9
194
1.25
2.1
200
1.50
2.2
209
1.75
2.3
219
2.00
2.5
229
a) Obtained with 0.1 M ammonium acetate buffer, source temperature =
258 C and probe position — 1.6 cm.
b) Source pressures obtained from a Granville-Phillips thermogauge.

128
correspond to an equal amount of ammonium acetate or ribavirin exiting
the probe at the various flow rates. Therefore, areas were quantitated
over the appropriate number of scans which correspond to an equal
amount of sample exiting the vaporizer probe for each flow rate.
The effects of flow rate on the ion intensities of pure ammonium
acetate are shown in Figure 5.12. The ion intensities of the positive
and negative ions of ammonium acetate increase dramatically with
increasing flow rates. Recall that this increase is not simply due to
the higher rate of delivery of sample into the source, since a number
of scans equivalent to the same amount of sample have been integrated.
The effects of flow rate on the ion intensities of ammonium acetate
with 100 ppm ribavirin present are shown in Figure 5.13. Ion
intensities show similar behavior to pure ammonium acetate buffer, and
generally increase with flow rate with exception to the intensities at
1.5 and 1.75 mL/min. The effects of flow rate on the positive ion
intensities of ribavirin are shown in Figure 5.14. The intensities of
all the positive ions generally increase with flow rate. The (M+H)+
increases with each flow rate setting, while the ion intensities of the
remaining positive ions stabilize at flow rates greater than 1.25
mL/min. The negative ions of ribavirin behave similarly, showing an
increase in ion intensities with increasing flow rates (Figure 5.15).
It is evident that flow rate is a crucial parameter to consider for
the production of ions by thermospray. Flow rates above 1.0 mL/min
yield sufficient positive ion and negative ion signals so that useful
mass spectra can be obtained. It is difficult to conclude whether flow
rates greater than 2.0 mL/min would result in either an increase or
decrease in ion intensities which would imply the existence of an

Figure 5.12
A plot showing the effects of flow rate on the ion
intensities of pure ammonium acetate. The results of 4
replicate measurements are shown.

Ion Intensity (counts)
130

Figure 5.13
A plot showing the effects of flow rate on the ion
intensities of ammonium acetate with 100 ppm ribavirin
present. The results of 4 replicate measurements are shown.

Ion Intensity (counts)
132

Figure 5.14
A plot showing the effects of flow rate on the positive ion
intensities of 100 ppm ribavirin. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)
134

Figure 5.15
A plot showing the effects of flow rate on the negative ion
intensities of 100 ppm ribavirin. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)
50000000 -i
Flow Rate (mL/min)
136

137
optimum flow rate and perhaps an optimum pressure. Experiments dealing
with higher flow rates could not be performed due to the fact that our
mass spectrometer cannot accommodate flows above 2.0 mL/min and still
maintain acceptable operating pressures (<1 X 10'^ Torr) in the mass
analyzer region.
The thermospray behavior of the ions of ammonium acetate appear to
be unaffected by the presence of a sample compound, ribavirin with
changes in flow rate. Changes in flow rates would be expected to
greatly affect the droplet population and pressure in the thermospray
source and thus, would be predicted to have the greatest effect on the
direct ion evaporation and/or Cl processes. Since the presence of more
droplets at higher flows was corrected for by the number of scans
added, Cl processes should be more affected by increased flow rates
than the direct ion evaporation processes. From our results it
appears that the formation of the major pseudo-molecular ions of
ribavirin, (M+H)+ and (M+C^COO)", is favored by high flow rates. This
behavior may be due to the presence of more vapor (higher source
pressure) which would favor the Cl process. Another possibility may be
that the increased pressures enhance the direct ion evaporation
processes resulting in an increase in ion intensity with increasing
flow rates. Ion intensities of several other ions of ribavirin also
increase with flow rate, with some stabilizing at flow rates greater
than 1.25 mL/min.
Source Pressure
Source pressure is not an instrumental parameter which the typical
user of thermospray would consider adjusting. This is because there

138
exists no convenient and reproducible method of selecting a particular
source pressure. As a result, nearly all of the thermospray work
reported in the literature has been performed at the pressure
determined by the amount of pumping provided on the source by the
mechanical pump and cold trap. Source pressures are not typically
reported, since the commercially available thermospray sources do not
come equipped to measure source pressure. In these experiments the
source pressure was adjusted by obstructing the source pump-out line
with a valve, as the corresponding source pressure measurements were
made in the same manner as described in Chapter II. With changes in
source pressure, all other instrumental parameters remained constant.
The effects of source pressure on the ion intensities of pure
ammonium acetate are shown in Figure 5.16. All the ion intensities of
ammonium acetate are maximum at intermediate source pressures of 4-6
Torr except for the (CH3COO)* ion which shows a maximum ion intensity
at 7.5 Torr. The effects of source pressure on the ion intensities of
ammonium acetate with 100 ppm present are shown in Figure 5.17. The
ion intensities of the ammonium acetate ions shows behavior similar to
the ions of pure ammonium acetate with maximum intensities occurring at
source pressures of 6-8 Torr. The effects of source pressure on the
positive ion intensities of ribavirin are shown in Figure 5.18. The
fragment ion at m/z 113 is optimum at high source pressures while the
m/z 130 fragment ion shows two maxima at 6 Torr and 3.5 Torr. The ion
intensity of the (M+NH4)+ adduct ion is maximum at source pressures of
2-3 Torr while the (M+H)+ and (2M+H)+ ions are maximum at 5-6 Torr and
4-5 Torr, respectively. The negative fragment and pseudo-molecular
ions of ribavirin show a similar behavior with source pressure (Figure

Figure 5.16
A plot showing the effects of source pressure on the ion
intensities of pure ammonium acetate. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)
o NH¿
140

Figure 5.17
A plot showing the effects of source temperature on the ion
intensities of ammonium acetate with 100 ppm ribavirin
present. Each data point is the average of 4 replicate
measurements.

Ion Intensity (counts)
142

Figure 5.18
A plot showing the effects of source pressure on the positive
ion intensities of 100 ppm ribavirin. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)
144

145
5.19). The fragment ion at m/z 111 is optimum at high source pressures
while the m/z 171 fragment ion shows two maxima at 6 Torr and 3.5 Torr.
The ion intensity of the (M-H)' ion is optimum at high source pressures
while the ion intensities of the remaining pseudo-molecular ions
(M+CH3COO)', (M+lll)’, and (2M-H)" are optimum at 5 Torr.
The presence of a sample compound, ribavirin, is shown to have a
slight affect on the thermospray ionization behavior of ammonium
acetate with changes in source pressure. A shift in the ammonium
acetate ion intensity maxima from 4-6 Torr to 6-8 Torr occurs when
ribavirin is present. The formation of the (CH3C00)' ion was
especially favored at high source pressures when obtained from either
pure buffer or buffer present with ribavirin. This increase in the
(Ci^COO)" ion may be due to increased collisions which may occur at
higher source pressures resulting in the fragmentation of the
(CH3COOH•CH3COO)* ion to produce the acetate ion. Another possibility
which could help to explain this increase in (CH3COO)* ion intensity
with increasing source pressure may result from more favorable ion
evaporation conditions occurring at higher source pressures.
The respective positive and negative fragment ions of ribavirin at
m/z 113 and 111 show the same behavior with source pressure as their
ion intensities increase with increasing source pressures. The
corresponding ammonia and acetate adducts of these fragment ions, 130+
and 171", also show the same behavior with source pressure as both
their ion intensities are maximum at 3.5 and 6 Torr. The presence of
two maxima may suggest the occurrence of two competing processes in the
ionization mechanism, for example, direct ion evaporation of the adduct
of the fragment versus chemical ionization of the fragment with the

Figure 5.19
A plot showing the effects of source pressure on the negative
ion intensities of 100 ppm ribavirin. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)
o 111
Source Pressure (Torr)
147

148
corresponding reagent ion (NH^4- or CH3COO'). The positive and negative
pseudo-molecular ions of ribavirin are optimum at intermediate source
pressures except for the (M-H)’ ion which is optimum at high source
pressures.
These results indicate that for ribavirin there exists optimum
source pressures for fragment ion and pseudo-molecular ion formation.
Since these experiments were performed under conditions of constant
flow rate by throttling the pump-out line by partially closing the
valve, increased source pressures (more vapor present) and longer
residence times for the vapor in the source resulted. Therefore, we
expect that these increases in source pressures would affect the Cl
process more than the direct ion evaporation process. From the
processes of a Cl-type mechanism it can be postulated that with an
increase in source pressure, a greater concentration of neutral gas-
phase sample molecules and reagent ions may be allowed to "accumulate"
in the source resulting a more efficient ionization process. Still,
these results are inconclusive as to which of the two ionization
mechanisms are responsible for the ionization of ribavirin.
Probe Position
Changing the probe position within the thermospray source is the
most straight-forward and simplest method of determining the effects of
ion sampling position on thermospray ionization. With the probe
inserted completely into the source the distance between the probe tip
and the center of the ion sampling cone was 0.6 cm. It was possible to
move the probe tip up to 2.1 cm away from the center of the sampling
cone without shutting down the vacuum system of the mass spectrometer.

149
As the probe position was changed all instrumental parameters remained
constant.
The effects of probe position on the ion intensities of pure
ammonium acetate are shown in Figure 5.20. With increasing ion
sampling distances the ion intensities of ammonium acetate remain
relatively constant. The effects of probe position on the ion
intensities of ammonium acetate with 100 ppm ribavirin present are
shown in Figure 5.21. Again, the ion intensities of the ammonium
acetate ions remain relatively constant. The effects of probe position
on the positive ions of ribavirin are shown in Figure 5.22. The ion
intensities of the fragment ions at m/z 113 and 130 decrease slightly
with increasing probe distances, while the ion intensities of the
pseudo-molecular ions increase with increasing probe distances. The
negative ions of ribavirin show similar behavior with probe position
(Figure 5.23). As was the case with the positive fragment ions the ion
intensities of the negative fragment ions at m/z 111 and 171 decrease
with increasing probe distances. The (M+lll)' adduct ion also exhibits
the same type of behavior with probe position as the negative fragment
ions. The ion intensities of the pseudo-molecular ions (M-H)' and
(M+CH3COO)'increase with increasing probe distances and are optimum at
distances greater than 1.1 cm while the ion intensity of the (2M-H)"
ion remains relatively constant.
In these studies the presence of a sample compound, ribavirin, does
not significantly affect the thermospray ionization behavior of
ammonium acetate. However, probe position does seem to affect the
positive and negative thermospray ion formation of ribavirin.
Distances less 1.1 cm favor the production of fragment ions while

Figure 5.20
A plot showing the effects of probe position on the ion
intensities of pure ammonium acetate. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)

Figure 5.21
A plot showing
intensities of
present. Each
measurements.
the effects of probe position on the ion
ammonium acetate with 100 ppm ribavirin
data point is the average of 4 replicate

Ion Intensity (counts)
3000000 n
2500000 -
2000000 -
1500000 t
1000000 -
° NHa
4
â–¡ nh/-h2o
a ch3conh2nh4+
0 ch3coo~
* CH3COOH-CH3COO~
500000 -
n
O
A — A
S o
u
0.
i ttti 111 ip ii i 111 111 111 111 1111111191111111 i rn'Tn |'i 11 rri i 11
4 0.6 0.8 1.0 1.2 1.4 1.
1111 11 i n 111111 M I I i | ill i r i n i]
6 1.8 2.0 2.2
Probe Position (cm)
153

Figure 5.22
A plot showing the effects of probe position on the positive
ion intensities of 100 ppm ribavirin. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)
155

Figure 5.23
A plot showing the effects of probe position on the negative
ion intensities of 100 ppm ribavirin. Each data point is the
average of 4 replicate measurements.

Ion Intensity (counts)
157

158
distances greater 1.1 cm favor the production of most of the pseudo -
molecular ions at a flow rate of 1.5 mL/min. It is interesting to
consider that at distances less 1.1 cm the thermospray plume would be
expected to contain more droplets than at distances greater than 1.1 cm
where the plume would be expected to contain smaller droplets and more
vapor. If this is the case, then these results indicate that sampling
the thermospray plume where there is a large population of droplets
would favor the detection of fragment ions while sampling the
thermospray plume where there is less droplets and more vapor would
favor the detection of pseudo-molecular ions. This would imply that at
small distances most of the droplets sampled are too large and afford
little opportunity for direct ion evaporation or Cl processes to occur.
However, droplets which are sprayed onto the source walls may evaporate
sample molecules which pyrolyze and produce decomposition product
molecules which can become ionized by Cl with ammonium acetate reagent
ions. At distances further from the probe tip the droplets are smaller
and the direct ion evaporation and Cl processes are more favorable.
Summary of the Effects of Instrumental Parameters
From these studies we have determined the effects of instrumental
parameters on thermospray ionization. Particular changes in any of the
instrumental parameters will definitely affect the relative abundances
in the mass spectra of ribavirin. Operating at conditions where
changes in a particular parameter produces a minimal change in ion
intensity of a particular ion or ions should result in minimal changes
in the mass spectra and hence, increased reproducibility. This would
be analogous to operating at a wavelength corresponding to maximum

159
absorption in a UV absorption experiment in order to minimize the
effects of wavelength drift. However, the selection of instrumental
parameters where the fluctuation of all the ion intensities are minimal
would appear to be difficult to achieve since the fragment ions and
pseudo-molecular ions typically show different behavior with changes in
instrumental parameters. Also, it is likely that different optimum
conditions exist for different sample compounds.
The results of the tip temperature and probe position studies
indicate that fragment ions may be produced from relatively large
droplets (presumably to condensation on the source walls followed by
pyrolysis of sample molecules which are then evaporated and ionized by
Cl) while pseudo-molecular ions may be produced from smaller droplets
(direct ion evaporation) and/or vapor (Cl reactions). Thus, various
tip temperatures can be selected to produce conditions of vaporization
and which favor the formation of fragment and pseudo-molecular ions.
Likewise, the probe position can be adjusted so that a desired portion
of the thermospray plume can be sampled for fragment ions and pseudo-
molecular ions. The source temperature studies showed similar behavior
of fragment and pseudo-molecular ion formation with temperature;
however, excessive temperatures (> 300 C) were shown to increase the
ion intensities of the fragment ions of ribavirin which may be due to
thermal decomposition of the pseudo-molecular ions. The results of the
flow rate experiments were rather difficult to interpret since this
parameter affected many of the other parameters and the magnitude of
the contributions of these other parameters may change with flow rate.
However, it is evident that the ion intensities do increase with
increasing flow rates (up to 2.0 mL/min). The studies dealing with

160
source pressure indicate that there exist optimum source pressures for
both fragment ion and pseudo-molecular ion formation.
From these results the distinction between the direct ion
evaporation mechanism and Cl mechanism in thermospray ionization of a
sample molecule is difficult to make. We had hoped that the
experiments dealing with flow rate and source pressure would provide
more conclusive information on these two processes. Changes in flow
rate mostly affect the droplet population and thus, would seem to have
the major effect on ion evaporation. However, an increase in droplets
would invariably result in the production of more reagent ions which
could take part in Cl reactions. Changes in source pressure via
obstruction of the pump-out line mostly affect the amount of vapor
present in the source and thus, would seem to have a major role in
affecting Cl processes. However, the possibility of pressure affecting
the direct ion evaporation process of sample molecules remains.
From these studies, relationships of the various instrumental
parameters have been developed which relate to the various thermospray
processes studied (vaporization, droplet evaporation, and direct ion
evaporation/CI). These parameters can be categorized as either
independent or dependent. An independent parameter is one which is
free from the influence of other parameters. Flow rate is an example
of an independent parameter. This is because changes in any of the
other parameters will not result in a change of flow rate. Probe
position is also another example of an independent parameter. A
dependent parameter is one which can be determined by other parameters.
Source pressure is an example of a dependent parameter since an
increase in flow rate will result in the increase in source pressure;

161
although source pressure could actually be "controlled" to a certain
extent, it could not be adjusted to any desired value. Tip temperature
and source temperature are examples of parameters which can be either
dependent or independent. Even though these two parameters are
affected by changes in other parameters, they are easily controlled and
can be adjusted to any desired value. Therefore, tip temperature and
source temperature can be controlled independently of the other
parameters if needed.

CHAPTER VI
SOLUTION CHARACTERISTICS
This chapter describes five separate experiments in which the
effects of various solution characteristics on the thermospray
ionization behavior of selected model compounds was determined. In
these studies solution characteristics such as mobile phase, sample
solvent, sample concentration, buffer concentration, pH, and mixture
components were varied systematically to determine their effect on
thermospray ionization. These solution characteristics directly affect
different processes of thermospray. Below is a discussion of each of
these experiments.
Mobile Phase and Sample Solvent
In these studies various combinations of mobile phases and sample
solvents were used to demonstrate the role of a volatile electrolyte in
the thermospray ionization of ribavirin. Methanol, water, and 0.1 M
ammonium acetate in water were selected for use as mobile phases and
sample solvents, resulting in a total of nine possible mobile phase-
solvent combinations. All instrumental parameters were held constant
(flow rate = 1.5 mL/min; tip temperature - 209 C; source temperature =
257 C; probe position — 1.6 cm) as 250 /¿L of 100 ppm ribavirin
standards were flow-injected.
The results of the mobile phase-solvent studies are shown in Table
VI. When methanol was used as the mobile phase the indicated source
pressure was 1.5 Torr, while water and ammonium acetate mobile phases
resulted in source pressures of 2.3 Torr. The ion intensities of the
162

163
Table VI.
Summary
of the mobile phase
-solvent studies.3
Mobile
Phase
Solvent
Source Pressure^
CTorr’)
Ion Intensity for 25
245+
4g Ribavirin
303-
MeOH
MeOH
1.5
c
c
h2o
1.5
c
c
NH40Ac
1.5
4.1 X 103
2.1 X 105
h2o
MeOH
2.3
c
c
H20
2.3
c
c
NH4OAC
2.3
4.5 X 105
7.0 X 106
NH40Ac
MeOH
2.3
1.0 X 105
1.6 X 106
h2o
2.3
8.1 X 105
1.4 X 107
NH40Ac
2.3
9.0 X 105
1.5 X 107
a) Obtained at flow rate = 1.5 mL/min; tip temperature = 209 C; source
temperature - 257 C; probe position -1.6 cm.
b) Source pressures obtained from a Granville-Phillips thermogauge.
c) Ion intensities less than three times the background.

164
(M+H)+ and (M+CH3C00)" were maximum when ammonium acetate was used as
both the mobile phase and the solvent. Any combination of mobile phase
and solvent in which ammonium acetate was not present in either
resulted in no detectable ion signals (signals greater than three times
the background).
The positive ion and negative ion mass spectra of ribavirin with an
ammonium acetate sample solvent and three different mobile phases are
shown in Figures 6.1 and 6.2. In both the positive and negative ion
mass spectra, differences in the relative abundances were observed with
the different mobile phases. There were differences for the reagent
ions relative to ribavirin ions and the ribavirin ions relative to each
other. The positive ion mass spectrum obtained with methanol as the
mobile phase (Figure 6.1a) shows the appearance of a Cl^OH-NH^ ion at
m/z 50 as the base peak. This particular spectrum contains many low
abundance ions (i.e. 175, 190, 285, 315) which were not observed with
the other mobile phases. The remaining positive mass spectra of
ribavirin obtained with water (Figure 6.1b) and ammonium acetate
(Figure 6.1c) have the (M+H)+ ion of ribavirin as the base peak. With
a water mobile phase the relative abundance of the fragment ion at m/z
130 was highest (16%). With ammonium acetate as the mobile phase the
relative abundance of the acetamide impurity at m/z 77 increased to
over 20%. The negative ion mass spectra were dominated by the abundant
ammonium acetate buffer ion, (CH3COOH•CH3COO)', at m/z 119 and the
(M+CH3COO)‘ion of ribavirin. With either methanol (Figure 6.2a) or
ammonium acetate (Figure 6.2c) as the mobile phase, the 119‘ was the
base peak. The (M+CH3COO)’ ion was the base peak when water was used
as the mobile phase (Figure 6.2b). As in the positive spectrum,

Figure 6.1
Positive ion thermospray mass spectra of ribavirin with 0.1 M ammonium
acetate buffer as the sample solvent and mobile phases of a) methanol,
b) water, and c) 0.1 M ammonium acetate.

% Relative Abundance
% Relative Abundance
100.0
% Relative Abundance
166

Figure 6.2
Negative ion thermospray mass spectra of ribavirin with 0.1 M ammonium
acetate buffer as the sample solvent and mobile phases of a) methanol,
b) water, and c) 0.1 K ammonium acetate.

% Relative Abundance
% Relative Abundance
e-00i
tVE 50 100 150 200 250 300 350 400 450 500
% Relative Abundance
168

169
aqueous mobile phases resulted in the largest relative abundance of the
negative fragment ion (m/z 111).
Figures 6.3 and 6.4 show the positive and negative ion mass spectra
of ribavirin obtained with an ammonium acetate mobile phase and three
different sample solvents. Again, variations in the relative
abundances were observed. Little change in the ammonium acetate ions
were observed except for the presence of the (CH30H'NH4)+ ion at m/z 50
in the positive mass spectrum when methanol was used as the sample
solvent (Figure 6.3). The (NH4'H20)+ ion of ammonium acetate was the
base peak when methanol was used as the sample solvent while the (M+H)+
ion of ribavirin was the base peak when either water or ammonium
acetate was used as the sample solvent. Note that the ratio of
abundances for the (NH4-H20)+ and (M+H)+ ions change with sample
solvents. The negative mass spectra show very little change in the
ammonium acetate relative abundances with sample solvent (Figure 6.4).
In all three spectra the (C^COOH-CH3COO)' ion was the base peak;
however, the relative abundance of the (M+CH3COO)' changes with the
sample solvent.
The vaporization and droplet evaporation processes would be
expected to be generally similar for thermospray plumes consisting of
water or ammonium acetate in water, the formation of various droplets
followed by their evaporation in the ion source. Any differences would
likely occur for thermospray plumes consisting of methanol since it has
a lower boiling point than water and has differences in surface
tension. However, solutions which contain only methanol or water or
combinations of both did not produce ions by thermospray. From this
reasoning these results indicate that it is the presence of ammonium

Figure 6.3
Positive ion thermospray mass spectra of ribavirin with 0.1 M ammonium
acetate buffer as the mobile phase and sample solvents of a) methanol,
b) water, and c) 0.1 M ammonium acetate.

% Relative Abundance
% Relative Abundance
100.3
m esc 00c
% Relative Abundance

Figure 6.4
Negative ion thermospray mass spectra of ribavirin with 0.1 M ammonium
acetate buffer as the mobile phase and sample solvents of a) methanol,
b) water, and c) 0.1 H ammonium acetate.

280 258 300 350 400 450 500
% Relative Abundance
% Relative Abundance
100.0
n/£ 53 130 150 200 250 300 350 400 450 500
% Relative Abundance
cn ®
oj
CD Q)
o
I
co
O
o
o
a:
ó
co1
O
O
O
l
173

174
acetate which has the greatest effect on the droplet charging process
of thermospray. Solutions such as methanol and water, which do not
contain a volatile electrolyte will not be able to generate sufficient
amounts of charge on its droplets for direct ion evaporation to occur
and will not therefore produce ions by the thermospray technique.
Sample Concentration
To determine the effects of sample concentration on the positive
ion and negative ion thermospray mass spectra, a series of standard
solutions of ribavirin were filled into a 25 /iL sample loop and flow-
injected into the mass spectrometer as both positive and negative mass
spectra were obtained. Figures 6.5 and 6.6 show the resulting positive
ion and negative ion mass spectra of ribavirin. With a change in
concentration the relative abundances of the positive and negative ions
of ribavirin change. Generally, at higher concentrations a greater
relative abundance of pseudo-molecular ions were present, while at
lower concentrations a greater relative abundance of fragment ions were
present. The positive ion mass spectra of decreasing concentrations of
ribavirin shows the increase in the relative abundance of the 136+ ion
which we believe to correspond to (CH3CONH2NH4•CH3CONH2)+ arising from
an acetamide impurity in the ammonium acetate. Also, upon inspection
of the ion intensity profile of this 136+ ion, no peak was observed to
be present corresponding to the time of the injection. The profile
contained a steady ion current with some random fluctuation close to
the baseline. At 250 and 100 ng of ribavirin injected, the m/z 130
fragment ion of ribavirin was observed as the base peak. The positive
ions of ribavirin (m/z 130, 245, and 262) are detected with the full

Figure 6.5
Positive ion thermospray mass spectra of various amounts of ribavirin,
a) 2500 ng, b) 1000 ng, c) 250 ng, d) 100 ng, and e) blank.

03!
CD
Q_ o cr
% Relative Abundance
to
2
O
3 4.1
O'
(H*m

Figure 6.6
Negative ion thermospray mass spectra of various amounts of ribavirin,
a) 2500 ng, b) 1000 ng, c) 250 ng, d) 100 ng, and e) blank.

CD
Q. o Cr
% Relative Abundance
0)
00
(OóocHO+n>

179
scan mass spectra down to 100 ng. Note that the fragment ion of
ribavirin (m/z 130) was also observed in the mass spectrum of the blank
(25 nL of ammonium acetate flow-inj ected) . The appearance of the ion
at m/z 279 is believed to be the phthalic anhydride adduct of the 130+
fragment ion of ribavirin. It seems likely that this molecule was
formed via a plasticizer from the polyethylene sample bottles. The
negative ion mass spectra show similar behavior as the positive spectra
with decreasing amounts of ribavirin injected. The relative abundances
of the ammonium acetate ions (CH3COOH • CH3COO • 2H2O) ' and
(2CH3COOH-CH3COO)' at m/z 155 and 179, respectively, increase while the
relative abundance of the (H+CH3COO)' ion decreases with decreasing
amounts of ribavirin injected. The negative ions of ribavirin (m/z 171
and 303) are also detected with the full scan mass spectra down to 100
ng. Note that the fragment ion of ribavirin at m/z 171 (112+CH3C00")
was also observed in the blank.
To further illustrate the behavior of the thermospray ions of
ribavirin with changes in sample concentration, the ion intensites of
the major ions of ribavirin are plotted versus the amount of ribavirin
injected (Figures 6.7 and 6.8). From these calibration curves it can
be seen that the ion intensities of ribavirin do not increase
proportionally with concentration. The most dramatic example of this
behavior can be seen in Figure 6.5 where the ion intensity of the
positive fragment ion of ribavirin, (112+NH4)+, is greater than the
(M+H)+ ion of ribavirin at low amounts (< 400 ng) ; however, with
increasing amounts of ribavirin, the ion intensity of the (M+H)+ ion
appears to increase at a faster rate than the fragment ion until ca.
1000 ng. Ratios of various positive and negative ion intensities were

Figure 6.7
Calibration plots for the major positive ions of ribavirin
and the RIC.

Ion Intensity (counts)
00

Figure 6.8
Calibration plots for the major negative ions of ribavirin
and the RIC.

Ion Intensity (counts)
183

184
calculated and are listed in Table VII. The positive and negative
RIC's are approximately equal at each concentration. Ratios of the ion
intensities of 245+ to 303" reveal that nearly equal number of these
ions are produced below 250 ng and that a greater number of 303" ions
are produced above 250 ng. Ratios of the selected positive and
negative ion intensities show the most change between 100 ng and 250 ng
while these ratios are essentially constant at amounts greater than
1000 ng.
Buffer Concentration
In these studies the effects of buffer concentration on thermospray
ionization were determined. Solutions of various concentrations of
pure ammonium acetate buffers, as well as ribavirin standards prepared
in these buffer solutions, were run by thermospray. Each solution was
continuously fed into the mass spectrometer at a flow rate of 1.5
mL/min and positive and negative ion mass spectra were obtained. All
parameters were held constant (tip temperature - 208 C, source
temperature = 255 C, source pressure - 2.2 Torr, and probe tip position
- 1.6 cm). The effects of buffer concentration on the resulting ion
intensity of ammonium acetate and ribavirin are described below.
The intensities of the major positive ions of various
concentrations of pure ammonium acetate solutions are shown in Figure
6.9. The most dramatic changes in the positive ion intensities were
the decrease of the hydrated ammonium ions, m/z 36 (NH4+,H20) and m/z
54 (NH4+-2H20) and increase of the m/z 35 (NH4+-NH3) and m/z 77
(CH3C0NH2NH4+) ions with increasing buffer concentration. The
intensities of these ammonium acetate positive ions obtained from

Table VII. Ratios of various positive and negative ion intensities of ribavirin.
Ribavirin
+RIC/-RIC
245+/303"
130+/245+
262+/245+
171-/303"
243-/303"
100 ng
1.0
0.8
4.3
0.4
0.8
0.01
250 ng
1.2
0.9
2.4
0.7
0.3
0.01
1000 ng
1.1
0.6
0.5
0.5
0.1
0.03
2500 ng
1.2
0.6
0.6
0.5
0.1
0.01

Figure 6.9
Positive ion intensities of the major ions of ammonium
acetate at various concentrations.

Ion Intensity (counts)

188
solutions of ribavirin standards prepared in various concentrations of
ammonium acetate show similar behavior with varying buffer
concentration (Figure 6.10). The positive ion intensities of ribavirin
are shown in Figure 6.11. The most notable changes in ion intensities
were the increase of the ammonium adducts of the triazole ring fragment
at m/z 130 and ammonium adduct of the ribavirin molecule at m/z 262
with increasing buffer concentration.
The negative ion intensities for the major ions of various
concentrations of pure ammonium acetate solutions are shown in Figure
6.12. The ion intensity of the acetate dimer (CH3COOH•CH3COO') at m/z
119 increases while the ion intensity of the ion at m/z 77 (CH3COO'
•H2O) decreases with increasing buffer concentration. The ion
intensity of the acetate ion (CI^COO") at m/z 59 remains nearly
unchanged with a change in buffer concentration. The intensities of
these ammonium acetate negative ions obtained from solutions of
ribavirin standards prepared in various concentrations of ammonium
acetate show similar behavior with buffer concentration (Figure 6.13).
The negative ion intensities of ribavirin (Figure 6.14) were dominated
by the abundant acetate adduct ion at m/z 303 (M+CH3C00‘) which was a
maximum at a buffer concentration of 0.1 M.
Table VIII shows that the summation of the ion intensities of pure
ammonium acetate are nearly equal to the combined ion intensities of
ammonium acetate (with ribavirin) and ribavirin at each buffer
concentration for both the positive and negative ions. This type of
behavior is what would be expected for chemical ionization (Cl): a
decrease in the ion intensities of the reagent ions (ammonium acetate)
when in the presence of neutral sample molecules. Note that the

Figure 6.10
Positive ion intensities of the major ions of ammonium
acetate obtained from 100 ppm ribavirin standards prepared in
various concentrations of ammonium acetate.

Ion Intensity (counts)

Figure 6.11
Positive ion
intensities of 100 ppm ribavirin prepared in
various concentrations of ammonium acetate.

Ion Intensity (counts)

Figure 6.12
Negative ion intensities of the major ions of ammonium
acetate at various concentrations.

Ion Intensity (counts)
194

Figure 6.13
Negative ion intensities of the major ions of ammonium
acetate obtained from 100 ppm ribavirin standards prepared in
various concentrations of ammonium acetate.

0.01 0.05 0.10 0.50 1.00
Ammonium Acetate (M)
Ion Intensity (counts)
—*• IV) OJ
o o o
o o o
o o o
o o o
o o o
o o o
I I I I I I I I I I 1 I I I I I I I I I I I I Ill 1 I I I I I—I—I
•-()
o
o
I
961
4000000

Figure 6.14
Negative ion intensities of 100 ppm ribavirin prepared in
various concentrations of ammonium acetate.

0.01 0.05 0.10 0.50 1'00
Ammonium Acetate (M)
Ion Intensity (counts)
i
K>
cn
o
cn
o
O
o
o
o
O
o
o
o
O
o
o
o
O
o
o
o
O
o
o
o
III i | I I I I I I I I I I I I I l I I I I I l I l I I I I I I I I ! I I I I I I I I I
861
2500000

Table VIII. Positive and negative ion intensities of pure ammonium acetate
versus 100 ppm ribavirin.
Positive Ion Intensities of Pure Ammonium Acetate vs. 100 ppm Ribavirin.
Buffer
Pure
100 ppm Ribavirin
Cone.
Ammonium Acetate3
Ammonium Acetate
Ribavirin13
Sum
0.01 M
7.8
X
105
2.5
X
105
6.5
X 105
9.0
X
105
0.05 M
1.1
X
106
4.1
X
105
7.1
X 105
1.1
X
106
0.10 M
1.0
X
106
3.9
X
105
7.3
X 105
1.1
X
106
0.50 M
1.0
X
106
4.7
X
105
8.6
X 105
1.3
X
106
1.00 M
1.2
X
106
6.1
X
105
8.4
X 105
1.4
X
106
Negative Ion Intensities of Pure Ammonium Acetate vs. 100 ppm Ribavirin.
Buffer
Pure
100
ppm Ribavirin
Cone.
Ammonium Acetate0
Ammonium Acetate
Ribavirin0
Sum
0.01 M
3.2
X
106
1.1
X
106
1.7
X 106
2.8
X 105
0.05 M
4.8
X
106
2.4
X
106
2.2
X 106
4.6
X 106
0.10 M
5.0
X
106
2.2
X
106
2.4
X 106
4.6
X 106
0.50 M
5.3
X
106
3.2
X
106
2.1
X 106
5.3
X 106
1.00 M
6.5
X
106
4.2
X
106
2.0
X 106
6.2
X 106
199

a)
b)
c)
d)
Ions summed are
Ions summed cure
Ions summed are
Ions summed are
18+ 35+, 36+, 54+, 59+, 60+, 77+, and 78+.
113 , 130+, 245+, 262+, and 489+.
59", 77", and 119".
Ill-, 171“, 243", 303”, 355”, and 487“.
200

201
summation intensities for ribavirin stay nearly constant for a constant
ribavirin concentration, independent of buffer concentration. Although
this type of behavior is indicative of Cl, the possibility of buffer
concentration affecting the direct evaporation of sample ions cannot be
overlooked.
From these results it is evident that changes in buffer
concentration may affect not only the resulting mass spectra of
ammonium acetate but also the resulting mass spectra of ribavirin.
Higher concentrations of ammonium acetate generally promote the
formation of ammonium adduct ions in the positive ion mode and acetate
adduct ions in the negative ion mode. Also, with higher concentrations
of ammonium acetate the ion intensities of the hydrated species of
ammonium acetate decrease.
Although not conclusive, these results suggest that Cl is a major
process occurring during the thermospray ionization of sample
molecules, whereby neutral molecules are formed in the gas phase (by
evaporation from droplets) and then chemically ionized by reagent ions
from ammonium acetate. In our previous studies, the ionization
behavior of ammonium acetate has shown some dependency on whether it is
pure in solution or present with a sample compound which may account
for a Cl type of mechanism. However, the effects operating conditions
and other various parameters described previously may also affect the
direct ion evaporation mechanism, thereby preventing the distinction of
direct ion evaporation processes versus Cl processes.

202
pH Study
In this study, the effect of pH on thermospray of both ammonium
acetate and a model compound, histidine, were determined so that more
insight could be gained into the contributions of direct ion
evaporation and Cl processes for sample ionization by thermospray.
Solutions of 0.1 M ammonium acetate buffer and standard solutions of
histidine prepared in 0.1 M ammonium acetate buffer were adjusted to pH
values of 3, 7, and 11 (as indicated by pH meter) and then run by
thermospray. Each solution was continuously fed into the mass
spectrometer at a flow rate of 1.5 mL/min and positive and negative ion
mass spectra were obtained. All parameters were held constant (tip
temperature 208 C, source temperature 255 C, and probe tip position 1.6
cm). The effects of pH on the resulting ion intensity (25 scans) and
on the mass spectra of ammonium acetate and histidine are described
below.
The structures of ammonium acetate and histidine at pH 6.0 are
shown in Figure 6.15. Ammonium acetate is a 1:1 salt of a weak base,
acetate (pKa = 4.76) and a weak acid, ammonium (pKa = 9.25). Histidine
is a basic amino acid with three acid/base groups, a-COOH (pKa = 1.82),
CÍ-NH3 (pKa = 9.17), and a secondary amine contained in the ring portion
of the molecule (pKa = 6.0). Compounds which contain an acid/base
group or groups within their structure will be affected by a change in
pH resulting a change in concentration of the conjugate acid-base pair.
The Henderson-Hasselbalch equation (eqn. 6.1), a logarithmic
transformation of the equation for the acid dissociation constant (Ka),
makes it possible to calculate the ratios of a proton-donor [HA] and
proton-acceptor [A'] pair at any pH.

Figure 6.15
The structures of a) ammonium acetate and b) histidine at pH 6.0.

204
O
II
a) H3C-C-O' NH4+
H
b)
CH„ —
HNW^NH
C -
I
NH,
COO"

205
pH = pKa + log fproton acceptor 1 (6.1)
[proton donor]
Three situations may occur for a given conjugate acid-base pair.
The first situation occurs when pH is less than pKa, resulting in a
greater fraction of the proton donor species. A second situation
occurs when pH is greater than pKa> resulting in a greater fraction of
the proton acceptor species. And finally, when pH equals pKa, an equal
fraction of the proton acceptor and proton donor results. These
situations for ammonium acetate and the a-COOH, CÍ-NH3, and secondary
amine groups of histidine are summarized in Tables IX and X.
Table XI shows the positive and negative RIC values for ammonium
acetate at the three pH's. The RIC for the buffer at pH 3 was lower
than the RIC's for the buffers at pH 7 and 11. The RIC's of the
buffers at pH 7 and 11 were essentially equal. The negative ion RIC
was 3 times greater than the positive ion RIC for the buffer at pH 3.
The negative ion RIC values were over 8 times greater than the positive
ion RIC values for the buffers at pH 7 and 11.
The positive ion and negative ion mass spectra of the buffers are
shown in Figures 6.16 and 6.17. The positive ion mass spectrum of
ammonium acetate changes dramatically with a change in pH. At pH 3 the
base peak (and only peak above background) is the ion at m/z 78
corresponding to CH3C00H-NH4+. At pH 7 the spectrum contained abundant
ions at m/z 18, 36, 54, 60, 77, and 78 corresponding to NH4+, NH4+-H20,
NH4+-2H20, CH3CONH3+, CH3C0NH2NH4+, and CH3COOH-NH4+, respectively. At
pH 11 the spectrum contained abundant ions at m/z 18, 35, 36, 53, 60,
and 77 corresponding to NH4+, NH3NH4+, NH4+-H20, NH3NH4+-H20,

206
Table IX. Forms of the acid/base groups of ammonium acetate at various
pH's.
Acid/Base
Grouü
dH 3
dH 4.76
dH 7
dH 9.25
dH 11
Acetate
COOH
C00H/C0CT
COO'
COO'
COO'
Ammonia
nh4+
nh4+
nh4+
nh4+/nh3
nh3

207
Table X. Forms of the acid/base groups of histidine at various pH's.
Acid/Base
Grouo
dH 1
dH 1.82
dH 3
pH 6
pH 7
dH 9.17
oH 11
a COOH
COOH
COOH/COO*
COO'
COO'
COO'
COO*
COO'
a NH3
nh3+
nh3+
nh3+
nh3+
nh3+
NH3+/NH2
nh2
/
/
/
/ /
/
/
/
R Group
HN+
HN+
HN+
HN+/N
N
N
N
\
\
\
\ \
\
\
\

208
Table XI. Positive and negative RIC values of 0.1 M ammonium acetate.
pH of
ammonium acetate +RIC (counts') -RIC (counts) +RIC/-RIC
8.4
X
105
2.5
X
106
0.33
2.1
X
106
1.7
X
107
0.12
o
CM
X
106
1.7
X
107
0.12
11

Figure 6.16
Positive ion thermospray mass spectra of 0.1 M ammonium acetate at pH
a) 3, b) 7, and c) 11.

210
ch3cooh-nh¿
b)
NH4*'H2°
;00.a
rvE
ch.conh-nh/
7
7
18
34
1 53
1
T—\ - r ■ -l ■"! 'T— i'—! ■ l 1 '■ I 1 ‘ i 1 ■! 1 I ■ T * ) ■ |
50 toe 150 :ca :5a
c)
NHjNH*
50
100

Figure 6.17
Negative ion thermospray mass
a) 3, b) 7, and c) 11.
spectra of 0.1 M ammonium acetate at pH

212
CH3COOH-CH3COO
a) !«•«■
ch3coo~
2CH3COOH-CH3COO~
59
1
—1- 1 • 1 ■ 1

213
CH3CONH3+, and CH3CONH2NH4+. The negative ion mass spectra of ammonium
acetate shows very little change with pH. The ions at m/z 59 and 119
which correspond to C^COO'and CH3COOHâ– CH3COO', respectively, dominate
the spectra. At pH 7 an ion at m/z 155 was present which corresponds
to CH3COOH■ CH3C00" • 2H2O while at pH 11 ions at m/z 154 and 155 were
present which correspond to CH3COOH • C^COCT • NH3 -^0 and CH3COOH ■ C^COO"
•2H2O, respectively, plus m/z 62 for which no reasonable structure is
apparent.
Table XII shows the positive ion and negative ion RIC values for
100 ppm solutions of histidine prepared in the three pH buffers. As is
the case with ammonium acetate, the RIC values for the solutions at pH
3 were 4 to 10 times lower than the RIC's for the solutions at pH 7 and
11, which were essentially equal. The negative RIC was about 3 times
greater than positive RIC for the histidine standard in the buffer of
pH 3. The negative RIC values were over 7 times greater than the
positive RIC values for the histidine standards in buffers of pH 7 and
11.
The positive ion and negative ion thermospray mass spectra of
histidine at pH 3, 7, and 11 are shown in Figures 6.18 and 6.19.
Abundant positive ions at m/z 156, 198 and 311 which correspond to
(M+H) + , (M'+H) + , and (2M+H)+ were present in each of the mass spectra
(where M' is presumed to be an impurity with a molecular weight of
197). With only 2 ions corresponding to histidine, the only difference
between the spectrum is the increased abundance of the dimeric ion at
higher pH's. The negative ion mass spectra exhibited more change with
pH than the positive ion spectra. At pH 3 the spectrum contained
abundant ions at m/z 154, 179, 196, 214, and 309 corresponding to (M-

214
Table XII. Positive and negative RIC values of 100 ppm histidine
in 0.1 H ammonium acetate.
pH of
ammonium acetate -fRIC (counts) -RIC (counts’) +RIC/-RIC
8.4
X
105
2.4
X
106
0.36
3.5
X
106
2.7
X
107
0.13
3.6
X
106
2.7
X
107
0.13
11

Figure 6.18
Positive ion thermospray mass spectra of 100 ppm histidine in 0.1 M
ammonium acetate at pH a) 3, b) 7, and c) 11.

3'U
o*e;
e-eei
(0
oc: e;: eo: ost *r. e:
** *- * 1 1
1
861
ez
-1 - ■ -«- ■ - ■
9c M
Ilf
jH+ni)
-
K1
3'U
p-wi (Q
*ei ret «-
lie
ik e;i c: e:
»ii’ut'iin!'i'’ti'i!i|i'i',T ""i1.11 Wn i',r i,1
+ (H + WZ)
/HN-HOOOCHO
-e*e;
Lp’ooi (B
91Z
stl
(H+ni>

Figure 6.19
Negative ion thermospray mass spectra of 100 ppm histidine in 0.1 M
ammonium acetate at pH a) 3, b) 7, and c) 11.

218
ü) íea.a-i
(M-H)'
134
CH3COOH-CH3COO~
ch3coo
(M*CH3COO>
214
(2M-H)~
:as
b) 1W-8
30.9
(M’-H)
i?e
(M+M'-H)
331
i- ; â–  i â–  V' i 11 i 'i ' r ' i i m â– ! â–  ; â–  t
lúe 1:0 :ae 2 50
2Z9
400

219
H)', (M''-H)‘, (M'-H)‘, (M+59)", and (2M-H)' where M' ' is an impurity
with a molecular weight 180. At pH 7 the spectrum contained abundant
ions at m/z 154, 196, 309, and 351. The ion at m/z 351 corresponds to
(M+M'-H)". At pH 11 the spectrum contained abundant ions at m/z 154,
190, 196, 217, 309, and 351. The ions at m/z 190 and 217 correspond to
(M'''-H)‘ and (M+62)' where M' ' ' is an impurity with molecular weight
191.
These results demonstrate the effects of pH on thermospray ion
formation of both ammonium acetate and histidine. At low pH the
formation of reagent ions from the ammonium acetate buffer was
suppressed for both positive and negative ions. At pH 7 and 11 the ion
intensities of the buffer solutions were essentially equal. The same
result was observed with the ion intensities of the histidine
solutions. At each pH the ion intensities for the histidine solutions
were greater than those for the corresponding buffer solutions. The
ratio of positive to negative ions was dependent on pH. Solutions
adjusted to pH 3 had a positive to negative ion ratio of 0.33-0.36
while solutions adjusted to pH 7 and 11 had ratios of 0.12-0.13.
The positive ion mass spectrum of ammonium acetate was affected
more by pH than the negative ion spectrum with different ions
dominating the mass spectrum at each pH. The negative ion spectra
contained the same ions (m/z 59 and 119) at each pH with a new ion
appearing at m/z 62 at pH 11. The positive ion spectra of histidine
did not show much change with pH. The same ions corresponding to
(M+H)+, (M+18)+, and (2M+H)+ were contained in each spectrum. The
negative ion spectra of histidine produced different adduct ions with a
change in pH.

220
Thermosprav Ionization Behavior in a Mixture
This study was performed to determine the ability of thermospray to
ionize components of simple mixtures without interferences or matrix
effects. Here, the thermospray ionization behavior of two- and three-
component mixtures was compared to that of solutions of pure
components. If the same response (equal peak areas for each
characteristic ion) for the pure component and the component contained
in the mixture were obtained, then this would indicate that the
presence of other components have little effect on the thermospray
processes. However, if differing responses were obtained for the pure
standard and the mixture sample, then this would indicate that the
processes of thermospray are indeed affected by the presence of other
components of a mixture. We expect the vaporization, droplet
evaporation, and droplet charging processes of thermospray to remain
unchanged with these simple two- and three-component mixtures. The
processes which we predict would be most affected by other components
contained in a mixture are the direct ion evaporation and Cl processes.
The mixture experiments performed with allantoin, 2-hydroxypurine, and
2,6-diamino-8-purinol are discussed below.
Standard stock solutions of each compound were prepared in 0.1 M
ammonium acetate buffer. A one-to-three dilution of each stock
solution (diluted in 0.1 M ammonium acetate) was prepared for use as
the pure solutions. For the two-component mixture studies equal
aliquots were taken from each of the two desired stock solutions and
1
combined with an equal aliquot of 0.1 M ammonium acetate. For the
three-component mixture studies equal aliquots were taken from each of

221
the three stock solutions and combined in one vial for use as the
mixture. All instrumental parameters were kept constant: flow rate =
2.0 mL/min; tip temperature - 228 C; source temperature - 300 C; source
pressure — 2.4 torr; probe position = 1.6 cm; sample volume = 25 /¿L.
Samples were run in triplicate while both positive and negative ion
full scan mass spectra were obtained simultaneously.
Typical thermospray spectra were observed for the three compounds,
as positive (M+H)+ and (M+HN4)+, and negative (M-H)' and (M+59)" ions
were observed. All three compounds formed (M+H)+ and (H-H)‘ ions.
Allantoin and 2-hydroxypurine formed (M+NH4)+ ions while only 2,6-
diamino-8-purinol produced a (M+CH3COO)' ion.
The results of the two-component mixture studies are shown in
Tables XIII-XV. The largest % errors were observed with the allantoin
and 2-hydroxypurine mixture. The ion intensities of the allantoin ions
in the mixture were lower that the ion intensities of the standard
while the ion intensities of the 2-hydroxypurine ions in the mixture
were higher than the ion intensities of the standard. This suggests
that a competitive process for ionization occurs between the two
compounds with the ionization of 2-hydroxypurine favored over
allantoin. The al1antoin/2,6-diamino - 8 - purinol and 2-
hydroxypurine/2,6-diamino-8-purinol mixtures show better agreement
(<20% error) between the mixture and standards. In each of the two-
component mixture studies, the summation of the positive ions of the
components in the mixture equals (within experimental error) the
summation of the positive ions of the pure compounds. The same was
true for negative ions. This result suggests that even though
competitive processes may exist which suppress the ionization of one

Table XIII. Summary of the two-component mixture study for allantoin
hydroxypurine (triplicate measurements).
Ion Intensity
Standard Mixture ‘
Compound
Ion
Allantoin
(715 ng)
159+
(M+H) +
176+
(M+NH4)
157”
(M-H) ”
2-Hydroxy¬
purine
(500 ng)
137+
(MfH) +
154+
(M+NH4)
135”
(M-H) ”
X Positive Ions
X Negative Ions
8.6 X 104 ± 1.5 X 104
1.7 X 105 ± 1.2 X 104
2.6 X 106 ± 5.7 X 105
4.2 X 105 ± 1.6 X 105
8.6 X 103 ± 5.3 X 103
2.0 X 106 ± 7.6 X 105
6.8 X 105 ± 1.9 X 105
4.6 X 106 ± 1.3 X 106
7.6 X 104 ± 2.6 X 104
1.3 X 105 ± 4.2 X 104
2.1 X 106 ± 4.8 X 105
5.8X 105 ± 1.2 X 105
1.5 X 104 ± 4.0 X 103
2.7 X 106 ± 6.1 X 105
8.0 X 105 ± 1.9 X 105
4.8 X 106 ± 1.1 X 106
a) % Error = fmixture - standard) X 100%
standard
and 2-
Error3
-12%
-24%
-19%
+38%
+74%
+35%
-18%
-4%
222

Table XIV. Summary of the two-component mixture study for allanto in and 2,6-
diamino-8-purinol (triplicate measurements).
Ion Intensity
Compound
Ion
Standard
Mixture
% Error3
Allanto in
159+
8.6
X
104
±
1.5
X
104
8.3
X
(-•
0
H-
2.9
X
104
-3%
(715 ng)
(MfH) +
176+
1.7
X
in
0
1—i
±
1.2
X
104
1.5
X
105 ±
5.2
X
104
-12%
(MfNH4) +
157“
2.6
X
106
±
5.7
X
105
2.2
X
106 ±
4.3
X
105
-15%
(M-H) “
2,6-diamino- 167+
3.0
X
105
±
1.0
X
105
2.9
X
105 ±
5.2
X
104
-3%
8-purinol
(M+H)+
(560 ng)
165“
1.4
X
105
±
5.0
X
103
1.4
X
105 ±
3.2
X
104
<1%
(M-H) “
225“
2.3
X
105
±
1.0
X
105
2.2
X
105 ±
5.6
X
104
-4%
(M+CH3OOO)
Z Positive
Ions
5.6
X
105
±
1.3
X
105
5.2
X
105 ±
1.3
X
105
-7%
Z Negative
Ions
3.0
X
106
±
6.8
X
105
2.6
X
106 ±
5.2
X
105
-13%
a) % Error = fmixture - standard] X 100%
standard
223

Table XV. Summary of the two component mixture study for 2-hydroxypurine and
2,6-diamino-8-purinol (triplicate measurements).
Ion Intensity
Compound
Ion
Standard
2-Hydroxy-
137+
4.:
2 X 105
± 1.
6
X ]
purine
(M+H) +
(500 ng)
154+
8.6
X
103 ±
5.3
X
10
(MfNH4) +
135“
2.0
X
106 ±
7.6
X
101
(M-H) “
2,6-diamino- 167+
3.0
X
+1
in
0
<—1
1.0
X
101
8-purinol
(MfH) +
(560 ng)
165"
1.4
X
103 ±
5.0
X
10
(M-H) “
225"
2.3
X
105 ±
1.0
X
101
(M+CH3OOO)
X Positive
Ions
7.3
X
-H
in
0
<—1
2.6
X
10!
X Negative
Ions
2.4
X
106 ±
8.6
X
10!
Mixture
% ETror3
3.3 X 105 ± 4.3 X 104 +10%
1.5 X 105 ± 6.5 X 103 +7%
2.7 X 105 ± 4.1 X 104 +17%
+8i
a) % Error = Ímixture - standard) X 100%
standard
224

225
compound while enhancing the ion formation of another, approximately
the same number of total sample ions (positive or negative) are being
formed.
The results of the three-component mixture study are shown in Table
XVI. The ion intensities for the components contained in the mixture
were slightly higher than those for the pure standards for 2-
hydroxypurine and 2,6-diamino-8-purinol and slightly lower for
allantoin. The largest % error was observed with the (M+NH4)+ ion of
2-hydroxypurine (40%). The remaining ions showed less than 20% error
between the mixture and standards. The summation of the positive ion
intensities in the mixture equal (within experimental error) the
summation of the positive ion intensities of the pure compounds. This
also suggests that competitive ionization processes exist in this
three -component mixture.
Summary of the Effects of Solution Characteristics
From these investigations we have demonstrated the effects of
various solution characteristics on thermospray ionization processes.
The mobile phase and sample solvent studies showed the importance of a
volatile electrolyte and its role during the droplet charging process.
Ion intensities of ribavirin were maximum when ammonium acetate was
used as both the mobile phase and the solvent. Ion intensities of
ribavirin showed a dependence on sample concentration which seems to
suggest that sample concentration has an effect on the direct ion
evaporation process. Changes in buffer concentration affect the
resulting mass spectra of both ammonium acetate and ribavirin. Higher
concentrations of buffer generally promote the formation of ammonium

Table XVI. Summary of the three-component mixture study for alian to in, 2-
hydroxypurine and 2,6-dioamino-8-purinol (triplicate measurements).
Ion Intensity
ComDound
log
Standard
Mixture
%Errora
Allantoin
159+
8.6 X 104 ± 1.5 X 104
8.6 X
104 ± 1.4 X
104
<1%
(715 ng)
(M+H) +
176+
(M+NH4)+
1.7 X 105 ± 1.2 X 104
1.4 X
105 ± 2.7 X
104
-18%
157“
2.6 X 106 ± 5.7 X 105
2.2 X
106 ± 4.0 X
105
-15%
2-Hydroxy-
137+
4.2 X 105 ± 1.6 X 105
5.4
X 105 ±1.2
X 10
4 +28%
purine
(500 ng)
(M+H) +
154+
(M+NH4)+
8.6 X 103 ± 5.3 X 103
1.2 X
104 ± 1.5 X
103
+40%
135" 2.0 X 106 ± 7.6 X I05 2.2 X 106 ± 6.3 X 105 +10%
(M-H)-
2,6-diamino-
167+
3.0
X
105
±
1.0
X
IT)
o
pH
3.7
X
105 ±
2.9
X
104
8-purinol
(M+H) +
(560 ng)
165“
1.4
X
103
±
5.0
X
103
1.7
X
105 ±
2.9
X
103
(M-H)-
225"
2.3
X
105
±
1.0
X
105
2.8
X
105 ±
4.2
X
104
(M+ai3ooo)‘
+23%
+21%
+22%
£ Positive Ions
£ Negative Ions
9.8
X
105 ±
2.9
X
105
1.1 X
106 ±
1.9
X
105
+13%
5.0
X
106 ±
1.5
X
105
4.8 X
106 ±
1.1
X
105
+4%
a) %Error = fmixture - standard1 X 100%
standard
226

227
adduct ions in the positive ion mode and acetate adduct ions in the
negative ion mode. From the buffer concentration experiments it was
also shown that the both the positive and negative ammonium acetate and
ribavirin ions behave in a manner typical of Cl: a decrease in the ion
intensities ammonium acetate (reagent ions) in the presence of neutral
sample molecules. The effects of pH on the mass spectra of ammonium
acetate and histidine was demostrated. However, more insight into the
contributions of the direct ion evaporation process was not obtained.
The studies dealing with the thermospray ionization behavior of two-
and three-component mixtures indicate that even with simple mixtures,
competitive ionization processes may exist.

CHAPTER VII
CONCLUSIONS AND FUTURE WORK
From these studies we have determined the effects of various
parameters on thermospray ionization. Particular changes in any of
these parameters will affect particular processes of thermospray,
resulting in a change in the overall sensitivity and the relative
abundances in the mass spectra. Operating at conditions where changes
in a particular parameter produce a minimal change in the ion intensity
of a particular ion or ions should result in minimal changes in the
mass spectra. The selection of instrumental parameters in order to
minimize the fluctuation of ion intensities for all ions would appear
to be difficult, since the fragment ions and pseudo-molecular ions
typically showed different behavior with changes in the various
parameters. Also, it is likely that different optimum conditions exist
for different sample compounds.
The experiments with thermospray plumes in vacuum showed that
droplet-rich and vapor-rich regions exist within a thermospray plume.
Droplets are generally abundant in the regions of the plume close to
the probe tip (<1 cm) , while vapor predominates regions of plume
further from the probe tip. The water droplets produced by thermospray
were mostly negatively charged with maximum current measured at
distances close to the plume (<1 cm).
The experiments dealing with the instrumental parameters of
thermospray ionization showed the effects of various conditions of
vaporization, droplet evaporation, and ion sampling position on the
thermospray behavior of ammonium acetate and ribavirin. Results of the
tip temperature, source temperature, and probe position studies showed
228

229
that the formation of fragment ions are favored by larger droplets at
regions close to the probe tip (<1 cm) while pseudo-molecular ion
formation is favored by smaller droplets at regions further away from
the probe tip (>1 cm) . The source temperature study also showed that
excessive temperatures (>300 C) increases fragment ion formation,
possibly due to thermal decomposition of the pseudo-molecular ions.
The flow rate results were rather difficult to interpret since this
parameter affected many of the other parameters. However, it was
evident that increased flow rates (up to 2.0 mL/min) result in
increased ion intensities. The studies dealing with source pressure
showed that optimum source pressures exist for fragment ion and pseudo-
molecular ion formation.
The distinction between the direct ion evaporation mechanism and Cl
mechanism in thermospray ionization of a sample molecule is difficult
to make. We had hoped that the experiments dealing with flow rate,
source pressure, and solution characteristics would provide more
conclusive information into these two processes. However, these two
processes are difficult to isolate and study independent of one
another. Although our results are inconclusive, they do indeed
indicate that Cl most likely plays a major role in sample molecule
ionization by thermospray.
Our recommendations for future work in thermospray fundamentals are
directed towards methods which will help to better distinguish between
the direct ion evaporation and Cl sample ionization mechanisms.
Studies dealing with several classes of model compounds (rather than
one class) should provide more data so that the generalizations made
here can be confirmed. Studies dealing with gas-phase basicities

230
should provide valuable insight into the role of gas-phase ion/molecule
reactions in thermospray ionization. In these experiments selected
classes of compounds could be used whose gas-phase and solution-phase
basicities are known. The most interesting compounds would be those
which show different behavior in solution and in the gas-phase.
Compounds which show an improvement in thermospray performance with
increasing gas-phase basicity would indicate the dominance of gas-phase
ion/molecule reactions with thermospray ionization. Compounds which
show an improvement in thermospray performance with increasing
solution-phase basicity would indicate the dominance of direct ion
evaporation processes with thermospray ionization.
Another study which should provide more information would require
the use of external source of ionization. In these experiments
compounds in which the thermospray ionization behavior is known would
serve as model compounds. An external source of ionization such as a
filament or discharge would be used as compounds dissolved in water are
thermosprayed with water mobile phases (no volatile electrolytes used).
The source could also be configured in such a way as to allow for the
introduction of a suitable reagent gas such as ammonia. The formation
of (M+H)+ and (M+NH4)+ ions would confirm the formation of intact
neutral sample molecules by thermospray without the presence of a
volatile electrolyte. The use of a labeled isotope of a particular
reagent gas (i.e. ND3) may provide even more interesting results. If
(M+ND4)+ species are observed this would also indicate that the Cl
mechanism plays a major role in sample molecule ionization by
thermospray.

231
Instrumentality, it would be interesting to determine the effects of
modifications of source geometry, in particular, a modification which
can regulate the amount of droplet collisions or turbulent flow within
the plume. The insertion of a rotatable stainless steel "flap" into
which droplets could collide, resulting in more turbulent flow within
the source, would seem to be the easiest and most straightforward way
of achieving this condition. The dependence of fragment and pseudo-
molecular ion formation on various orientations of this flap could
provide an alternative way of achieving more fragmentation by
thermospray without the use of a repeller or a tandem mass
spectrometer.

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BIOGRAPHICAL SKETCH
Mike Seung Chong Lee was born in Washington DC on September 21,
1960. He attended various public schools in Princes Georges and
Montgomery Counties. In 1978, he graduated from Sherwood High School
in Montgomery County, Maryland. From 1978 to 1982 he attended the
University of Maryland where he graduated with a B.S. degree in
chemistry. In the fall of 1982 he began his graduate school studies at
the University of Florida under the direction of Dr. Richard A. Yost.
In August, 1985, he received the Master of Science degree in analytical
chemistry. He remained at the University of Florida to complete work
on the Ph.D. in analytical chemistry.
236

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.
Associate 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.
James D. Winefordnqn:
^Graduate Research Professor
*of Chemistry
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
i.O
John G). Dorsey
Ass^elate Professor of

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.
Professor of Environmental
Engineering Sciences
This dissertation was submitted to the Graduate Faculty of the
Department of Chemistry in the College of Liberal Arts and Sciences and
to the Graduate School and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy.
December, 1987
Madelyn Lockhart
Dean, Graduate School

UNIVERSITY OF FLORIDA
3 1262 08554 0234




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