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Characterization and enhancement of sample introduction and ion transmission in combined gas chromatography/tandem mass spectrometry

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Characterization and enhancement of sample introduction and ion transmission in combined gas chromatography/tandem mass spectrometry
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Hail, Mark Edward
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viii, 173 leaves : ill. ; 28 cm.

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Electric potential ( jstor )
Flow velocity ( jstor )
Inlet pressure ( jstor )
Ionization ( jstor )
Ions ( jstor )
Mass spectrometers ( jstor )
Mass spectroscopy ( jstor )
Quadrupoles ( jstor )
Reactants ( jstor )
Velocity ( jstor )
Chemistry thesis Ph. D ( lcsh )
Dissertations, Academic -- Chemistry -- UF
Gas chromatography ( lcsh )
Mass spectrometry ( lcsh )
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Thesis:
Thesis (Ph. D.)--University of Florida, 1989.
Bibliography:
Includes bibliographical references (leaves 167-172).
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Typescript.
General Note:
Vita.
Statement of Responsibility:
by Mark Edward Hail.

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CHARACTERIZATION AND ENHANCEMENT OF SAMPLE INTRODUCTION AND
ION TRANSMISSION IN COMBINED
GAS CHROMATOGRAPHY/TANDEM MASS SPECTROMETRY













By

MARK EDWARD HAIL


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


1989




CHARACTERIZATION AND ENHANCEMENT OF SAMPLE INTRODUCTION AND
ION TRANSMISSION IN COMBINED
GAS CHROMATOGRAPHY/TANDEM MASS SPECTROMETRY
By
MARK EDWARD HAIL
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULLFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1989


To my loving wife, best pal, and big toe, Amy.


ACKNOWLEDGMENTS
I would like to express sincere gratitude to my graduate research
director, Dr. Richard A. Yost, for his guidance and friendship over the
past four years. Rick deserves some sort of medal, or perhaps more
appropriately, a new prescription for his glasses, for all of the reading
and reviewing of manuscripts and thesis chapters that he has done during
the past few months. I would like to thank the members of my graduate
committee, Drs. James D. Winefordner, Anna Brajter-Toth, Samuel 0.
Colgate, and Henri A. Van Rinsvelt. I would also like to thank Dr. John
G. Dorsey (a fellow Lexingtonian) for his helpful discussions.
I would like to acknowledge the U. S. Air Force Engineering and
Services Center at Tyndall Air Force Base and the U. S. Army Chemical
Research Development and Engineering Center at Aberdeen Proving Grounds
for providing the financial support for this work.
No graduate student would be very productive without the help of the
departmental staff. For this reason, I thank Chester Eastman, Vern Cook,
and Dailey Burch of the machine shop, and Mark Ross, Steve Miles, and Dean
Schoenfeld of the electronics shop. I extend special thanks to Russ
Pierce of the electronics shop, who was particularly helpful in answering
questions and troubleshooting problems in the electronics lab and in my
research.
I would like to thank my friends who made this period of my life so
enjoyable, including all of the Yosties, and post-Yostie Jodie Johnson.
Special thanks to my two office mates and soon-to-be coworkers, Ken
iii


Matuszak and Dave "Cool One" Berberich. I would also like to thank those
that dwelled with me in that most humble abode known as the thunderdome,
where one man enters and two people leave. In order of appearance (and
in some cases disappearance), thanks to Todd "Spud" Gillespie, Stephen
"Judas" Brooks, Mark "Citizen" Barnes, and Jerry "Citrus Connection"
Grnewald.
I would like to thank my family, Mom, Dad, Carl Jr. Beccy, and Steve,
who have been so supportive.
Finally, I would like to thank my wife, Amy, the person who was always
there to pat me on the back, who did my laundry and fed my face when I
didn't have time, and who was always there for entertainment just by being
herself.
iv


TABLE OF CONTENTS
page
ACKNOWLEDGMENTS iii
ABSTRACT vii
CHAPTERS
1 INTRODUCTION 1
Short GC Columns in GC/MS and GC/MS/MS 1
Short GC Columns at Sub-ambient Pressures 2
Mixture Analysis by GC, GC/MS and GC/MS/MS 4
Organization of Dissertation 10
2 THEORETICAL AND PRACTICAL ASPECTS OF SHORT OPEN
TUBULAR COLUMNS AT SUB-AMBIENT PRESSURES IN
GAS CHROMATOGRAPHY/MASS SPECTROMETRY 12
Introduction and Theory 12
Experimental Section 15
Calculations 15
Mass Spectrometry 15
Gas Chromatography 16
Average Velocity Measurement 19
Results and Discussion 20
Effect of the Pressure Drop 20
Sub-ambient Inlet Pressures 26
Experimental vs. Theoretical Performance 37
Choice of the Carrier Gas 41
Conclusions 43
3 CONCEPTS FOR PORTABLE GAS CHROMATOGRAPHIC
INSTRUMENTATION IN GC/MS: FLOW RATE
PROGRAMMING AND DIRECT COLUMN HEATING 46
Introduction 46
Flow Rate Programming 47
Direct Resistive Heating of Al-Clad Capillary Columns ... 48
GC Probe 51
Experimental Section 52
Experimental Conditions 52
Flow Control System 53
Direct Resistive Heating 57
v


GC Probe Design 61
Results and Discussion 64
Flow Rate Programming 64
Direct Resistive Heating 77
GC Probe Performance 85
Conclusions 89
4 GAS CHROMATOGRAPHIC SAMPLE INTRODUCTION INTO THE COLLISION
CELL OF A TRIPLE QUADRUPOLE MASS SPECTROMETER
FOR MASS-SELECTION OF REACTANT IONS FOR
CHARGE EXCHANGE AND CHEMICAL IONIZATION 90
Introduction and Theory 90
Experimental Section 93
Mass Spectrometry 93
Gas Chromatography 95
GC/MS Interface 96
Results and Discussion 98
Ionization Energy Measurements 98
Simultaneous Structural and Molecular Weight Information 101
Selectivity of Ionization 106
Detection Capabilities of GC/MSR/MS 112
Chromatographic Integrity 116
Conclusions 116
5 CHARACTERIZATION AND OPTIMIZATION OF ION TRANSMISSION
IN TRIPLE QUADRUPOLE MASS SPECTROMETRY 118
Introduction 118
Experimental Section 119
Instrumentation 119
Procedures 125
Results and Discussion 126
Mass-dependent Optimization 126
Quadrupole Mass Filters 135
Ion Transmission Characteristics of the Center Quadrupole 140
Ion Transmission at High Collision Energies 153
Conclusions 157
6 CONCLUSIONS AND FUTURE WORK 159
Conclusions 159
Suggestions for Future Work 161
Reduced Column Pressures with Conventional GC Detectors . 161
Directly-heated Open Tubular Columns 162
Resonance Excitation and Ion Storage in Q2 164
LITERATURE CITED 167
BIOGRAPHICAL SKETCH 173
vi


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
CHARACTERIZATION AND ENHANCEMENT OF SAMPLE INTRODUCTION
AND ION TRANSMISSION IN COMBINED
GAS CHROMATOGRAPHY/TANDEM MASS SPECTROMETRY
By
Mark Edward Hail
May 1989
Chairman: Richard A. Yost
Major Department: Chemistry
Various theoretical and practical aspects of the use of short open
tubular columns in combined gas chromatography/mass spectrometry (GC/MS)
and tandem mass spectrometry (GC/MS/MS) have been investigated, including
the advantages of low-pressure operation with short and/or wide-bore open
tubular columns. It is shown both theoretically and experimentally that
short columns with vacuum outlet require sub-atmospheric inlet pressures
if optimum gas velocities are to be obtained. The use of sub-ambient
inlet pressures is also shown to improve the sensitivity when short
columns are used with electron ionization in GC/MS.
Several considerations are addressed for the design of portable GC
instrumentation. In particular, exponential flow rate programming as well
as resistively-heated aluminum-clad open tubular columns have been
evaluated for low-power, high-throughput analysis in GC/MS. Aluminum-clad
vii


open tubular columns are heated directly by applying a voltage across the
capillary, and the column temperature is sensed by measuring the column
resistance. These investigations have led to the development of a compact
gas chromatograph probe for use in GC/MS.
A gas chromatograph has been interfaced to the collision cell of a
triple quadrupole tandem mass spectrometer for performing mass selected
ion-molecule reactions. Reactant ions are selected with the first
quadrupole and are allowed to react in the second quadrupole collision
cell with the effluent from a short open tubular GC column. The ion-
molecule reaction product ions are mass-analyzed by the third quadrupole.
The advantages of using mass-selected reactions for controlling the
selectivity of charge exchange and chemical ionization are demonstrated.
In addition, this configuration is shown to provide both structural
information and molecular weight information in the same chromatogram by
alternating between different reactant ions.
The ion transmission characteristics of a triple quadrupole mass
spectrometer (TQMS) have been studied. The unique high level of computer
control of the TQMS has allowed for rapid characterization and
understanding of the ion optical parameters. Strategies for obtaining
optimum ion transmission and minimization of mass dependencies in the ion
optics for single MS experiments as well as for MS/MS experiments are
discussed.
viii


CHAPTER 1
INTRODUCTION
In this dissertation a variety of concepts are presented for the
enhancement of sample introduction and ion transmission in analytical mass
spectrometry (MS) and tandem mass spectrometry (MS/MS). Due to the
inherent speed and selectivity of MS and MS/MS, considerable attention was
devoted to the study of rapid gas chromatographic (GC) separation
techniques. A new tandem mass spectrometric method, in which the
selectivity of ion-molecule reactions is exploited for gas chromatographic
mixture analysis, is also described. Finally, the results of a detailed
characterization of the ion optical parameters of a triple quadrupole
tandem mass spectrometer are presented.
Short GC Columns in GC/MS and GC/MS/MS
In this research group, it has been found that short GC columns can be
used for the extremely rapid analysis of complex mixtures by taking
advantage of the inherent selectivity of MS, and particularly MS/MS [1-
3], In the initial results that were obtained with very short (e.g., 50
cm) packed columns, it was found that the .sensitivity and selectivity
obtainable with short-column GC in conjunction with MS/MS in many cases
exceeded that of high-resolution GC/MS (HRGC/MS) [1], In addition, it
was shown that 100 injections per hour could be performed with the short-
column method, while only 2 injections per hour could be made with the
HRGC/MS method employing an 18 m capillary column [1], Later, short open
1


2
tubular capillary columns were evaluated (including an investigation of
the associated chromatographic theory) due to their inherent advantages
over packed columns [2,3]. These studies began to demonstrate the
advantages of vacuum outlet operation with short open tubular columns.
It was found that there were added benefits other than speed of analysis
that were important. For instance, because of the high gas velocities and
low operating pressures associated with short open tubular columns under
vacuum outlet conditions, it was found that solutes tended to elute at
temperatures well below their boiling points. Thus, it was discovered
that short-column open tubular GC in conjunction with MS or MS/MS was
ideal for the analysis of involatile, thermally labile, or polar solutes
[2,3], More recently, McClennen et al. have taken advantage of this and
have demonstrated that large involatile and/or polar molecules can be
analyzed by examining the Curie-point pyrolysis products of these
molecules with short-column GC/MS [4,5],
Short GC Columns at Sub-ambient Pressures
Many of the chromatographic separation techniques currently in
widespread use require relatively high operating pressures, namely, high-
pressure liquid chromatography (HPLC), supercritical fluid chromatography
(SFC), and even high-resolution capillary open tubular gas chromatography
(HRGC). In contrast, research in the area of low-pressure separation
techniques has been limited. More than 25 years ago, Giddings
demonstrated that the minimum analysis times in gas chromatography could
be achieved when the column outlet was connected to a vacuum [6] .
However, to date, a majority of the research done in high-speed GC has
dealt with the use of small diameter columns and the development of high-


3
speed injectors required for use with these columns [7-13], Recently,
interest in vacuum outlet operation has been renewed, as Leclercq and
coworkers have published a number of articles that have addressed the
theory and advantages of vacuum outlet operation for reducing analysis
times in open tubular GC [14-17],
Vacuum outlet operation in gas chromatography results in lower column
pressures, which increases the diffusivity of the solute in the gas phase
and leads to increased optimum carrier gas velocities and shorter analysis
times [14]. The advantages of vacuum outlet operation are inherently
obtained when gas chromatography is combined with mass spectrometry
(GC/MS), provided the column is inserted directly into the vacuum of the
mass spectrometer. Consideration of the theory indicates that low outlet
pressure operation has a more pronounced effect if wide-bore columns [16]
and/or short columns [3] are used. As shorter column lengths are used to
minimize analysis times, lower inlet pressures are required if optimum
chromatographic performance is to be obtained. The use of very short
lengths or wide-bore columns is limited when conventional inlet pressures
are used because of the gas load imposed on the mass spectrometer.
Alternatively, the inlet can be operated at sub-ambient pressures to
obtain optimum performance and/or to reduce the flow rate of carrier gas
entering the mass spectrometer vacuum system [18]. Unfortunately, the
value of sub-atmospheric column inlet pressures has not been widely
appreciated, due in part to the difficulty of operating traditional GC
injection ports at vacuum.


4
Mixture Analysis by GC. GC/MS. and GC/MS/MS
Application of short columns with conventional GC detectors has been
limited to the analysis of relatively simple mixtures due to the non-
selective nature of the detectors and the reduced resolving power of short
columns. However, as stated previously, the inherent selectivity of MS
and MS/MS can be utilized to atone for the chromatographic resolution lost
when shorter columns are used. Perhaps a better comprehension of the
additional selectivity of the mass spectrometric detection methods over
conventional detection methods in gas chromatography can be obtained by
considering the possible cases which result in interferences. These
coincidences are summarized in Table 1.1, which shows a comparison of GC
employing flame ionization detection (FID) as well as GC with various mass
spectrometric detection methods. A schematic comparison of the three mass
spectrometric techniques, all of which can be performed on the triple
quadrupole tandem mass spectrometer used in this work, is shown in Figure
1.1.
Mixture overlap will readily occur when non-selective detection methods
(e.g., FID) are used in complex mixture analysis, unless a column of
sufficient length is used to chromatographically separate all possible
interferences from the analyte of interest. In many cases, no column,
regardless of length, would be sufficient to accomplish this goal; thus,
other extraction and sample cleanup procedures must be used in addition
to the already time-consuming chromatographic analysis. In addition,
since chromatographic resolving power increases only with the square root
of the column length, while analysis times increase proportionally with
column length, increasing the column length to gain the necessary


5
Table 1.1. Cases leading to mixture overlap (interferences) in gas
chromatographic mixture analysis with flame ionization detection and
various methods of mass spectrometric detection. The '?'s in the table
indicate the conditions under which targeted mixture components will be
obscured by interferences.
Analytical Method
GC/FID
GC/MS
Coincidences
GC/MS/MS
Resulting in Interferences
GC/MSR/MS3
(1) coelute
(1) coelute
(1)
coelute
(1)
coelute
4-
4
4
4
?
(2) produce
(2)
produce
(2)
react to form
ions of
ions of
product ions
same m/z
same m/z
of same m/z
4
4
4
?
(3)
fragment to
?
daughter ions
of same m/z
1
?
a GC/MSR/MS = GC/mass-selected reaction/MS, see Figure 1.1.


Figure 1.1. Schematic comparison of three different combined gas chromatography/mass spectrometric methods
which may be performed on a triple quadrupole mass spectrometer: (a) GC/MS, (b) GC/MS/MS, and (c) GC/mass-
selected reaction/MS (GC/MSR/MS). The new technique described in this thesis, GC/MSR/MS, involves introduction
of the GC column into the second quadrupole collision cell (Q2), rather than into the ion source. Also note
the differences in the two MS/MS approaches. In traditional MS/MS, a sample (parent) ion is mass-selected
with Q1 and is allowed to fragment via CAD in Q2, whereas in MSR/MS, Q1 is used to mass-select a reactant ion
for reaction with sample molecules in Q2.


SAMPLE
REACTANT
CAS
(a)
SAMPLE
REACTANT
CAS
REACTANT
G AS < EST"
IONI
ZATION
ION
SOURCE
GC/MS
Q1 Q2 Q3
Q1 MASS
SPECTRUM
EM
GC/MS/MS
Q 1
Q2
CAD
Q3
DAUGHTER
SPECTRUM
E M
COLLISION
GAS
GC/MSR/MS
Q 1
Q2
CE or Cl
Q3
PRODUCT
SPECTRUM
SAMPLE


8
resolution tends to be very costly in terms of the time required for an
analysis.
As shown in Table 1.1, the possibilities for interferences in mixture
analysis are greatly reduced when mass spectrometric detection is used.
In GC/MS (Figure 1.1 (a)), other component(s) of the mixture must coelute
with the analyte of interest and must yield ions of the same m/z as the
analyte of interest in order for interferences to occur. Even more
selectivity is obtainable with GC/MS/MS (Figure 1.1 (b)). In order for
mixture overlap to occur in GC/MS/MS, undesirable interferents must not
only coelute and be ionized to produce the same nominal mass ions, but the
interfering parent ions must also fragment via collisionally-activated
dissociation (CAD) to produce all of the same daughter ions as the
analyte.
Described in this dissertation is a new GC/tandem mass spectrometric
technique, GC/mass-selected reaction/MS (GC/MSR/MS), which involves mass-
selection of reactant ions and reaction of these ions with the effluent
from a gas chromatographic column. It can be seen from Figure 1.1 (c)
that this method involves introduction of the GC column into the second
quadrupole collision cell (Q2) rather than into the ion source. This
method takes advantage of the intrinsic selectivity of ion-molecule
reactions. Hence, for overlap to occur in mixture analysis with
GC/MSR/MS, interferences must react in Q2 to produce all of the same m/z
product ions as the analyte of interest.
It can be seen from the above comparisons that the added resolution
elements available with the mass spectrometric techniques in many cases
obviate the need of high-resolution chromatographic separation in mixture
analysis. Therefore, the use of short open tubular columns and the


9
concomitant short analysis times are readily justified.. In addition, the
capability to perform any of the three GC/MS techniques (GC/MS, GC/MS/MS,
or GC/MSR/MS) certainly demonstrates the versatility of the triple
quadrupole mass spectrometer (TQMS).
Although instrumentation for MS/MS tends to be rather expensive (e.g.,
$350,000 for a TQMS) when compared to other analytical techniques (e.g.,
$15,000 for a GC), it has been pointed out that MS/MS is a cost-effective
technique because of the short analysis times that are possible [19].
This is most likely the reason for the increasing popularity as well as
the widespread availability of commercial instrumentation for MS/MS. The
MS/MS technique has certainly benefited from recent advances in computers
and electronics, as these progressions have led to the development of a
wealth of new, exciting, computer-controlled instrumentation. One such
instrument is the Finnigan MAT TSQ70, which was used throughout this work.
This instrument, with its unique level of computer control, has allowed
a detailed study of the ion optical parameters and their effects on ion
transmission in triple quadrupole mass spectrometry. In fact, many of the
undesirable ion optical effects (e.g., mass dependencies) can be avoided
due to the capability of this instrument to vary any the ion optical
parameters as a function of mass within each mass spectral scan. These
types of studies have provided a more thorough understanding of the ion
optical parameters that effect the performance of this instrument, which
will ultimately improve the analytical capabilities during its use.


10
Organization of Dissertation
This dissertation is divided into six individual chapters. This
chapter has served to present an overall perspective as well as the
purpose of the work. Global conclusions and suggestions for future
research are found in Chapter 6. Chapters 2-5 make up the body of the
dissertation. Due to the widely varying material presented in this
dissertation, there is no particular chapter devoted entirely to the
description of experimental conditions. Rather, each chapter (of Chapters
2-5) consists of a separate introduction, experimental section, discussion
of results, and conclusions.
In Chapter 2, the advantages of utilizing low column pressures with
short open tubular columns in GC/MS are examined by consideration of the
theoretical chromatographic relationships. In particular, the advantages
of operating the column inlet, as well as the column outlet, at sub-
atmospheric pressures are stressed. Experimental data are presented that
demonstrate the practicality of the combination of short GC columns with
mass spectrometry.
In Chapter 3, considerations for portable and/or high-throughput sample
introduction schemes for GC/MS are presented, including investigations of
exponential flow rate programming and direct resistive heating of
aluminum-clad open tubular columns. A portable direct insertion gas
chromatograph probe is described that incorporates these concepts.
The fourth chapter describes the GC/MSR/MS technique, including a
brief discussion of the utility of selective ion-molecule reactions for
mixture analysis. This work represents the first experiments ever
performed in which samples are introduced from a GC column into the center
quadrupole collision cell of a TQMS. Experimental data are presented that


11
demonstrate the potential selectivity, detection and screening
capabilities of this method. A method is also described which allows for
the extremely rapid measurement of ionization energies on the computer-
controlled TQMS.
In Chapter 5, the effects of various ion optical parameters of the
TQMS for MS and MS/MS are investigated. In particular, it is shown that
the mass-dependencies in the ion optics of the computer-controlled TQMS
can be minimized, since all of the ion optical parameters may be varied
as a function of mass during a mass spectral scan. The insights gained
in these experiments have yielded a better understanding of the complex
inter-relation of the parameters that affect the performance and
reproducibility of the results obtained with this instrument.


CHAPTER 2
THEORETICAL AND PRACTICAL ASPECTS OF SHORT OPEN TUBULAR COLUMNS
AT SUB-AMBIENT PRESSURES IN GAS CHROMATOGRAPHY/MASS SPECTROMETRY
Introduction and Theory
In this chapter, various theoretical and practical aspects of the use
of short open tubular columns in GC/MS are discussed, including the
advantages of low-pressure operation with short and/or wide-bore open
tubular columns. It is shown that short columns with vacuum outlet
require sub-atmospheric inlet pressures if optimum gas velocities are to
be obtained. Since sub-atmospheric inlet pressures are not obtainable
with most gas chromatographic inlet systems, a computer-controlled flow
control system has been investigated that allows the injection port to be
operated at sub-ambient pressures with either split or splitless
injections. Plate heights have been experimentally determined at sub
ambient pressures and have been compared with theoretical predictions.
The effects of extra column band broadening are evident in the
experimental data and demonstrate the need for high-speed injection
systems if maximum performance is to be obtained with short columns. The
use of sub-ambient inlet pressures is also shown to improve the
sensitivity when short columns are used with electron ionization in GC/MS.
Finally, it is shown that hydrogen carrier gas can be used to obtain the
fastest analyses without altering the relative abundances of ions obtained
with methane chemical ionization mass spectrometry.
12


13
The theoretical dependence of the plate height, H, on the average gas
velocity, v, in open tubular gas chromatography is given by the Golay
equation [20]
H = B/v + (Cg+Ct)v
(2.1)
where B is a term relating to the longitudinal diffusion of the solute
zone, and Cg and are terms relating to mass transfer in the gas phase
and liquid stationary phase, respectively. For open tubular columns
employing a thin-film stationary phase, mass transfer in the liquid phase
is negligible and Cg dominates [14]. The Golay equation for thin-film
columns corrected for gas compressibility [21] can be rewritten in terms
of the column parameters yielding [14]
2D Qf (l+6k+llk2)r2uQf
H : + (2.2)
v0 24(l+k)2Dgo
where Dg Q is the diffusion coefficient of the solute in the gas phase at
the outlet pressure, f is the Giddings correction factor, is the gas
velocity at the column outlet, k is the partition ratio, and r is the
column radius. The Giddings correction factor accounts for the
decompression of the carrier gas when a pressure gradient exists and is
given by
9(P4-1)(P2-1)
f
8(P3-!)2
(2.3)


14
where P is the inlet to outlet pressure ratio, Pi/PQ. The gas
decompression factor approaches a value of 9/8 when large pressure
gradients are employed, as in GC/MS. Most commonly, H is expressed in
terms of the average velocity, u. If this is done, uQ and Dg Q of eq 2.2
are replaced by the average velocity, v, and an average diffusion
coefficient, Dg. An average diffusion coefficient can be estimated from
[14]
D
9
P19.1
(2.4)
where Dg 1 is the diffusion coefficient at 1 atmosphere pressure, P1, and P
is the average column pressure. The length-average column pressure, P,
is dependent on the pressure gradient and is given by [14]
2P0(P3-1)
P (2.5)
3(P2-1)
which reduces to 2Pt-/3 for vacuum outlet conditions.
As shown by eq 2.4, lowering the average column pressure increases the
diffusion coefficient. An increase in the diffusion coefficient has been
shown to result in an increase in the optimum gas velocity and decreased
analysis times [3,14-18], In addition, a larger Dg results in a slower
rate of increase of H (from the Cg term) as the gas velocity is increased
beyond the optimum. This means that even higher gas velocities can be
used without severe losses of efficiency.


15
There are a number of ways of decreasing the average column pressure.
Lower average column pressures are readily obtained if the column outlet
is at vacuum as in GC/MS. At lower outlet pressures, the greatest gains
in terms of speed of analysis (as compared to normal atmospheric outlet
operation) can be obtained if short and/or wide-bore columns are used
[3,16]. Lower average column pressures can also be obtained if low-
viscosity carrier gases are used [22]; hence, the use of hydrogen as a
carrier gas will always result in the shortest analysis times.
Experimental Section
Calculations
All theoretical calculations were done on an IBM PC/XT or a PC/AT, with
programs written in TurboBASIC (Borland International). The results of
the calculations (e.g. H vs. u data) were stored on disk in ASCII format,
which allowed the data to be retrieved and plotted with a commercially
available plotting program (Grapher, Golden Software, Inc.).
Mass Spectrometry
A Finnigan MAT TSQ70 triple quadrupole mass spectrometer was used in
these studies. Electron energies of 70 and 100 eV were used for electron
ionization (El) and chemical ionization (Cl), respectively. The
appropriate interchangeable ion volumes were used to obtain El and Cl
spectra. In order to obtain true ion source pressures, indicated ion
source pressures were calibrated with a capacitance manometer. This was
done by connecting a capacitance manometer (MKS Instruments model 127AA)
to the Cl ion source via the side flange (normally used for GC/MS transfer
line) of the TSQ70. The capacitance manometer was used to calibrate the


16
ion source thermocouple gauge at various pressures of methane reagent gas.
Ion source pressures were determined as a function of flow rate of helium
and hydrogen carrier gases by connecting a mass flow controller (MKS
1159A) to the Cl ion source and recording the true pressures read from the
capacitance manometer. All ion source pressures reported are the absolute
pressures obtained from the calibrations. Ion source pressures for
chemical ionization were typically 0.2-0.5 torr. The ionizer temperature
was 150C. The preamplifier gain was set to 108 V/A; the electron
multiplier was operated at 1000 V.
Gas Chromatography
A Varian model 3400 GC equipped with a split/splitless injector with
a 2 mm i.d. glass liner was used. Bonded phase fused silica open tubular
columns were used with an inner diameter of 0.25 mm and a 0.25 im film
thickness (DB-5, J&W Scientific). Column lengths were typically 3 m. All
columns were inserted directly into the ion source. Column efficiencies
were measured with n-pentadecane (C15H32) with the column and interface at
100C (k = 32) and an injection temperature of 200C. For all other
applications temperature programming was used with the interface at 150C.
Column temperatures were programmed at a rate of 25C/min from 50C to
150C, after an initial hold period of 12 seconds.
A schematic of the gas chromatographic system, modified to allow for
either normal pressure-regulated operation or flow-controlled operation,
is shown in Figure 2.1. With valve 1 (VI) rotated to position A, the GC
can be operated in the normal configuration with inlet pressures above 1
atmosphere. With VI rotated to position B, flow regulation is used,
allowing inlet pressures above or below one atmosphere to be used. In


CA R R I
GAS
PRESSURE
REGULATOR
ME C HA NICAL
E R
Figure 2.1. Schematic diagram of gas chromatographic flow control system that allows inlet pressures above
or below one atmosphere to be used in the injection port.


18
addition, computer control of the flow controller allows the flow rate of
the carrier gas to be programmed with time if desired (see Chapter 3).
The mass flow controller (MFC) used was a MKS model 1159A with a type
246 power supply/digital readout. The output flow rate of the MFC, which
has a range of 0-100 mL/min for N2, is set by a 0-5 V control voltage from
the TSQ70. The TSQ70 has two spare digital-to-analog converters (DACs),
or user outputs, one of which was used to control the flow rate of the
MFC.
In order to achieve the sub-ambient inlet pressures, a 30 L/min
mechanical pump (Alcatel model 8A) was connected to the split line of the
injection port. A Bourdon tube pressure gauge (Omega Engineering model
30V/30) capable of indicating pressures above or below one atmosphere was
connected to the septum sweep of the GC injection port. Valve V2 is a
solenoid valve that can be opened or closed under computer control. The
inlet pressure in the vacuum inlet mode is determined by the flow rate of
carrier gas from the MFC and the adjustment of the needle valve (V3). The
occurrence of air leaks in the system was monitored by observing the
intensity of the N2+ (m/z 28) and 02+ (m/z 32) ions. It was necessary to
replace the plastic fittings and viton ferrules of the Varian 3400 carrier
gas lines with stainless steel fittings to minimize leakage of air into
the system. Leakage of air through the septum was minimized by replacing
the septum after frequent use (e.g., after 30 injections). Once these
steps were taken, no additional increase in the intensity of the air peaks
was observed when vacuum inlet operation was used instead of normal
atmospheric operation.
During a split injection, samples are injected directly into the low-
pressure injection port with valve V2 open; thus, most of the sample is


19
swept away by the mechanical pump. One of the problems of vacuum inlet
GC that has limited its use in the past is that only split injections
could be performed. This is certainly a limitation, especially when
analyzing samples with analytes present at trace levels. Splitless
injection is commonly used for trace analysis [23], With the flow control
system shown in Figure 2.1, the advantages of vacuum inlet GC, as well as
splitless operation, can be utilized. A splitless injection is made with
V2 initially closed, with the injection port slightly above atmospheric
pressure. After allowing ample time for the sample to enter the column,
the inlet pressure is then rapidly reduced by opening V2. Based on the
internal volume of the injection port (0.24 mL), at a typical flow rate
of 5 mL/min, the injection port is completely flushed with carrier gas in
approximately 3 s. Thus, leaving V2 closed for any period longer than 3
s should be sufficient time to insure that all the sample has entered the
column. Nevertheless, in all experiments utilizing splitless injections
reported here, V2 was left closed for 12 s or more.
Average Velocity Measurement
It was found that the average velocity of the carrier gas could not be
accurately measured by conventional means with the short columns that were
used. Traditionally, the average velocity is determined from the dead
time, which is the time required for the elution of unretained air or
methane. A 3 m x 0.25 mm i.d. column at 50C and 1 atmosphere inlet
pressure of helium carrier gas has a gas velocity of 240 cm/s and a dead
time of 1.25 s. Dead times that are this short are nearly impossible to
measure with a reasonable degree of accuracy. An alternative method is
to measure the gas flow rate and calculate the gas velocity. This is not


20
entirely straightforward in GC/MS. One can obtain a measure of the column
flow rate by measuring the gas flow rate exiting the forepump of the
vacuum system, since all of the carrier gas exiting the column must exit
through this pump. However, the pulsations produced by the pump limit
the accuracy and precision of this measurement, especially at low flow
rates. A more accurate indication of the average gas velocity when short
columns are used can be obtained by measurement of the inlet pressure.
If the inlet pressure, P1-, is known, the average velocity under vacuum
outlet conditions can be calculated from the Poiseuille equation [14]
v = 3Pir2/32r7L (2.6)
where r is column radius, rj is the viscosity of the carrier gas at the
column temperature, and L is the column length. With the pressure gauge
that was used, inlet pressures ranging from -14.7 psig to 30 psig could
be measured with a rated accuracy of 3% [24].
Results and Discussion
Effects of the Pressure Drop
As was stated previously, lower average column pressures result in
increased gas-phase diffusion coefficients and increased optimum gas
velocities. Lower column pressures are obtained in GC/MS, since the
outlet of the column is at vacuum. Figure 2.2 shows Golay plots
(calculated from eq 2.2) demonstrating the effect on the plate height of
using vacuum outlet or atmospheric outlet with 0.25 mm i.d. open tubular
columns with lengths of 3 m and 30 m. As shown in the figure, the most
significant increase in the optimum velocity is obtained with the short


Plate Height
21
0.10
^0.08
E
u
^0.06
0.04
0.02
0.00
Average Velocity (cm/s)
Figure 2.2. Theoretical dependence of plate height on the average carrier
gas velocity for 3 m and 30 m 0.25 mm i.d open tubular columns under
vacuum outlet and atmospheric outlet conditions. (Conditions: thin-film
stationary phase; He carrier gas; k 10; 100C; D 0.27 cm2/s; =
2.28 x 10'* poise)


22
column operated with vacuum outlet. The speed of analysis is ultimately
dependent on the gas velocity and the column length. Under vacuum outlet
conditions, analysis times are reduced by a factor of 30, if a 3 m column
is used instead of a 30 m column, and both are operated at their optimum
gas velocities. The reduced analysis times are due to a reduction of the
column length by a factor of 10, as well as the increase in the optimum
gas velocity by a factor of 3 for the shorter column. In addition, since
chromatographic resolution is proportional to the square root of the
number of theoretical plates, the resolution lost by using a column that
is 10 times shorter is only a factor of the square root of 10 (i.e.,
3.16). As shown in Figure 2.2, the theoretical minimum plate heights
obtained with vacuum outlet operation are slightly greater than those
which are obtained with atmospheric outlet pressure. This can be
attributed to the gas decompression term, f, in eq 2.2, which results in
an increase of the minimum plate height by a factor of 9/8 (in the worst
case) when vacuum outlet is employed instead of atmospheric outlet
operation [14].
A better understanding of the effects of the outlet pressure on the
plate height for a short column can be obtained from an investigation of
the pressure drop. The pressure, p(x), at each point, x, along an open
tubular column can be calculated from the inlet pressure, P1-, the outlet
pressure, P and the column length, L [25]:
P(x) = [(P02-P,.2)x/L + P,.2]1/2 (2.7)
This equation was used to calculate the pressure at each point along a 3
m x 0.25 mm i.d. open tubular column with various outlet pressures. As


23
shown in Figure 2.3 (a), the pressures calculated for outlet pressures of
100 torr and 1 torr are virtually the same over most of the length of the
column. Figure 2.3 (b) shows the theoretical dependence of the plate
height on the average gas velocity for the same outlet pressures used in
Figure 2.3 (a). Note that the effects of the outlet pressure on the plate
heights calculated are nearly the same with outlet pressures of 100 torr
and 1 torr. It is apparent from these data that any GC detector that
could be operated at the reduced pressures would offer significant
advantages in terms of speed of analysis. In addition, if operation at
1 torr were not possible, operation with outlet pressures of 100 torr
should yield nearly the same chromatographic results (at least with the
length and diameter column used in this example). Another advantage of
the low outlet pressure operation is that chromatograms obtained on a
stand-alone GC (e.g., with flame ionization or electron capture detection)
could be reproduced on a GC/MS system. If this could be done, routine
chromatographic optimization and methods development could be performed
on the low outlet pressure GC for analyses to be performed later on the
GC/MS system. This would obviate the use of valuable instrument time on
a GC/MS system for chromatographic methods development. Investigations
in these areas are currently underway in our laboratory.
The effect of the outlet pressure on the optimum velocity is even more
pronounced for short wide-bore columns. Figure 2.4 shows the effect of
using vacuum outlet or atmospheric outlet on 3 m columns with inner
diameters of 0.53 mm and 0.25 mm. As shown in the figure, the use of
wide-bore columns when compared to narrow-bore columns of the same length
results in a more significant increase of the optimum velocity with vacuum
outlet operation. However, it is evident from Figure 2.4 that a lower


24
Po=760 torr
Po=400 torr
Po=100 torr
P0= 1 torr
Figure 2.3. (a) Calculated pressures along a 3 m x 0.25 mm i.d. open
tubular column with an inlet pressure of 860 torr of helium and various
outlet pressures. (b) Theoretical dependence of the plate height on the
average gas velocity for the same outlet pressures used in (a) The
conditions were the same as those listed with Figure 2.2.


25
Figure 2.4. Theoretical dependence of the plate height on the average gas
velocity for 3 m x 0.25 mm i.d. and 3 m x 0.53 mm i.d. open tubular
columns with atmospheric or vacuum outlet operation. The conditions were
the same as those listed with Figure 2.2.


26
minimum plate height can be obtained with the narrow-bore column. Since
the maximum number of theoretical plates is equal to column length divided
by the minimum plate height, more theoretical plates can be obtained with
the narrow-bore column. If a constant number of theoretical plates are
required, analysis times will be shorter for the narrow-bore column. For
example, approximately 5600 plates can be obtained with the 3 m x 0.53 mm
i.d. column at an optimum velocity of 182 cm/s. The same number of
theoretical plates and optimum velocity can be obtained with a 1.4 m x
0.25 mm i.d. column. Based on the lengths of the two different diameter
columns used to generate 5600 plates, the analysis times should be faster
for the narrow-bore column by a factor of 2.1 (i.e., 3 divided by 1.4)
Sub-ambient Inlet Pressures
The optimum gas velocity for thin-film open tubular columns operated
under vacuum outlet conditions, uQpt ygc can be calculated from [14]
(P13(l+k)gD,|lH.,),'z
(2LKllk2+6k+l))1/2
(2.8)
where Hmin is the minimum plate height obtained at u t. As can be seen
from eq 2.8, the optimum velocity is inversely proportional to the square
root of the column length; thus, a reduction of the column length results
in higher optimum velocities. In addition, the optimum inlet pressure
needed to obtain the optimum gas velocity is given by [14]
(72P1Lr7Dg1)
(Hmin)1/2r
1/2
P.
i,opt,vac
(2.9)


27
Thus, short columns and/or columns with larger inner diameters require
lower inlet pressures. In addition, optimum inlet pressures are predicted
to be lower for low-viscosity carrier gases. This is demonstrated in
Figure 2.5 (a) which shows a plot of optimum velocity vs. column length
and Figure 2.5 (b) which shows a plot of optimum inlet pressure vs. column
length for 0.25 mm i.d. open tubular columns with both He and H2 carrier
gases. As shown in the figures, short columns with vacuum outlet require
sub-atmospheric inlet pressures, if optimum gas velocities and hence,
minimum plate heights, are to be obtained. Most GCs designed to be used
with open tubular capillary columns utilize pressure regulators that are
referenced to one atmosphere, and are incapable of operating with the
injection port at reduced pressures. Because of this limitation, the
advantages of short-column vacuum outlet GC have never been fully
exploited.
Sub-ambient inlet pressures are readily obtained with the flow-
regulated system used in this work. The chromatograms shown in Figure 2.6
demonstrate the increased resolution obtained on a 3 m x 0.25 mm i. d. open
tubular column when the injection port was operated at a reduced pressure.
The chromatogram in Figure 2.6 (a) was obtained with an inlet pressure of
940 torr and a gas velocity of 297 cm/s. For the chromatogram in Figure
2.6 (b), the inlet pressure was kept at 940 torr for the initial 12 s,
after which the pressure was reduced to 380 torr and a gas velocity of 120
cm/s. Note that the two components unresolved chromatographically in
Figure 2.6 (a) exhibit baseline resolution in Figure 2.6 (b). Moreover,
only 35 s of additional time was required for the chromatogram in Figure
2.6 (b).


28
,500 q
CO
E
^400^1
: l
; I
: l
O300
CP
>
:' i
co
o 200
O
100:
CL
O
(a)
:* \
l
' \
\ \
Subatmospheric Inlet
0
| l i l I
5 10
I *
15
"T- I >
20
i~i
25
Column Length (m)
Ho
Figure 2.5. Calculated optimum velocity (a) and optimum inlet pressure
(b) as a function of column length for 0.25 mm i.d. open tubular columns
with helium and hydrogen carrier gases illustrating need for sub-
atmospheric inlet pressures with short columns.


Figure 2.6. GC/MS chromatograms obtained with a 3 mx 0.25 mm i.d. open
tubular column at two different inlet pressures: (a) P( = 940 torr, v = 297
cm/s, (b) Pi = 380 torr, u = 120 cm/s. Individual traces are for the
molecular ions of acenaphthylene (m/z 152) and tetradecane (m/z 198) as
well as the reconstructed ion current (RIC) from m/z 35 to 300.


Relative Intensity Relative Intensity
30
700%.
Retention Time (minis)


31
It has been suggested that chromatographic analyses be performed at the
optimum practical gas velocity (OPGV) rather than the optimum velocity
[26,27]. The OPGV has been defined as the velocity that yields the
highest number of theoretical plates per unit time [27]. The number of
theoretical plates per unit time can be determined by dividing the number
of theoretical plates by the retention time (i.e., N/tR). Thus, if N/tR
is plotted as a function of the average gas velocity, the OPGV will be
represented by the maximum on the curve. Figure 2.7 shows examples of
these calculated curves demonstrating the determination of OPGVs for 3 m
and 30 m 0.25 mm i.d. columns operated with atmospheric and vacuum outlet.
For this comparison, the value of the partition ratio (k) was chosen to
be 30. Although this is far from the optimum partition ratio of 1.76
suggested by Guiochon [22], a partition ratio of 30 is a typical value
with the inherent low operating temperatures employed with short columns.
Since chromatographic resolution is proportional to the square root of the
number of theoretical plates, the percent resolution lost (^Riost) by
operation at any gas velocity other than the optimum is given by
%Riost = looa (VNopt)1/2) (2-10)
where NQpt is the number of theoretical plates obtained at the optimum
velocity and is the number of theoretical plates obtained at an average
velocity, v.
As shown by the results of this comparison summarized in Table 2.1,
vacuum outlet operation results in the shortest analysis times, regardless
of whether the columns are operated at their optimum velocities or optimum


32
Figure 2.7. Examples of calculated plots to determine optimum practical
gas velocities (OPGVs) for 3 m and 30 m 0.25 mm i.d. open tubular columns
operated with vacuum or atmospheric outlet. For each plot, the number of
theoretical plates generated per second has been normalized to a maximum
of 1002. The maxima of the curves indicate the OPGVs, while the stars
indicate the optimum velocities. The conditions were the same as those
of Figure 2.2, except k 30.


Table 2.1. Comparison of calculated optimum conditions
operated with atmospheric and vacuum outlet.3
Column
Length
Outlet
%tb
0PGVc
t d
R,opt
k,0PGV
(m)
(cm/s)
(cm/s)
(min)
(min)
3
atm
42
121
3.7
1.3
3
vac
122
161
1.3
1.0
30
atm
29
46
53.4
33.7
30
vac
39
50
39.7
31.0
3 same conditions as those of Figure 2.2, except k 30.
b calculated by iteration of eq 2.2
c obtained from maxima of curves of Figure 2.7
d analysis time obtained with uQpt
e analysis time obtained with OPGV
f %resolution lost by operation at OPGV, rather than uQpt
for 3 m and 30 m 0.25 mm i.d. open tubular columns
N b
opt
(s'1)
N c
l0PGV
nopgvAr
(s1)
IR.o
12814
57.7
7352
94.3
24
11408
146.3
10004
166.7
6
124003
38.7
97244
48.1
9
114031
47.9
99944
53.7
6
OJ
OJ
calculated from eq 2.10


34
practical velocities. As stated previously, atmospheric outlet operation
results in a lower minimum plate height (typically 10% lower), and thus
more theoretical plates at the optimum velocity, than vacuum outlet
operation. For example, if operated at their optimum velocities, a 3 m
column with atmospheric outlet would yield 5.6% better resolution than a
3 m column with vacuum outlet. However, the analysis times with the 3 m
column would be 2.8 times shorter if vacuum outlet was used instead of
atmospheric outlet. Moreover, if operated at their optimum practical gas
velocities, columns with vacuum outlet would result in faster analyses and
more theoretical plates (and hence, better resolution) than columns with
atmospheric outlet. For example, analysis times are 0.3 min shorter on
the 3 m column with a 15% improvement in the resolution if vacuum outlet
at its OPGV is used instead of atmospheric outlet at its OPGV. It can be
seen from Table 2.1, that for any column length, vacuum outlet operation
yields the shortest analysis times and the highest number of theoretical
plates per unit time, regardless of whether optimum velocities or optimum
practical velocities are used.
There are other significant advantages of utilizing reduced inlet
pressures with short columns in GC/MS. It has previously been reported
that short columns result in narrower bands and higher sample
concentrations at the detector than long columns [3]; thus, at least from
a theoretical perspective, short columns should provide the best possible
detection limits. In practice, short columns operated at conventional
inlet pressures result in high flow rates of carrier gas into the ion
source. In fact, sub-ambient inlet pressures are a necessity if short
wide-bore columns are to be used at all in GC/MS, since conventional inlet
pressures result in analyzer pressures that are excessive for the normal


35
operation of the mass spectrometer (see Chapter 3). It is possible that
the advantage of low detection limits obtainable with short columns in
GC/MS might be lost if excessive flow rates (which can disrupt the
ionization process in the ion source) are used. Experience in this
laboratory has shown that the sensitivity obtainable with chemical
ionization (Cl) is not greatly affected by the high flow rates of a
different carrier gas (e.g., He or H2) (see Chapter 3). However, when
electron ionization (El) is used the sensitivity decreases dramatically
with increasing flow rates of carrier gas into the ion source, especially
when flow rates above ca. 10 mL/min are used. Thus, lower inlet pressures
are preferred for use with El and short columns. The increased
sensitivity of the vacuum inlet operational mode is demonstrated in Table
2.2, which shows the integrated peak areas of the molecular ion of n-
hexylbenzene obtained with different injection methods and inlet
pressures. As shown in the table, splitless followed by low-pressure
operation at 560 torr yielded the best sensitivity. The vacuum split
injection is inherently less sensitive, since less sample enters the
column and is detected. For the splitless injections, the peak areas were
significantly lower when the higher inlet pressures (and flow rates) were
employed. For example, the peak area obtained by splitless operation with
an inlet pressure of 560 torr was approximately two orders of magnitude
greater than that obtained at 2155 torr.
The effects of splitless operation on retention times was also studied.
Traditionally, both split and splitless operation are performed at
constant inlet pressure. Thus, retention times are the same regardless
of which mode is used, provided that the same inlet pressures are used.
In the splitless method described here, the initial inlet pressure during


36
Table 2.2. Comparison of peak areas for four different injection of n-
hexylbenzene obtained with different injection methods and inlet pressures
of He carrier gas.
Inj ection
Method
Inlet
Pressure
(torr)
Average
Velocity
(cm/s)a
Flow
Rate
(mL/min)b
Peak Area of
M+ (m/z 162)
splitc
560
176
2.6
2.56 x 106
splitlessd
560
176
2.6
2.09 x 107
splitless
940
297
5.0
7.95 x 106
splitless
2155
679
30.0
2.11 x 105
a average velocities calculated with eq 2.6
b indicated flow rates from flow controller
c split ratio =10:1
d inlet pressure = 940 torr for initial 12 s, final inlet pressure = 560
torr


37
injection is much higher than that during the remainder of the elution.
Since a change in the inlet pressure results in a change in the gas
velocity, it was expected that the retention times for split and splitless
operation would not be the same. In order to determine the effects of the
large pressure change, the retention times for split injections and
splitless injections for a mixture of alkylbenzenes were compared. The
results are shown in Table 2.3. The split injection was performed at a
constant inlet pressure of 560 torr. The splitless injections were
performed with an initial pressure of 940 torr and a final pressure of 560
torr. For splitless operation, the split valve was left closed for 12 s
in one case and for 30 s in another. As shown in Table 2.3, the large
change in inlet pressure does not greatly affect the retention times, even
when a splitless time of 30 s is used. This is most likely due to
trapping of the solutes at the head of the column at the initial column
temperature of 50C.
Experimental vs. Theoretical Performance
Figure 2.8 shows a comparison of experimental and calculated (from eq
2.2) plate heights obtained with a 3.1 m x 0.25 mm i.d. open tubular
column operated with H2 carrier gas and the column outlet at vacuum. As
can be seen in the figure, the experimental performance at higher
velocities is not as good as predicted by theory. The differences in the
experimental and theoretically calculated data can be attributed to extra
column variances. Extra column contributions are the result of band
broadening due to instrumental time constants and, thus are not accounted
for in the Golay equation (eq 2.1). The extra column variances result in
the addition of a constant band width to the theoretical band width. This


38
Table 2.3. Comparison of retention times for components of an
alkylbenzene mixture utilizing vacuum inlet split and splitless injection.
Injection Method
Split3 Splitless, 12sb Splitless, 30 sb
Component Retention times (mints')
n-hexylbenzene
1:17
1:14
1:10
n- octylbenzene
2:16
2:14
2:14
n-decylbenzene
3:13
3:12
3:13
n-dodecylbenzene
4:08
4:08
4:09
3 inlet pressure = 560 torr
b inlet pressure 940 torr for initial 12 s, final inlet pressure = 560
torr


39
Inlet Pressure (torr)
Average Velocity (cm/s)
Inlet Pressure (torr)
(b)
Figure 2.8. Comparison of theoretical and experimental data obtained for
a 3.1 m x 0.25 mm i.d. open tubular column at 100C with hydrogen as a
carrier gas: (a) peak widths at 10% height and (b) plate heights. The
experimental data agree well with theory if an extra column variance (oec2
= 0.073s2) is included in the calculations.


40
is demonstrated in Figure 2.8 (a), which shows the expected (theoretical)
peak widths and the experimentally observed peak widths for n-pentadecane
on a 3.1 x 0.25 mm i.d. open tubular column. The extra column band
broadening results in an additional 0.4 s to the theoretical band width
at 10% peak height. Guiochon et al. have elaborately discussed the role
of extra column effects and have stressed their importance in high-speed
GC applications where short and/or small diameter columns are used [8].
They demonstrated that an additional term (Du2) can be added to the Golay
equation which can be used to describe the apparent plate height, Happ
with:
Happ B/u + Cv + Du2
(2.11)
D aec2/(l+k)2L
(2.12)
where aec2 is the extra column variance. When eq 2.11 was used with an
extra column variance of 0.073 s2, the resulting curve was an excellent
description of the experimental data, as shown in Figure 2.8 (b) As
shown in the figure and by eqs 2.11 and 2.12, the extra column
contributions become more significant as shorter column lengths or higher
velocities are used. It is believed that the instrumental variances
observed here arise from sample injection rather than detection. The n-
pentadecane used for the plate height determinations was manually injected
as a liquid into the hot injection port. There is expected to be a finite
time required for vaporization of the sample and entrance of the solute
onto the column, which could result in the observed peak broadening. The
residence times of samples eluting from the GC column into the ion source
is not expected to cause a significant amount of band broadening. In


41
addition, the data were collected utilizing a scan time of 0.05 s over the
m/z of interest, which resulted in at least 40 data points per GC peak.
Therefore, no significant time constant was introduced during the data
collection. Because of the extra column effects, the use of short columns
dictates careful consideration of injection technique and injector design
if the maximum performance is to be obtained.
Choice of the Carrier Gas
The importance of the choice of the carrier gas for minimizing analysis
times has previously been discussed by Giddings [6]. Hydrogen, which has
the largest gas viscosity to diffusivity ratio, is predicted to yield the
fastest analyses. This can be seen from the experimentally determined
plate heights for H2 and He on a 3.1 mx 0.25 mm i.d. open tubular column,
as shown in Figure 2.9. Hydrogen is preferred, since the optimum velocity
is higher and the Golay curve is flatter than that of helium. The dashed
line parts of the curves of Figure 2.9 indicate where sub-ambient inlet
pressures were used. The use of H2 results in lower optimum inlet
pressures than He as predicted by eq 2.9 and shown in Figure 2.5 (b).
Even though H2 is predicted to yield the fastest chromatographic
analyses, the effects of its use as a carrier gas on resulting mass
spectra have not been previously studied. When H2 is used a carrier gas,
H3+ ions are produced in the ion source. The presence of these ions does
not appear to affect El spectra and in an El source do not cause a
significant amount of chemical ionization (i.e., no increase in the MH+/M+
ratio was observed). This is most likely due to the fact that pressures
in the El ion source are not high enough to cause a significant amount of
reaction to occur. However, the reaction of Hj+ ions with sample molecules


42
Figure 2.9. Comparison of experimentally determined dependence of the
plate height on average velocity for helium and hydrogen carrier gases
with a 3.1 m x 0.25 mm i.d. open tubular column at 100C. The dashed
lines indicate where sub-atmospheric inlet pressures are required. The
curves were calculated with eq 2.11.


43
in a chemical ionization (Cl) source is possible due to the higher
operating pressures. It was also predicted that the H3+ ions would alter
the mass spectra normally obtained with methane Cl. On the contrary, the
presence of H2 carrier gas does not appear to affect methane Cl spectra.
As shown in Figure 2.10 (a) and (b), the Cl spectrum of 2,4
dimethylaniline obtained with 0.5 torr H2 and 0.2 torr CH^ (b) is the same
as that obtained with 0.2 torr CH^ and no H2 (a). In fact, as soon as
methane is introduced in the ion source with H3+ ions present, the Hj+ ions
disappear. This is apparently due to the fact that the hydrogen reagent
ions (Hj+) are used in the protonation of methane. This is not surprising,
since methane has a higher proton affinity than hydrogen [28], It is
expected that similar behavior would be observed with other commonly used
proton transfer Cl reagent gases (e.g., isobutane, ammonia, etc.) because
of the low proton affinity of hydrogen. In the absence of methane,
hydrogen can be used as both the carrier gas and the Cl reagent gas, as
shown in Figure 2.10 (c) The spectra that are obtained with hydrogen Cl
exhibit more fragmentation than those obtained with methane Cl, which is
due to the greater exothermicity of the proton transfer reaction between
the H3+ reagent ions and the sample molecules. With hydrogen Cl, no adduct
ions are observed as is normally the case with methane Cl. The absence
of these adducts is often desirable, since their presence may complicate
interpretation of the mass spectra.
Conclusions
A better understanding of the influence of the pressure drop on the
chromatographic behavior of open tubular columns in GC/MS has allowed for
better utilization of the inherent advantages associated with short


Relative Abundance
44
Positive ion Cl spectra of 2,4-dimethylaniline (MW 121)
m/z
Figure 2.10. Effect of using hydrogen carrier gas on the chemical
ionization mass spectra of 2,4 dimethylaniline: (a) 0.2 torr methane and
no hydrogen present, (b) 0.2 torr methane with 0.5 torr hydrogen present,
(c) 0.5 torr hydrogen and no methane present.


45
columns. The large pressure drop associated with vacuum outlet GC should
not be considered a disadvantage. On the contrary, the low-pressure
outlet results in increased optimum gas velocities and decreased analysis
times. It has been shown that vacuum outlet operation yields the shortest
analysis times, regardless of whether optimum velocities or optimum
practical velocities are used. In addition, the chromatographic
resolution obtained when optimum practical gas velocities are used is
predicted to be higher for vacuum outlet than for atmospheric outlet
operation. As shorter column lengths are used with vacuum outlet
operation, lower inlet pressures are needed to obtain optimum performance.
Sub-ambient inlet pressures can be obtained with a relatively simple flow
control system as described here. The capability of splitless operation
has greatly extended the utility of vacuum inlet operation. The lower
inlet pressures with short columns are also shown to provide the best
sensitivity when electron ionization is used, since the flow rates are
lower. The use of hydrogen carrier gas results in even lower optimum
inlet pressures and higher optimum velocities than helium. Hydrogen can
be used as the chemical ionization reagent gas or it can be used in the
presence of other commonly used proton transfer chemical ionization
reagent gases without affecting the mass spectra.


CHAPTER 3
CONCEPTS FOR PORTABLE GAS CHROMATOGRAPHIC
INSTRUMENTATION IN GC/MS: FLOW RATE
PROGRAMMING AND DIRECT COLUMN HEATING
Introduction
The object of any chromatographic analysis is the separation and
identification of components in mixtures. However, chromatographic
mixture analysis is often complicated by the "general elution problem"
[29]. This predicament results both in poor resolution in the early part
of a chromatogram due to coeluting peaks, and in severe peak broadening
near the end of a chromatographic run. In gas chromatography, temperature
programming is often used to circumvent this problem. In temperature-
programmed GC (TPGC), the column temperature is increased (usually
linearly) during the course of the analysis. Since the temperature range
covered can be up to 300C (and higher depending on the stationary phase),
TPGC offers a wide dynamic range. In analyses where temperature
programming is used, the rate at which samples can be analyzed is not only
dependent on the time required by the separation, but also on the time
required to equilibrate (cool) the column for the next injection. Since
the column temperature is changed by changing the temperature of a large
gas chromatograph oven, the rate at which a GC column can be heated or
cooled in TPGC is usually slow (typically 50C/min or less depending on
the manufacturer of the GC oven). Moreover, the need to heat a GC oven by
conventional convection heating, only to cool it back down, requires
46


47
significant amounts of electrical power, making temperature programming
unattractive for portable instrumentation.
In this chapter, two approaches to solving these problems are
presented. The first approach involved investigation of flow rate
programming as a robust alternative to temperature programming. The
second approach examined was direct resistive heating of metal-clad
capillary GC columns, as opposed to oven (convection) heating. Direct
resistive heating is especially exciting, and should be widely applicable
in portable GC instrumentation for environmental or process monitoring.
The concepts developed in these studies have led to the development of a
portable GC probe, which appears to be particularly attractive for future
applications involving portable mass spectrometers or as a convenient
analytical tool for that could be used in any mass spectrometry
laboratory.
Flow Rate Programming
Flow programming of the carrier gas has previously been examined as
an alternative to temperature programming [25,29-35]. Use of this
technique was limited in the early years of application due to the high
inlet pressures required with packed columns. The high permeability of
short and/or wide-bore open tubular columns allows for high flow rates to
be achieved at relatively low inlet pressures [34], In addition, it has
more recently been shown that exponential flow rate programming under
isothermal conditions results in peak distributions that are very similar
to those obtained with linear temperature programming [29,33-35], One
advantage of programmed flow GC is that the sample throughput is high,
since it is not necessary to wait for the column to cool for the next


48
injection. Although resetting of the column flow rate is required, this
step is essentially instantaneous due to the high permeability of open
tubular columns [34]. This technique might prove attractive for a field
instrument that could not be rapidly heated and cooled, or in the
laboratory where high-throughput analyses are desired. Flow rate
programming is well suited for the analysis of thermally labile compounds,
since the column can be operated isothermally at relatively low
temperatures. It has also been pointed out that bleed of the liquid
stationary phase from the column is reduced since lower temperatures are
required [34]. This could prove to be an important factor in the
reduction of chemical noise at the detector, or could help to extend the
life of GC columns. One disadvantage of the method that is evident from
GC theory, is that chromatographic efficiency is reduced if the flow rate
is increased beyond the optimum. However, as was shown in Chapter 2,
optimum flow rates are higher when the column outlet is at vacuum as in
GC/MS. In addition, the rate of increase of the plate height is much less
for short columns than for long columns, when higher than optimum
velocities are used. Nevertheless, as will be shown in this chapter, the
maximum allowable flow rate is ultimately dependent on the pumping speed
and associated performance of the mass spectrometer.
Direct Resistive Heating of Al-clad Capillary Columns
Another alternative that avoids many of the limitations of oven
heating is to heat the column directly instead of by convection.
Recently, aluminum-clad fused silica open tubular (Al-clad FSOT) capillary
columns have become commercially available (SGE, Quadrex). The potential
advantages of directly-heated capillary GC columns have been suggested


49
[36], but never demonstrated. In Chapter 4 it will be shown that these
directly-heated columns can be used as simple transfer lines for
introducing samples via GC into the collision cell of a triple quadrupole
mass spectrometer. In this chapter, a method is described for heating
these columns directly by passing an electrical current through the thin
aluminum cladding coated on the outer surface of the column. A method of
sensing the temperature of the Al-clad capillary directly, by measuring
the column resistance instead of using external thermocouples or other
temperature sensors, is also presented.
The principles of resistive heating are relatively simple. When a
voltage is dropped across an Al-clad capillary, the column temperature
increases depending on the amount of power dissipated. The power
dissipated, P, is dependent on the current through the column, i, and the
voltage across the column, V.
P = iV
(3.1)
The current is dependent on the voltage drop and the electrical
resistance, R.
i = V/R (3.2)
The electrical resistance is dependent on the length, diameter, and
thickness of the Al coating on the outside of the capillary. The amount
of heat (or energy) required to increase the temperature of any substance
by an increment AT is given by [37]


50
Q = mCAT (3.3)
where Q is the amount of heat (or energy, J or W-s), m is the mass of the
material (g), C is the specific heat of the material (J/gC), and AT is
the change in temperature of the material (C). In addition, since the
heat is produced resistively, the amount of energy consumed over a given
time period At is given by
Q = PAt (3.4)
From eqs 3.3 and 3.4 it is apparent that objects of large mass require
more heat or more power over a given time period to reach a given
temperature. Al-clad FSOT GC columns are coated with a very thin aluminum
film, and have a very small thermal mass. The low thermal mass of these
columns allows them to be rapidly heated and cooled with much less
electrical power than is normally required with chromatograph ovens.
Since no large oven is needed, direct resistive heating is extremely
attractive for use in portable GC instrumentation.
The low thermal mass of the Al-clad columns does present some problems
if conventional methods are used to measure the column temperature.
Currently, there are no thermocouples or resistance thermometers available
that are small enough to accurately measure the temperature of these
columns. Even if small sensors are used, placing the sensor against the
column causes a local cold spot and leads to inaccurate temperature
measurement. An alternative method of measuring the column temperature
that avoids this problem is to use the column itself as the temperature
sensor. The resistance of any metallic conductor is linearly related to


51
its temperature over a wide temperature range and is given by [38]
Rt = R0(l + oT) (3.5)
where Ry is the resistance (0) at temperature T (C), RQ is the resistance
at 0C, and a is the temperature coefficient of resistivity of the metal
(C*1). In fact, eq 3.5 is the basis of operation of Pt resistance
thermometers, which are widely used in scientific applications. The
resistance of a column is readily calculated during resistive heating if
the current through the column and the voltage across the column are
measured. Once a calibration of temperature versus resistance has been
performed, a determination of the column resistance can be used as a
direct measure of the column temperature.
GC Probe
The concepts discussed above have been applied in the development of
a compact GC probe, which resembles a conventional direct insertion probe,
and interfaces with a commercially available mass spectrometer. A GC
probe utilizing packed columns operated isothermally has previously been
described [39], The GC probe described here allows the temperature to be
rapidly cycled for high-throughput GC/MS analyses requiring very little
electrical power. Removal or insertion of the probe from the mass
spectrometer is through the normal probe inlet assembly. This facilitates
changing of the column or allows for rapid changing to other techniques
(e.g., solids probe, FAB, etc.) with minimal effort. Once the probe is
removed from the mass spectrometer, the column can be rinsed with solvent
without removing the column from the probe. This is attractive in cases


52
when particularly "dirty" samples are being analyzed, as may be the case
for environmental monitoring in the field.
Experimental Section
Experimental Conditions
Mass spectrometry. A Finnigan MAT TSQ70 triple quadrupole mass
spectrometer was used for this work. The instrument was tuned with FC43
(perfluorotributylamine) with the GC probe in place. The electron
multiplier was maintained at 800-1200 V with the preamplifier gain set at
O
10 V/A. Electron energies of 70 eV and 100 eV were used for electron
ionization (El) and chemical ionization (Cl), respectively. Normal Q1
mass spectra were acquired for all MS experiments. Methane reagent gas
was used in all Cl experiments with ion source pressures of 0.2-0.5 torr.
The same removable ion volumes commonly used with the direct insertion
probe were used with the GC probe.
Flow rate programming. All experiments with flow rate programming
were performed on a Varian model 3400 GC equipped with a split/splitless
injector. Open tubular GC columns, approximately 3 m in length with an
internal diameter of 0.25 mm and a 0.25 /m film thickness (DB5, J&W
Scientific) were used. Injection temperatures were typically 220C;
column temperatures are reported with each analysis.
GC probe. Wide-bore (0.53 mm i.d.) Al-clad FSOT columns with a non
polar 1.0 /m film BP-1 stationary phase (SGE) were used with the GC probe.
Narrow-bore columns (0.33 mm i.d., 0.5 /m film, BP-5 stationary phase)
were evaluated in terms of heating requirements; however, for all
chromatograms obtained with the GC probe, wide-bore columns were used,
which allowed for a mechanically simple injection port.


53
Flow Control System
Flow rate programming. For many of the experiments performed (e.g.,
exponential flow rate programming and sub-ambient inlet pressure GC) it
was necessary to be able to control the column flow rate instead of the
column inlet pressure. Several methods for programming the column flow
rate have appeared in the literature [25,29-33]. For example, a very
simple method of flow programming was achieved by manually manipulating
a needle valve in the splitter line [33]. However, due to the imprecision
and inconvenience of this method more reliable methods were developed.
Others have achieved flow control via feedback from a pressure transducer
with a motor-controlled regulator valve [29,34,35]. This type of system
first requires a calibration step to determine the dependence of flow rate
on inlet pressure. After the flow function is determined and programmed
into the computer, the pressure transducer detects any difference between
the inlet pressure and the setpoint flow. If a difference is detected,
a DC motor adjusts the regulator until the correct flow is achieved. The
main disadvantage of this method is the slow response time, since several
seconds are required for the motor to reposition the valve. The response
time of the flow controller becomes even more important for short columns
since the analysis times are typically only a few minutes long. For this
reason it was decided that commercially available mass flow controllers
would be most suitable for these applications. These types of transducers
incorporate thermal mass flow sensors to determine mass flow rates of
gases. Sensors placed at both ends of a laminar flow tube detect
differences in heat transferred along the tube, and directly relate this
difference to the flow rate of the input gas. The measured flow rate is
compared to the setpoint value and is adjusted by an electromagnetically


54
controlled automatic valv. The transducer and controller valve are
contained in a single package measuring approximately 1/2 in. x 5 in. x
5 in. An external power supply/readout unit provides the appropriate
power, real-time readout of the measured flow, and capability for computer
interfacing. The advantages of this system are fast response (500 ms for
the transducer), excellent precision (0.2% of full scale), and high
accuracy (0.5% of full scale). The flow controller is interfaced to the
mass spectrometer electronics via one of two digital-to-analog converters
(DACs, known as user outputs) that are variable over the range 5 V DC
via the TSQ70 trackball or Instrument Control Language (ICL) procedure.
A control voltage range of 0-5 V is used to adjust the flow controller
flow rate over its full range.
One concern about the use of a mass flow controller was its ability
to regulate flow when the output was connected to the vacuum of the mass
spectrometer, as it might be in short-column or vacuum inlet GC. In order
to test the performance of the transducer in this configuration, the flow
controller was utilized to control the input of collision gas into the
collision cell of the TSQ70. In fact, this configuration worked so well
that a flow controller dedicated for control of collision gas pressure has
since been installed.
For exponential flow rate programming experiments, the flow control
system was essentially the same as that described in Chapter 2. The only
difference was that, instead of a MKS 1159A mass flow controller, a MKS
1259B mass flow controller was used, which has a flow control range of 0-
20 mL/min for nitrogen and 0-29.6 mL/min for helium. It was later found
that the flow rate range of this flow controller limited the experiments
that could be performed. For example, during flow programming


55
experiments, the split and sweep valves had to be left closed during the
entire GC run to obtain the widest range of useable flow rates, which
limited analyses to splitless operation. A flow controller with a wider
flow rate range (0-100 mL/min for nitrogen) was eventually installed to
alleviate this problem. The flow function for an exponential flow program
is controlled automatically with the DAC user output of the TSQ70 with an
Instrument Control Language (ICL) program. The flow rate at time t, Ft,
is given by
kt
Ft = F0 + e
(3.6)
where FQ is the initial flow rate and k is a constant. In a flow rate
programming experiment, the initial and final flow rates, as well as the
time for the delay and end of the programming period are selected and
entered into the computer program. The constant, k, is calculated in the
computer program such that the flow rate at any time during the analysis
is determined by eq 3.6.
GC probe. Since short wide-bore columns (typically 3 m in length)
were used in the GC probe, the injection port could not be operated at
conventional injection port pressures (i.e., atmospheric pressure and
above) without a carrier gas flow rate exceeding the normal operating
pressure of the mass spectrometer. Instead, the injection port was
maintained at lower pressures (typically 100-200 torr) with the flow
control system described in Chapter 2. This system utilized a MKS model
1159A flow controller with a range of 0-100 mL/min for nitrogen and 0-145
mL/min for helium. The use of sub-atmospheric inlet pressures did require
some consideration of injection technique, since it was possible for the
sample to be prematurely aspirated from the syringe by the vacuum in the


56
injection port. It was found that the best way to avoid this problem was
to use a gas-tight syringe and to draw the sample as well as a small
volume (e.g., 1 /iL) of air into the syringe barrel prior to injection.
The column flow rate of He carrier gas used with the sub-ambient
inlet pressures was typically 3.5 mL/min. Flow rates were determined by
measuring the flow output of the flow controller with a soap bubble meter
prior to connecting the carrier gas line to the injection port. Davies
has shown that, under the conditions of vacuum outlet, flow rates can be
calculated by [40,41]
Q = 64^L3/9tQ2 (3.7)
where Q is the flow rate (mL/s), t is the retention time of the unretained
air peak (s) r¡ is the gas viscosity (poise) and L is the column length
(m). However, when short columns are used, the air peak times are so
short that they are not easily measured. For example, a 3 m column
operated at a typical gas velocity of 300 cm/s has an air peak time of 1
s. As discussed in Chapter 2, the average gas velocity can be calculated
if the inlet pressure is known. However, due to the very simple design
of the injection port of the GC probe, a conventional pressure gauge could
not be used to measure the inlet pressure. Therefore, it was more
convenient to measure flow rates instead. Once the flow rates were
determined, the inlet pressures and gas velocities were readily
calculated. Given that the average gas velocity is
u L/t0
(3.8)


57
Average velocities can be calculated if eq 3.8 is substituted into eq 3.7
yielding
v = (375Q/167tjL) 1/2 (3.9)
where Q is in mL/min, L is in m, r¡ is in poise, and u is in cm/s. The
inlet pressure (Pj, dynes/cm2) can be calculated from the Poisuelle
equation for vacuum outlet conditions [14]
P. 32ur/L/3r2 (3.10)
where r is the column radius in cm, and L is in cm. A 3 m wide-bore
column, operated at a typical flow rate of 3.5 mL/min of He at 25C, has
a calculated gas velocity of 204 cm/s and an inlet pressure of 146 torr.
Direct Resistive Heating
Electronics. A programmable DC power supply was constructed in-house
and was used to control the voltage across the Al-clad GC columns. The
electronic components for the power supply are contained in a box which
is approximately 10 in. x 6 in. x 3.5 in. A schematic of the overall
system electronics is shown in Figure 3.1, with details of the DC power
supply shown in Figure 3.2. The output of the power supply (up to 30 V)
is controlled by an input control voltage of 0-5 V DC. A DAC from the
TSQ70 was used to generate a control voltage for the power supply. The
control voltage (from the potentiometer or DAC) is fed into a voltage
follower with variable gain. The output of the op-amp provides a bias to
the base of power transistor that determines the output of the supply.


USER
Figure 3.1. Schematic diagram of electronics used for resistive heating and temperature sensing of Al-clad
capillary columns.


6A/ SB
I < ans)
Figure 3.2
Circuit diagram of programmable 30 V DC power supply designed for column heating.


60
The output of the power transistor is rated at 30 V and 30 A. However,
in the present configuration the output of the supply is limited by the
transformer, which is rated at approximately 6 A. Typically, the gain of
the voltage follower was set such that the power supply yielded the
maximum desired temperature at the maximum control voltage. This
versatility is important, since a change in the length or diameter of the
column changed the amount of power required for the column to reach a
particular temperature. No additional circuitry had to be designed to
measure the column voltages and currents. The voltage drop across the
column, as well as the voltage drop across a 0.261 current sensing
resistor were measured with an external devices sensor circuit already
present in the TSQ70 electronics. This circuit consists of five
differential amplifiers, an analog multiplexer, and an analog-to-digital
converter (ADC). The voltages to be determined were fed into two separate
differential amplifiers of the sensor circuit. This allows the voltages
to be displayed and/or plotted in real time with the computer-controlled
TSQ70 and the ICL. Programs were written in ICL that allowed the voltage
across the column to be increased linearly with time during the
acquisition of data. The linear voltage ramp was used to simulate the
linear temperature ramp commonly used in temperature programming.
However, it should be pointed out that any desired voltage ramp could be
created with this system (e.g., exponential, logarithmic, etc.), by a
simple modification of the ICL program.
Temperature sensing. The resistance of the column, obtained by
measurement of the voltage and current, was used as an indication of the
column temperature. Al-clad GC columns were "calibrated" by placing them
in a GC oven (Varian model 3400) and recording the resistances of the


61
columns as a function of GC oven temperature. A wide-bore Al-clad FSOT
column (0.53 mm i.d. x 3.14 m) and two different lengths (4.77 m and 3.05
m) cut from the same narrow-bore (0.33 mm i.d.) Al-clad FSOT column were
calibrated using this method. The nominal resistances of these columns
were very low (ca. 0.9 fi/m for the narrow-bore column and 0.5 Q/m for the
wide-bore columns), and thus were too small to be accurately and precisely
measured with a typical digital multimeter (DMM). In order to obtain
accurate resistance measurements for the calibration, the DC power supply
was used to apply a small voltage drop (e.g., 50 mV) across the column.
The current through the column and the voltage drop across the column were
measured with two DMMs (Fluke model 75). The resistances were then
calculated from the voltages and currents measured. The voltages applied
during calibration were not high enough to cause a significant amount of
column heating.
GC Probe Design
The GC probe is schematically illustrated in Figure 3.3. The 1.27 cm
x 28 cm stainless steel probe shaft was adapted from a Finnigan 4500
series ion volume insertion/removal tool. The probe shaft is threaded and
screws into one of the two 4 in. square end-blocks (machined from phenolic
material). The injection port is positioned in a milled slot on the other
end-block and is held firmly in place by a set screw. The column is
wrapped around a 2.5 in. diameter Teflon spool. The spool is held between
the two end-blocks and contains a slot that allows the column to be
inserted through the spool and down the length of the probe shaft. The
column is covered with Nextel glass braid insulation (Omega Engineering)
and is coiled around the Teflon spool. The insulation serves to


PHENOLIC
END-BLOCK
C u
CONNECTOR
PROBE
SHAFT
V I TON
O-RING
A I -CLAD
COLUMN
1/16"
UNION
hH
1 INCH
BANANA
JACK
Figure 3.3.
Illustration of direct insertion GC probe.
CARRIER
GAS
INLET INJECTION
END-BLOCK


63
electrically isolate each strand of the coiled column. A 1/16 in.
stainless steel union (Swagelok) with a graphitised vespel ferrule and a
Viton o-ring are used to form a vacuum-tight seal at the end of the probe
shaft. Approximately 2 cm of the column extends from the end of Swagelok
union. A copper connector slides over this last few cm of the column and
electrically connects the end of the column to the probe shaft.
Electrical leads from the DC power supply plug into the banana jacks
mounted on the probe assembly. It is very important to minimize the
contact resistance of the electrical connections to the column, since poor
connections result in power consumption and local heating at the point of
contact. In addition, large connectors have a large thermal mass and
dissipate heat near the contact. For these reasons, the electrical
connection to the head of the column was made with a small removable clip
lead. The probe shaft, which is used as a ground return, remains cool
during column heating, since the resistance of the probe is much less than
that of the columns used. The nominal resistance of the column in the
probe was always checked prior to applying power to insure good electrical
connections. The GC probe shaft is inserted through the probe inlet
assembly and the end of the probe fits firmly against the ion volume
contained in the ion source block.
A compact, low thermal mass injection port was fabricated from a 1/8
in. x 1/16 in. stainless steel tube reducing union (Swagelok). For the
introduction of carrier gas into the injection port, a hole was drilled
into the side of this fitting and a 1/16 in. o.d. stainless steel tube was
silver-soldered in place. The wide-bore column is inserted into the
smaller end of the union and is sealed with a nut and a graphitised vespel
ferrule. The top of the injection port contains an 1/8 in. nut and a


64
septum. A standard GC syringe (SGE), equipped with a 26-gauge needle, was
used to inject samples directly onto the wide-bore columns. The inside
of the injection port contains a stainless steel guide, which directs the
syringe needle to the column entrance. Upon injection, the syringe needle
enters the column and is inserted far enough that the sample is injected
past the point where the electrical connection is made. This minimizes
sample loss due to unvolatilized components. The injection port remains
cold when the column is heated, and thus does not have to be cooled
between injections.
Results and Discussion
Flow Rate Programming
Effect of carrier gas flow rates on mass spectrometer performance.
Exponential flow rate programming has been used in many instances with
conventional GC detectors [25,29-35]; however, the effects of the
resultant high (and varying) flow rates on the operation of the mass
spectrometer have not been studied. One problem that became apparent from
some initial experiments employing electron ionization (El) was the
reduction in the analyte signal (sensitivity) associated with high flow
rates of carrier gas into the mass spectrometer ion source. This was
briefly addressed in Chapter 2. Initially, it was unclear whether this
problem was linked to mass analysis, ion formation, or a combination of
both. It is possible that the increased pressure in the mass analyzer
(which causes a decrease in the mean free path of the ions) results in
scattering losses and a reduction of the number of ions reaching the
detector. Analyte signal may also be decreased due to inefficient ion
formation with El. This associated decrease in sensitivity with El has


65
also been observed in supercritical fluid chromatography/mass spectrometry
(SFC/MS) where the pressure in the ion source is elevated due to the
presence of the constantly eluting mobile phase [42].
It was discovered that the problem of reduced analyte sensitivity with
high carrier gas flow rates could be overcome by employing chemical
ionization (Cl) instead of El. Due to the presence of a reagent gas
(e.g., methane) at much higher operating pressures (typically 0.1-1 torr
for Cl vs. 10'4 torr for El), ion formation with Cl appears unaffected by
the increase in carrier gas flow rates. Figures 3.4 and 3.5 show a
comparison of the performance of El and methane Cl respectively under
identical changes in column flow rates. For these experiments, the
chromatograms were obtained in the splitless mode with a 3.2 m, 0.25 mm
i.d., DB-5 capillary column operated isothermally at 80C. The sample
injected was 30 ng each of C14 C15 and C16 n-alkanes. Figure 3.4 shows
the chromatograms of the major fragment ion (m/z 85) obtained with El.
For Figure 3.4 (a) the column flow rate was 8 mL/min and that for Figure
3.4 (b) was 20 mL/min. Both traces are plotted on the same time and
intensity scale for ease of comparison. The integrated peak areas for the
components at the two flow rates are given in Table 3.1. As shown in the
table, the m/z 85 peak areas for the three hydrocarbons in a particular
chromatogram are reproducible within the expected limits of error.
However, the areas of the peaks at the higher flow rate are approximately
a factor of four less than the peak areas obtained at the lower flow rate.
Although the resulting sensitivity for only one particular ion is
considered in this comparison, all of the sample ions were observed to
decrease by approximately the same amount. The major sample ions obtained
with methane Cl of n-alkanes are formed by hydride abstraction to yield


Electron Ionization
Q>
+-
J3
O
W
A
<
Retention Time (min:s)
Figure 3.4. Mass chromatograms of m/z 85 from 70 eV El of n-alkane mixture with (a) 8 mL/min and (b) 20 mL/min
He carrier gas flow rates. The sensitivity with El is drastically reduced when the higher flow rates are used.


Chemical Ionization
miz 197 f = 8 mLlmin
Retention Time (min:s)
Figure 3.5. Mass chromatograms of major ions
and (b) 20 mL/min He carrier gas flow rates.
obtained with methane Cl of n-alkane mixture with (a) 8 mL/min
The sensitivity with Cl is apparently unaffected by the increase
in flow rate.


68
Table 3.1. Integrated peak areas for comparison of El and Cl at different
He carrier gas flow rates.
Flow Rate
Ionization
Peak
Hydrocarbon
m/z
(mL/min)
Method
Area
C14
85
8
El
220694
C15
85
8
El
213729
C16
85
8
El
227111
CK
85
20
El
53244
C15
85
20
El
53148
C16
85
20
El
52292
CK
197
8
Cl
1893613
C15
211
8
Cl
1827399
C16
225
8
Cl
1617471
C14
197
20
Cl
1758975
C15
211
20
Cl
1804526
C16
225
20
Cl
1679119


69
(M-H)+ ions. Figure 3.5 shows the mass chromatograms of these ions for
the two flow rates. The resultant integrated peak areas for these ions
are shown in Table 3.1. As shown in the table, the variation in the peak
areas when the flow was increased from 8 to 20 mL/min is insignificant and
within the limits of experimental error. These results indicate that the
chemical ionization process is less affected by the increased He carrier
gas flow rates. This may be due to the fact that sample molecules in the
Cl ion source exist in a large excess of reagent molecules, thus the
variation of He flow rates does not significantly affect sample
ionization. The increased sensitivity (larger signal intensities and peak
areas) obtained with Cl for this alkane mixture can be attributed to the
differences in the amount of fragmentation obtained with El and Cl. The
El spectra exhibited a large amount of fragmentation, and thus resulted
in an abundance of fragment ions. In contrast, abundant pseudo-molecular
(M-H+) ions were obtained with Cl with less fragmentation than El. Thus,
the integrated peak areas for one selected ion were larger for Cl than for
El.
Once it was determined that El, but not Cl, sensitivity was degraded
by high flow rates of He carrier gas, it was postulated that increasing
the electron energy might help to improve the sensitivity of El with the
high flow rates of carrier gas. The electron energy, which is the
potential difference between the filament and the source block, represents
the kinetic energy of the electrons impinging upon the sample molecules.
The electron energy is usually set higher in Cl than in El (100 eV instead
of 70 eV) to insure that the electrons have sufficient energy to penetrate
the source region containing ca. 1 torr of reagent gas. However,
experiments have shown that an increase or decrease in electron energy


70
and/or optimization of any of the other ion optical parameters does not
improve the performance of El when relatively high flow rates are used.
These data indicate that Cl is the ionization method that should be used
if high flow rates (>10 mL/min) of carrier gas are to be used. This is
not a severe limitation since Cl is often more sensitive than El and is,
thus, usually preferred for trace analysis. In addition, since Cl is a
"softer" ionization method (less fragmentation than El), molecular weight
information is often more readily obtained. If more fragmentation were
desired, it might be possible to employ charge exchange ionization instead
of El or Cl, as shown in Chapter 4, and as has been done in SFC/MS [42].
Comparison of temperature programming and flow rate programming. In
order to illustrate the similarities and differences of linear temperature
programming and exponential flow rate programming, a series of
chromatograms was obtained under each set of conditions. In these
experiments, a 3 m x 0.25 mm i.d. DB-5 capillary column was used. The
sample was a GC test mixture obtained from J&W Scientific that is often
used for evaluating the chromatographic performance of non-polar bonded-
phase columns. The sample contained 23 ng/juL of each of seven components,
which are listed along with their boiling points in the caption of Figure
3.6.
Figures 3.6 and 3.7 show the separation of this mixture with
exponential flow rate programming and temperature programming,
respectively. Figure 3.6 (a) shows the chromatogram obtained with
isorheic (constant flow rate) operation at 5 mL/min of He carrier gas and
an isothermal column temperature of 65C. Figures 3.6 (b) and (c) show
the mixture separated isothermally at 65C using exponential flow rate
programming. The rate of the exponential was adjusted such that the final


Figure 3.6. Reconstructed ion chromatograms (methane Cl) of GC test
mixture obtained on a 3 m column with isothermal operation at 65C and
various flow conditions of He carrier gas: (a) isorheic at 5 mL/min; (b)
exponential flow program from 5 mL/min to 30 mL/min in 4 minutes; (c)
exponential flow program from 5 mL/min to 30 mL/min in 3 minutes. The
components of the mixture and their boiling points are: 1, 2-chlorophenol
(175C); 2, n-undecane (196C); 3, 2,4-dimethylaniline (214C); 4, 1-
undecanol (243C); 5, acenaphthylene (270C); 6, tetradecane (232C); 7,
pentadecane (271C).


72
(a)
(b)
(c)


Figure 3.7. Reconstructed ion chromatograms of GC test mixture performed
with isorheic operation at 5 mL/min He carrier gas and different
temperature program rates: (a) 2C/min; (b) 5C/min; (c) 15C/min. The
components of the mixture are identified in Figure 3.6.


74
(a)
(b)
(c)
Retention Time (min:s)
0:21


75
flow rate of 30 mL/min was reached after 4 minutes and 3 minutes for
Figures 3.6 (b) and (c), respectively. Figure 3.7 illustrates isorheic
separations of the mixture with temperature programming at rates of
2C/min, 5C/min, and 15C/min. It can be seen from Figures 3.6 and 3.7
that exponential flow rate programming yields similar results to linear
temperature programming.
It is evident from this comparison that a change in flow rate does not
influence retention as much as a change in temperature. For example, a
temperature program rate of 2C/min and a resulting temperature increase
of 8C over a period of 4 minutes (Figure 3.7 (a)) yielded nearly the same
results as an exponential flow program where the flow rate was increased
sixfold from 5 mL/min to 30 mL/min in 4 minutes (Figure 3.6 (b)). The
differences of temperature and flow rate programming are readily
understood if the thermodynamics of the chromatographic process are
considered. The retention time, tR, of a solute is given by [43]
tR = L(l+k)/u (3.11)
where v is the average carrier gas velocity, L is the column length, and
k is the partition ratio; thus, the retention time is inversely
proportional to the average velocity. It is also evident from eq 3.9 that
under vacuum outlet conditions the average velocity is proportional to the
square root of the column flow rate; therefore, an increase in flow rate
by a factor of 6 (e.g., from 5 to 30 mL/min) would decrease the retention
time by only the square root of 6 (or 2.4). In contrast, a change in
column temperature affects the equilibrium of the solute between the
mobile phase and stationary phase, and thus results in a change in the


76
partition ratio, k. The effect of the column temperature on the partition
ratio is given by [43]
In k -AG/RT In B (3.12)
where AG is the change in free energy for the evaporation of the solute
from the liquid stationary phase, T is the absolute temperature, R is the
gas constant, and B is the volume ratio of the mobile phase to the liquid
stationary phase. Consider n-pentadecane, which has a retention time of
311 s on the 3 m x 0.25 mm i.d. column at 65C. The associated values of
k, AG, and B for these conditions are 259, -7.9 kcal/mol, and 500,
respectively. In order to decrease the retention time of this compound
by a factor of 2.4 (the same factor afforded by a factor of 6 increase in
flow rate), the partition ratio factor (1+k) of eq 3.11 must also be
reduced by this amount (i.e., k must equal 108). From eq 3.12, the
temperature must be increased by only 27C (i.e., to 92C) to decrease the
partition ratio and the retention time by a factor of 2.4.
The calculations discussed above as well as the experimental data
demonstrate that a change in temperature has a greater effect on retention
than does a change in flow rate; therefore, the range of boiling points
spanned in a flow program will not be as large as that obtainable with
temperature programming. Nevertheless, the chromatograms obtained with
flow programming are certainly an improvement over isothermal operation
(Figure 3.6). The range of boiling points covered is still relatively
wide, as the components in the test mixture have boiling points ranging
from 175C (2-chlorophenol) to 271C (pentadecane). Perhaps the real
advantage of flow rate programming is the low elution temperatures. The


77
components of Figure 3.6 eluted as much as 200C below their boiling
points. This benefit might be of particular value in the separation of
thermally labile components or if limited power were available for the
heating of a portable GC.
Direct Resistive Heating
Calibration of columns for temperature sensing. Data obtained for the
calibration of the column resistance vs. temperature are shown in Figure
3.8. The column resistances were normalized by dividing by the length of
the column used in the calibration. Length normalization was necessary,
since the same length of column was not always used in the GC probe. Once
the calibrations were performed, the temperature of a resistively-heated
column could be determined from the relationship
T aR/L + b (3.13)
where T is temperature (C), R is the resistance (Q) L is the column
length (m) and a and b are the slope and y-intercept of the linear
calibration curve, respectively. The slopes, y-intercepts, and standard
deviations from the linear least squares fits of the calibration data are
shown in Table 3.2. As shown in Figure 3.8 and Table 3.2, essentially the
same calibrations were obtained for the two different lengths of the
narrow-bore column. This is an indication of the uniform thickness of the
Al coating. Since these columns were cut from the same original column,
no generalizations can be made about the column-to-column uniformity. As
shown in Figure 3.8, the resistance of the wide-bore column is less than
that of the narrow-bore column. This is due to the fact that there is a


Temperature (C)
78
Figure 3.8. Calibrations of column temperature vs. normalized resistance
(fl/m) for two different diameter Al-clad capillary columns. The
temperature of a resistively-heated column could be determined from the
calibration line with knowledge of the column resistance and the length.


79
Table 3.2. Results of linear least squares fits of temperature
calibrations shown in Figure 3.8.
column
slope s.d.a
Y-intercept s.d.b
r
c
0.33 mm i.d. x 3
0.33 mm i.d. x 4
0.53 mm i.d. x 3
05 m 276.7 0
77 m 280.4 0
14 m 522.2 1
7 -217.9 0
9 -220.8 0
7 -221.7 0
4
0.99991
5
0.99987
5
0.99986
aslope standard deviation, in units of C/iim'1
by-intercept standard deviation, in units of C
Correlation coefficient for the linear least squares fit


80
greater mass of Al per unit length on the wide-bore column, which can be
attributed to the greater surface area and/or thicker Al cladding on the
wide-bore column. Note that the y-intercepts obtained were practically
the same regardless of the column diameter. This can be better understood
by a rearrangement of eq 3.5
T VRo V (3.14)
which demonstrates that the y-intercept (1/a) is dependent only on the
temperature coefficient of resistivity (a), a fundamental property of the
metal. Due to the limited availability of these columns, it has not yet
been possible to compare calibrations from different columns of the same
diameter. If the column-to-column Al coating thicknesses were consistent,
then calibrations would not have to be repeated for each new column.
Rather, the calibrations would only need to be done for each new diameter
column that was used.
One advantage of this method of direct column temperature sensing,
since thermocouples or other temperature sensors are not needed, is the
simplicity of the electronic circuitry required. This could be an
important advantage in the design of portable instrumentation for
environmental or process monitoring. Another advantage that is
particularly important in this application is that measurement of the
column resistance is effectively a length-average temperature measurement.
The temperature measured at each point along the column may vary,
depending on the insulation and thickness of the Al coating at a
particular spot; thus, measurement of the column temperature at one of
these spots may not be an accurate representation of the average column


81
temperature. In addition, we have observed that temperature sensing by
conventional means (e.g., with a thermocouple placed in contact with the
column) causes the temperature of the column to drop near the point of
measurement and leads to inaccurate determinations. This direct
measurement of column temperature might be attractive for other
applications as well. For example, it is normally assumed that the
temperature of a column inside an oven directly follows the oven
temperature; this method could be used to directly measure the actual
temperature of an Al-clad GC column inside a chromatograph oven.
Temperature response characteristics. As mentioned previously, direct
resistive heating of Al-clad columns allows for extremely rapid cycling
of the column temperature due to their extremely low thermal mass. Figure
3.9 shows the temperature response of a 2.3 m x 0.53 mm i.d. Al-clad FSOT
column installed in the GC probe, after the voltage output of the
transistor of the power supply had been stepped from 0.5 V to 8.2 V and
then returned to 0.5 V after a period of 25 s. Note that the voltage
across the column (Figure 3.9 (a)) did not remain constant during the
heating period, but increased from 6.8 V to 7.5 V. Similarly, the column
voltage did not instantaneously return to its initial value of 0.45 V at
the end of the heating cycle. The slow response of the column voltage can
be attributed to an increase in the column resistance as the temperature
is increased. The output voltage of the transistor and the resistance of
the 0.261 0 resistor remain constant during the heating period; thus, as
the column resistance increases, a greater fraction of the transistor
output voltage is dropped across the column. This causes the column
voltage to slowly increase during heating and slowly decrease during
cooling.


Figure 3.9. Response characteristics for a resistively-heated 2.3 m x 0.53 mm i.d. Al-clad FSOT column during
a 25 s heating period: (a) applied voltage across the column, and (b) indicated temperature obtained from
column resistance measurement.


83
During the 25 s heating cycle, the column temperature increased from
43C to 150C. During the first second of the 25 s heating period, the
column temperature increased from 43C to 83C, which represents an
initial heating rate of 40C/s or 2400C/min. The heating and cooling
response of the column exhibits an exponential behavior, much like that
caused by impedance in an electrical circuit. However, the response times
for heating and cooling are not the same. In this example, the heating
response time of the column is defined as the time required to raise the
column temperature 90% of the way from 43C to 150C (i.e., to 139C).
As shown in Figure 3.9, this response time was found to be approximately
11 s. The temperature increase from 43C to 139C in 11 s represents an
average rate of increase of 8.7C/s or 524C/min. For comparison, the
ballistic heating rate of a typical gas chromatograph oven from 50C to
150C is only about 50C/min [44]. In addition, the maximum heating rate
for a typical on-column injector is only 150C/min [44]. Obviously, the
method of direct resistive heating offers considerable improvement in
terms of rapidity of response when compared to oven heating.
In applications where extremely rapid temperature ramps are used (as
in the example shown above), the speed of repetitive analyses will most
likely be limited by the time required to re-equilibrate (cool) the column
temperature between injections. During the first second of cooling, the
column temperature dropped from 152C to 112C, which is an initial
cooling rate of 40C/s or 2400C/min. Note that the initial heating and
cooling rates are the same. The response time for cooling the column,
defined as the time required to cool the column 90% of the way back down
to 43C from 150C (i.e., to 54C), was determined to be approximately 35
s. This represents an average rate of cooling of 2.7C/s or 165C/min.


84
For comparison, the temperature cool-down time for a typical gas
chromatograph oven over the same temperature range would be approximately
4 minutes (about 7 times longer) [44].
As shown in Figure 3.9, there is some thermal lag associated with the
heating and cooling these of columns. A small amount of thermal lag can
be attributed to the voltage regulation of the power supply. The power
supply regulates the voltage output of the transistor; thus, the voltage
across the column changes as the temperature (and hence the resistance)
of the column changes. Much of this thermal lag can be attributed to the
slow thermal response of the insulation that was used to electrically
isolate the column; thus, it is expected that these columns would exhibit
even faster thermal response if the glass braid insulation were
eliminated. This would be possible if the column could be wrapped around
a grooved spool, which would separate each strand of the column and
prevent electrical shorting. Nevertheless, even without any further
improvements, the temperature response characteristics are impressive when
compared to conventional GC ovens.
Another advantage of direct column heating is that very little
electrical power (or energy) is needed. For example, approximately 35 W
of electrical power applied for 25 s (0.875 kJ of energy) was required to
resistively heat the 2.3m column to 150C. For comparison, a typical GC
oven utilizes a 2400 W heater (240 V AC, 10 A) [44], Assuming an average
ballistic heating rate of 50C/min, 3 minutes would be required to heat
the oven from 50C to 150C. Approximately 432 kJ of energy would be
consumed, which is nearly 500 times more energy than that required by
direct resistive heating. In addition, a typical injector is heated
separately from the oven and requires an additional 100-300 W of power


85
[44] When the entire column is heated resistively and on-column
injection is used, as is the case with the GC probe, no additional
injector heaters are required. The power required to resistively heat a
column to a particular temperature is dependent on the thermal mass of the
column (eq 3.3). For columns of the same diameter and Al coating
thickness, the power needed to reach a particular temperature is directly
proportional to the column length. For example, a 23 m column would
require ten times more power than the 2.3 m column, or 350 W, to reach
150C.
One limitation of the present system is that the voltages sensed at
the lower column temperatures are too small to yield a high degree of
precision for the column resistance (and hence temperature) measurement.
This is the reason for the quantization noise (at low voltages and
temperatures) in the temperature profile of Figure 3.9. This limitation
could be remedied if an autoranging circuit was used to automatically
adjust the sensitivity of the resistance-sensing electronics.
GC Probe Performance
Figures 3.10 (a) and 3.10 (b) illustrate two different reconstructed
ion chromatograms that were obtained with the GC probe and two different
temperature ramps using a 3.19 m x 0.53 mm i.d. Al-clad FSOT column. A
0.5 jiL on-column injection was made in both cases. The concentration of
each component (identified in the figure) was 250 ng/^L in hexane. In
Figure 3.10 (a), the voltage across the column was programmed, after a
delay of 15 s, from 0 V (25C) to 8.6 V in 2 minutes. The voltage was
held constant for another 1.5 minutes, resulting in a final temperature
of 143C. In Figure 3.10 (b), the same initial voltage (temperature) was


Figure 3.10. Reconstructed ion chromatograms obtained with the GC probe
using a 3.19 m x 0.53 mm i.d. column and two different linear column
voltage programs: (a) final voltage of 8.6 V after 2 minutes, (b) final
voltage of 10.8 V after 1 minute. The components of the mixture are: 1,
2-chlorophenol; 2, 2,4-dimethylaniline; 3, n-undecane; 4, 1-undecanol; 5,
acenaphthylene; 6, tetradecane; 7, pentadecane.


87
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Retention Time (min)
(b)


88
used as in Figure 3.10 (a). The delay was 10 s and the final voltage was
10.8 V after 1 minute. The voltage was held constant at 10.8 V for an
additional 30 s, resulting in a final temperature of 168C.
On average, the peak widths at 50% height are approximately 3 s in
Figure 3.10 (a) and 2 s in Figure 3.10 (b) The earliest eluting peak (2-
chlorophenol) has a larger peak width in both cases. This can be
attributed to the deleterious effects of the solvent. During cold on-
column injection, components that are much less volatile than the solvent
are focused at the head of the column and exhibit very narrow band widths.
In contrast, the volatile, early eluting components (if not well separated
from the solvent front) can partition in the liquid solvent as well,
resulting in increased solute peak widths [45]. This problem can often
be avoided if smaller volumes of solvent are injected, or more reliably,
if cryofocusing is used to trap the solutes in a narrow band at the head
of the column [46]. The peak widths obtained here are extremely narrow
considering that the column that was used had a stationary phase film that
is normally considered thick (1.0 /m). In addition to increasing the
chromatographic resolution, the capability of obtaining narrow band widths
increases the sensitivity and lowers the limit of detection [3,14].
In another study, the reproducibility of retention times and peak
areas obtainable with the GC probe were measured. The retention times and
resulting integrated peak areas were measured for four on-column,
temperature-programmed injections of 150 ng tetradecane on a 3.3 m x 0.53
mm i.d. column. Tetradecane, which has a boiling point of 232C, eluted
at 125C with a retention time of 90.00 0.81 s (relative standard
deviation (RSD) of 0.9%). The integrated peak area of the m/z 198
molecular ion of tetradecane was determined with a precision of 3.5% RSD,


89
which is certainly within the limits of error for injections performed
with a standard syringe.
Conclusions
Several important concepts for low-power and/or high-throughput
analysis in GC/MS have been addressed. It has been shown that exponential
flow rate programming can be used to approximate linear temperature
programming. When chemical ionization is used, flow rates up to 30 mL/min
can be used without detrimental effects on instrument sensitivity.
Although the range of boiling points separated with flow programming is
less than that which can be obtained with temperature programming, flow
programming does represent a significant improvement over isothermal
operation. The inherent lower elution temperatures of flow rate
programming might also be attractive for the analysis of thermally labile
solutes.
The direct resistive heating of Al-clad FSOT columns has also been
demonstrated. This direct method of column heating obviates the need for
a large chromatograph oven, and should be widely applicable in portable,
low-power gas chromatographic instrumentation. This was demonstrated with
the design of a compact GC probe for high-throughput analyses in GC/MS and
GC/MS/MS. Measurement of the column resistance was shown to be a simple
yet accurate method of directly sensing the temperature of low thermal
mass Al-clad columns. It was demonstrated that linear voltage programming
of the Al-clad columns can be used to approximate linear temperature
programming. Future application of proportional-integral-differential
(PID) feedback control of the column temperature and elimination of the
column insulation is expected to further improve the thermal response
characteristics of direct resistive heating of these columns.


CHAPTER 4
GAS CHROMATOGRAPHIC SAMPLE INTRODUCTION INTO
THE COLLISION CELL OF A TRIPLE QUADRUPOLE MASS
SPECTROMETER FOR MASS-SELECTION OF REACTANT
IONS FOR CHARGE EXCHANGE AND CHEMICAL IONIZATION
Introduction and Theory
The maximum selectivity of any analytical technique can be realized
if all of the variable parameters of the method are considered. These
variable parameters can be considered as resolution elements that affect
the informing power (i.e., the amount of information available) in the
analytical method [47]. In mass spectrometry, two resolution elements
which may be varied are mass analysis and ionization. The informing power
of the mass analysis can be augmented by increasing the mass range,
increasing the mass resolution, or increasing the number of stages of mass
analysis (i.e., tandem mass spectrometry). Since the ionization method
is a resolution element, it too will affect the informing power. A
certain degree of "resolution" (or selectivity) can be obtained with
ionization by employing techniques that allow ionization of only the
desired components of a mixture, thereby excluding the ionization (and
hence detection) of undesirable interferents. This selectivity of
ionization is not frequently exploited, but can be controlled if careful
consideration is made for the selection of reactant ions for charge
exchange (CE) ionization or chemical ionization (Cl) [48], Traditionally,
ion-molecule reactions for CE or Cl are performed in a high-pressure
(e.g., 1 torr) ion source [48-50], However, there are limitations to
90


91
performing these reactions in the ion source. When a reactant gas is
introduced into the ion source, it is rare that only a single m/z reactant
ion is formed; due to the presence of other undesirable reactant ions,
the ionization process may not be well-controlled. In fact, many
ionization processes including CE, Cl, (each with various reactant ions)
and even El may compete with the ionization technique of interest. This
mixed-mode ionization certainly limits the selectivity of the ionization
process. In addition, instrumental parameters such as ion source pressure
affect the relative abundances of the various reactant ions; hence,
reproducible CE or Cl spectra are often not easily obtained.
A novel approach that eliminates the mixed-mode ionization discussed
above is to mass-select the desired reactant ion before allowing it to
react with the sample. Mass-selection of reactant ions has previously
been used for studying ion-molecule reactions and reactive collisions.
A variety of tandem mass spectrometers have been used, including sector
instruments [51], a double quadrupole instrument [52], quadrupole ion
traps [53-55] and triple quadrupole mass spectrometers [56-62] Crawford
and co-workers have used Kr as a charge exchange collision gas in a triple
quadrupole mass spectrometer (TQMS) for monitoring carbon monoxide in the
presence of hydrocarbons [63], However, to date there has been no
demonstration of the analytical utility of mass-selecting the reactant ion
for mixture analysis, or of the combination of mass-selected reactions
(MSRs) with GC. For this reason, a gas chromatograph has been interfaced
to the collision cell of a TQMS. In gas chromatography/mass-selected
reaction/mass spectrometry (GC/MSR/MS), reactant ions are selected with
the first quadrupole mass filter (Ql) and are allowed to react with
neutral sample molecules which elute from a GC column into the second


Full Text
BSgasia florea
126 08554 2743


CHARACTERIZATION AND ENHANCEMENT OF SAMPLE INTRODUCTION AND
ION TRANSMISSION IN COMBINED
GAS CHROMATOGRAPHY/TANDEM MASS SPECTROMETRY
By
MARK EDWARD HAIL
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULLFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1989

To my loving wife, best pal, and big toe, Amy.

ACKNOWLEDGMENTS
I would like to express sincere gratitude to my graduate research
director, Dr. Richard A. Yost, for his guidance and friendship over the
past four years. Rick deserves some sort of medal, or perhaps more
appropriately, a new prescription for his glasses, for all of the reading
and reviewing of manuscripts and thesis chapters that he has done during
the past few months. I would like to thank the members of my graduate
committee, Drs. James D. Winefordner, Anna Brajter-Toth, Samuel 0.
Colgate, and Henri A. Van Rinsvelt. I would also like to thank Dr. John
G. Dorsey (a fellow Lexingtonian) for his helpful discussions.
I would like to acknowledge the U. S. Air Force Engineering and
Services Center at Tyndall Air Force Base and the U. S. Army Chemical
Research Development and Engineering Center at Aberdeen Proving Grounds
for providing the financial support for this work.
No graduate student would be very productive without the help of the
departmental staff. For this reason, I thank Chester Eastman, Vern Cook,
and Dailey Burch of the machine shop, and Mark Ross, Steve Miles, and Dean
Schoenfeld of the electronics shop. I extend special thanks to Russ
Pierce of the electronics shop, who was particularly helpful in answering
questions and troubleshooting problems in the electronics lab and in my
research.
I would like to thank my friends who made this period of my life so
enjoyable, including all of the Yosties, and post-Yostie Jodie Johnson.
Special thanks to my two office mates and soon-to-be coworkers, Ken
iii

Matuszak and Dave "Cool One" Berberich. I would also like to thank those
that dwelled with me in that most humble abode known as the thunderdome,
where one man enters and two people leave. In order of appearance (and
in some cases disappearance), thanks to Todd "Spud" Gillespie, Stephen
"Judas" Brooks, Mark "Citizen" Barnes, and Jerry "Citrus Connection"
Grünewald.
I would like to thank my family, Mom, Dad, Carl Jr. , Beccy, and Steve,
who have been so supportive.
Finally, I would like to thank my wife, Amy, the person who was always
there to pat me on the back, who did my laundry and fed my face when I
didn't have time, and who was always there for entertainment just by being
herself.
iv

TABLE OF CONTENTS
page
ACKNOWLEDGMENTS iii
ABSTRACT vii
CHAPTERS
1 INTRODUCTION 1
Short GC Columns in GC/MS and GC/MS/MS 1
Short GC Columns at Sub-ambient Pressures 2
Mixture Analysis by GC, GC/MS and GC/MS/MS 4
Organization of Dissertation 10
2 THEORETICAL AND PRACTICAL ASPECTS OF SHORT OPEN
TUBULAR COLUMNS AT SUB-AMBIENT PRESSURES IN
GAS CHROMATOGRAPHY/MASS SPECTROMETRY 12
Introduction and Theory 12
Experimental Section 15
Calculations 15
Mass Spectrometry 15
Gas Chromatography 16
Average Velocity Measurement 19
Results and Discussion 20
Effect of the Pressure Drop 20
Sub-ambient Inlet Pressures 26
Experimental vs. Theoretical Performance 37
Choice of the Carrier Gas 41
Conclusions 43
3 CONCEPTS FOR PORTABLE GAS CHROMATOGRAPHIC
INSTRUMENTATION IN GC/MS: FLOW RATE
PROGRAMMING AND DIRECT COLUMN HEATING 46
Introduction 46
Flow Rate Programming 47
Direct Resistive Heating of Al-Clad Capillary Columns ... 48
GC Probe 51
Experimental Section 52
Experimental Conditions 52
Flow Control System 53
Direct Resistive Heating 57
v

GC Probe Design 61
Results and Discussion 64
Flow Rate Programming 64
Direct Resistive Heating 77
GC Probe Performance 85
Conclusions 89
4 GAS CHROMATOGRAPHIC SAMPLE INTRODUCTION INTO THE COLLISION
CELL OF A TRIPLE QUADRUPOLE MASS SPECTROMETER
FOR MASS-SELECTION OF REACTANT IONS FOR
CHARGE EXCHANGE AND CHEMICAL IONIZATION 90
Introduction and Theory 90
Experimental Section 93
Mass Spectrometry 93
Gas Chromatography 95
GC/MS Interface 96
Results and Discussion 98
Ionization Energy Measurements 98
Simultaneous Structural and Molecular Weight Information . 101
Selectivity of Ionization 106
Detection Capabilities of GC/MSR/MS 112
Chromatographic Integrity 116
Conclusions 116
5 CHARACTERIZATION AND OPTIMIZATION OF ION TRANSMISSION
IN TRIPLE QUADRUPOLE MASS SPECTROMETRY 118
Introduction 118
Experimental Section 119
Instrumentation 119
Procedures 125
Results and Discussion 126
Mass-dependent Optimization 126
Quadrupole Mass Filters 135
Ion Transmission Characteristics of the Center Quadrupole . 140
Ion Transmission at High Collision Energies 153
Conclusions 157
6 CONCLUSIONS AND FUTURE WORK 159
Conclusions 159
Suggestions for Future Work 161
Reduced Column Pressures with Conventional GC Detectors . . 161
Directly-heated Open Tubular Columns 162
Resonance Excitation and Ion Storage in Q2 164
LITERATURE CITED 167
BIOGRAPHICAL SKETCH 173
vi

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
CHARACTERIZATION AND ENHANCEMENT OF SAMPLE INTRODUCTION
AND ION TRANSMISSION IN COMBINED
GAS CHROMATOGRAPHY/TANDEM MASS SPECTROMETRY
By
Mark Edward Hail
May 1989
Chairman: Richard A. Yost
Major Department: Chemistry
Various theoretical and practical aspects of the use of short open
tubular columns in combined gas chromatography/mass spectrometry (GC/MS)
and tandem mass spectrometry (GC/MS/MS) have been investigated, including
the advantages of low-pressure operation with short and/or wide-bore open
tubular columns. It is shown both theoretically and experimentally that
short columns with vacuum outlet require sub-atmospheric inlet pressures
if optimum gas velocities are to be obtained. The use of sub-ambient
inlet pressures is also shown to improve the sensitivity when short
columns are used with electron ionization in GC/MS.
Several considerations are addressed for the design of portable GC
instrumentation. In particular, exponential flow rate programming as well
as resistively-heated aluminum-clad open tubular columns have been
evaluated for low-power, high-throughput analysis in GC/MS. Aluminum-clad
vii

open tubular columns are heated directly by applying a voltage across the
capillary, and the column temperature is sensed by measuring the column
resistance. These investigations have led to the development of a compact
gas chromatograph probe for use in GC/MS.
A gas chromatograph has been interfaced to the collision cell of a
triple quadrupole tandem mass spectrometer for performing mass - selected
ion-molecule reactions. Reactant ions are selected with the first
quadrupole and are allowed to react in the second quadrupole collision
cell with the effluent from a short open tubular GC column. The ion-
molecule reaction product ions are mass-analyzed by the third quadrupole.
The advantages of using mass-selected reactions for controlling the
selectivity of charge exchange and chemical ionization are demonstrated.
In addition, this configuration is shown to provide both structural
information and molecular weight information in the same chromatogram by
alternating between different reactant ions.
The ion transmission characteristics of a triple quadrupole mass
spectrometer (TQMS) have been studied. The unique high level of computer
control of the TQMS has allowed for rapid characterization and
understanding of the ion optical parameters. Strategies for obtaining
optimum ion transmission and minimization of mass dependencies in the ion
optics for single MS experiments as well as for MS/MS experiments are
discussed.
viii

CHAPTER 1
INTRODUCTION
In this dissertation a variety of concepts are presented for the
enhancement of sample introduction and ion transmission in analytical mass
spectrometry (MS) and tandem mass spectrometry (MS/MS). Due to the
inherent speed and selectivity of MS and MS/MS, considerable attention was
devoted to the study of rapid gas chromatographic (GC) separation
techniques. A new tandem mass spectrometric method, in which the
selectivity of ion-molecule reactions is exploited for gas chromatographic
mixture analysis, is also described. Finally, the results of a detailed
characterization of the ion optical parameters of a triple quadrupole
tandem mass spectrometer are presented.
Short GC Columns in GC/MS and GC/MS/MS
In this research group, it has been found that short GC columns can be
used for the extremely rapid analysis of complex mixtures by taking
advantage of the inherent selectivity of MS, and particularly MS/MS [1-
3], In the initial results that were obtained with very short (e.g., 50
cm) packed columns, it was found that the .sensitivity and selectivity
obtainable with short-column GC in conjunction with MS/MS in many cases
exceeded that of high-resolution GC/MS (HRGC/MS) [1], In addition, it
was shown that 100 injections per hour could be performed with the short-
column method, while only 2 injections per hour could be made with the
HRGC/MS method employing an 18 m capillary column [1], Later, short open
1

2
tubular capillary columns were evaluated (including an investigation of
the associated chromatographic theory) due to their inherent advantages
over packed columns [2,3]. These studies began to demonstrate the
advantages of vacuum outlet operation with short open tubular columns.
It was found that there were added benefits other than speed of analysis
that were important. For instance, because of the high gas velocities and
low operating pressures associated with short open tubular columns under
vacuum outlet conditions, it was found that solutes tended to elute at
temperatures well below their boiling points. Thus, it was discovered
that short-column open tubular GC in conjunction with MS or MS/MS was
ideal for the analysis of involatile, thermally labile, or polar solutes
[2,3], More recently, McClennen et al. have taken advantage of this and
have demonstrated that large involatile and/or polar molecules can be
analyzed by examining the Curie-point pyrolysis products of these
molecules with short-column GC/MS [4,5],
Short GC Columns at Sub-ambient Pressures
Many of the chromatographic separation techniques currently in
widespread use require relatively high operating pressures, namely, high-
pressure liquid chromatography (HPLC), supercritical fluid chromatography
(SFC), and even high-resolution capillary open tubular gas chromatography
(HRGC). In contrast, research in the area of low-pressure separation
techniques has been limited. More than 25 years ago, Giddings
demonstrated that the minimum analysis times in gas chromatography could
be achieved when the column outlet was connected to a vacuum [6] .
However, to date, a majority of the research done in high-speed GC has
dealt with the use of small diameter columns and the development of high-

3
speed injectors required for use with these columns [7-13], Recently,
interest in vacuum outlet operation has been renewed, as Leclercq and
coworkers have published a number of articles that have addressed the
theory and advantages of vacuum outlet operation for reducing analysis
times in open tubular GC [14-17],
Vacuum outlet operation in gas chromatography results in lower column
pressures, which increases the diffusivity of the solute in the gas phase
and leads to increased optimum carrier gas velocities and shorter analysis
times [14]. The advantages of vacuum outlet operation are inherently
obtained when gas chromatography is combined with mass spectrometry
(GC/MS), provided the column is inserted directly into the vacuum of the
mass spectrometer. Consideration of the theory indicates that low outlet
pressure operation has a more pronounced effect if wide-bore columns [16]
and/or short columns [3] are used. As shorter column lengths are used to
minimize analysis times, lower inlet pressures are required if optimum
chromatographic performance is to be obtained. The use of very short
lengths or wide-bore columns is limited when conventional inlet pressures
are used because of the gas load imposed on the mass spectrometer.
Alternatively, the inlet can be operated at sub-ambient pressures to
obtain optimum performance and/or to reduce the flow rate of carrier gas
entering the mass spectrometer vacuum system [18]. Unfortunately, the
value of sub-atmospheric column inlet pressures has not been widely
appreciated, due in part to the difficulty of operating traditional GC
injection ports at vacuum.

4
Mixture Analysis by GC. GC/MS. and GC/MS/MS
Application of short columns with conventional GC detectors has been
limited to the analysis of relatively simple mixtures due to the non-
selective nature of the detectors and the reduced resolving power of short
columns. However, as stated previously, the inherent selectivity of MS
and MS/MS can be utilized to atone for the chromatographic resolution lost
when shorter columns are used. Perhaps a better comprehension of the
additional selectivity of the mass spectrometric detection methods over
conventional detection methods in gas chromatography can be obtained by
considering the possible cases which result in interferences. These
coincidences are summarized in Table 1.1, which shows a comparison of GC
employing flame ionization detection (FID) as well as GC with various mass
spectrometric detection methods. A schematic comparison of the three mass
spectrometric techniques, all of which can be performed on the triple
quadrupole tandem mass spectrometer used in this work, is shown in Figure
1.1.
Mixture overlap will readily occur when non-selective detection methods
(e.g., FID) are used in complex mixture analysis, unless a column of
sufficient length is used to chromatographically separate all possible
interferences from the analyte of interest. In many cases, no column,
regardless of length, would be sufficient to accomplish this goal; thus,
other extraction and sample cleanup procedures must be used in addition
to the already time-consuming chromatographic analysis. In addition,
since chromatographic resolving power increases only with the square root
of the column length, while analysis times increase proportionally with
column length, increasing the column length to gain the necessary

5
Table 1.1. Cases leading to mixture overlap (interferences) in gas
chromatographic mixture analysis with flame ionization detection and
various methods of mass spectrometric detection. The '?'s in the table
indicate the conditions under which targeted mixture components will be
obscured by interferences.
Analytical Method
GC/FID
GC/MS
Coincidences
GC/MS/MS
Resulting in Interferences
GC/MSR/MS3
(1) coelute
(1) coelute
(1)
coelute
(1)
coelute
4-
4
4
4
?
(2) produce
(2)
produce
(2)
react to form
ions of
ions of
product ions
same m/z
same m/z
of same m/z
4
4
4
?
(3)
fragment to
?
daughter ions
of same m/z
1
?
a GC/MSR/MS = GC/mass-selected reaction/MS, see Figure 1.1.

Figure 1.1. Schematic comparison of three different combined gas chromatography/mass spectrometric methods
which may be performed on a triple quadrupole mass spectrometer: (a) GC/MS, (b) GC/MS/MS, and (c) GC/mass-
selected reaction/MS (GC/MSR/MS). The new technique described in this thesis, GC/MSR/MS, involves introduction
of the GC column into the second quadrupole collision cell (Q2), rather than into the ion source. Also note
the differences in the two MS/MS approaches. In traditional MS/MS, a sample (parent) ion is mass-selected
with Q1 and is allowed to fragment via CAD in Q2, whereas in MSR/MS, Q1 is used to mass-select a reactant ion
for reaction with sample molecules in Q2.

SAMPLE
REACTANT
CAS
(a)
SAMPLE
REACTANT
CAS
REACTANT
G AS < EST"
IONI¬
ZATION
ION
SOURCE
GC/MS
Q1 Q2 Q3
Q1 MASS
SPECTRUM
EM
GC/MS/MS
Q 1
Q2
CAD
Q3
DAUGHTER
SPECTRUM
E M
COLLISION
GAS
GC/MSR/MS
Q 1
Q2
CE or Cl
Q3
PRODUCT
SPECTRUM
SAMPLE

8
resolution tends to be very costly in terms of the time required for an
analysis.
As shown in Table 1.1, the possibilities for interferences in mixture
analysis are greatly reduced when mass spectrometric detection is used.
In GC/MS (Figure 1.1 (a)), other component(s) of the mixture must coelute
with the analyte of interest and must yield ions of the same m/z as the
analyte of interest in order for interferences to occur. Even more
selectivity is obtainable with GC/MS/MS (Figure 1.1 (b)). In order for
mixture overlap to occur in GC/MS/MS, undesirable interferents must not
only coelute and be ionized to produce the same nominal mass ions, but the
interfering parent ions must also fragment via collisionally-activated
dissociation (CAD) to produce all of the same daughter ions as the
analyte.
Described in this dissertation is a new GC/tandem mass spectrometric
technique, GC/mass-selected reaction/MS (GC/MSR/MS), which involves mass-
selection of reactant ions and reaction of these ions with the effluent
from a gas chromatographic column. It can be seen from Figure 1.1 (c)
that this method involves introduction of the GC column into the second
quadrupole collision cell (Q2) rather than into the ion source. This
method takes advantage of the intrinsic selectivity of ion-molecule
reactions. Hence, for overlap to occur in mixture analysis with
GC/MSR/MS, interferences must react in Q2 to produce all of the same m/z
product ions as the analyte of interest.
It can be seen from the above comparisons that the added resolution
elements available with the mass spectrometric techniques in many cases
obviate the need of high-resolution chromatographic separation in mixture
analysis. Therefore, the use of short open tubular columns and the

9
concomitant short analysis times are readily justified.. In addition, the
capability to perform any of the three GC/MS techniques (GC/MS, GC/MS/MS,
or GC/MSR/MS) certainly demonstrates the versatility of the triple
quadrupole mass spectrometer (TQMS).
Although instrumentation for MS/MS tends to be rather expensive (e.g.,
$350,000 for a TQMS) when compared to other analytical techniques (e.g.,
$15,000 for a GC), it has been pointed out that MS/MS is a cost-effective
technique because of the short analysis times that are possible [19].
This is most likely the reason for the increasing popularity as well as
the widespread availability of commercial instrumentation for MS/MS. The
MS/MS technique has certainly benefited from recent advances in computers
and electronics, as these progressions have led to the development of a
wealth of new, exciting, computer-controlled instrumentation. One such
instrument is the Finnigan MAT TSQ70, which was used throughout this work.
This instrument, with its unique level of computer control, has allowed
a detailed study of the ion optical parameters and their effects on ion
transmission in triple quadrupole mass spectrometry. In fact, many of the
undesirable ion optical effects (e.g., mass dependencies) can be avoided
due to the capability of this instrument to vary any the ion optical
parameters as a function of mass within each mass spectral scan. These
types of studies have provided a more thorough understanding of the ion
optical parameters that effect the performance of this instrument, which
will ultimately improve the analytical capabilities during its use.

10
Organization of Dissertation
This dissertation is divided into six individual chapters. This
chapter has served to present an overall perspective as well as the
purpose of the work. Global conclusions and suggestions for future
research are found in Chapter 6. Chapters 2-5 make up the body of the
dissertation. Due to the widely varying material presented in this
dissertation, there is no particular chapter devoted entirely to the
description of experimental conditions. Rather, each chapter (of Chapters
2-5) consists of a separate introduction, experimental section, discussion
of results, and conclusions.
In Chapter 2, the advantages of utilizing low column pressures with
short open tubular columns in GC/MS are examined by consideration of the
theoretical chromatographic relationships. In particular, the advantages
of operating the column inlet, as well as the column outlet, at sub-
atmospheric pressures are stressed. Experimental data are presented that
demonstrate the practicality of the combination of short GC columns with
mass spectrometry.
In Chapter 3, considerations for portable and/or high-throughput sample
introduction schemes for GC/MS are presented, including investigations of
exponential flow rate programming and direct resistive heating of
aluminum-clad open tubular columns. A portable direct insertion gas
chromatograph probe is described that incorporates these concepts.
The fourth chapter describes the GC/MSR/MS technique, including a
brief discussion of the utility of selective ion-molecule reactions for
mixture analysis. This work represents the first experiments ever
performed in which samples are introduced from a GC column into the center
quadrupole collision cell of a TQMS. Experimental data are presented that

11
demonstrate the potential selectivity, detection and screening
capabilities of this method. A method is also described which allows for
the extremely rapid measurement of ionization energies on the computer-
controlled TQMS.
In Chapter 5, the effects of various ion optical parameters of the
TQMS for MS and MS/MS are investigated. In particular, it is shown that
the mass-dependencies in the ion optics of the computer-controlled TQMS
can be minimized, since all of the ion optical parameters may be varied
as a function of mass during a mass spectral scan. The insights gained
in these experiments have yielded a better understanding of the complex
inter-relation of the parameters that affect the performance and
reproducibility of the results obtained with this instrument.

CHAPTER 2
THEORETICAL AND PRACTICAL ASPECTS OF SHORT OPEN TUBULAR COLUMNS
AT SUB-AMBIENT PRESSURES IN GAS CHROMATOGRAPHY/MASS SPECTROMETRY
Introduction and Theory
In this chapter, various theoretical and practical aspects of the use
of short open tubular columns in GC/MS are discussed, including the
advantages of low-pressure operation with short and/or wide-bore open
tubular columns. It is shown that short columns with vacuum outlet
require sub-atmospheric inlet pressures if optimum gas velocities are to
be obtained. Since sub-atmospheric inlet pressures are not obtainable
with most gas chromatographic inlet systems, a computer-controlled flow
control system has been investigated that allows the injection port to be
operated at sub-ambient pressures with either split or splitless
injections. Plate heights have been experimentally determined at sub¬
ambient pressures and have been compared with theoretical predictions.
The effects of extra column band broadening are evident in the
experimental data and demonstrate the need for high-speed injection
systems if maximum performance is to be obtained with short columns. The
use of sub-ambient inlet pressures is also shown to improve the
sensitivity when short columns are used with electron ionization in GC/MS.
Finally, it is shown that hydrogen carrier gas can be used to obtain the
fastest analyses without altering the relative abundances of ions obtained
with methane chemical ionization mass spectrometry.
12

13
The theoretical dependence of the plate height, H, on the average gas
velocity, v, in open tubular gas chromatography is given by the Golay
equation [20]
H = B/v + (Cg+Ct)v
(2.1)
where B is a term relating to the longitudinal diffusion of the solute
zone, and Cg and are terms relating to mass transfer in the gas phase
and liquid stationary phase, respectively. For open tubular columns
employing a thin-film stationary phase, mass transfer in the liquid phase
is negligible and Cg dominates [14]. The Golay equation for thin-film
columns corrected for gas compressibility [21] can be rewritten in terms
of the column parameters yielding [14]
2D Qf (l+6k+llk2)r2u0f
H :— + (2.2)
v0 24(l+k)2Dgo
where Dg Q is the diffusion coefficient of the solute in the gas phase at
the outlet pressure, f is the Giddings correction factor, vQ is the gas
velocity at the column outlet, k is the partition ratio, and r is the
column radius. The Giddings correction factor accounts for the
decompression of the carrier gas when a pressure gradient exists and is
given by
9(P4-1)(P2-1)
f
8(P3-!)2
(2.3)

14
where P is the inlet to outlet pressure ratio, ?i/’PQ. The gas
decompression factor approaches a value of 9/8 when large pressure
gradients are employed, as in GC/MS. Most commonly, H is expressed in
terms of the average velocity, u. If this is done, uQ and Dg Q of eq 2.2
are replaced by the average velocity, v, and an average diffusion
coefficient, Dg. An average diffusion coefficient can be estimated from
[14]
D
9
P1°9.1
(2.4)
where Dg 1 is the diffusion coefficient at 1 atmosphere pressure, P1, and P
is the average column pressure. The length-average column pressure, P,
is dependent on the pressure gradient and is given by [14]
2P0(P3-1)
P (2.5)
3(P2-1)
which reduces to 2Pt-/3 for vacuum outlet conditions.
As shown by eq 2.4, lowering the average column pressure increases the
diffusion coefficient. An increase in the diffusion coefficient has been
shown to result in an increase in the optimum gas velocity and decreased
analysis times [3,14-18], In addition, a larger Dg results in a slower
rate of increase of H (from the Cg term) as the gas velocity is increased
beyond the optimum. This means that even higher gas velocities can be
used without severe losses of efficiency.

15
There are a number of ways of decreasing the average column pressure.
Lower average column pressures are readily obtained if the column outlet
is at vacuum as in GC/MS. At lower outlet pressures, the greatest gains
in terms of speed of analysis (as compared to normal atmospheric outlet
operation) can be obtained if short and/or wide-bore columns are used
[3,16]. Lower average column pressures can also be obtained if low-
viscosity carrier gases are used [22]; hence, the use of hydrogen as a
carrier gas will always result in the shortest analysis times.
Experimental Section
Calculations
All theoretical calculations were done on an IBM PC/XT or a PC/AT, with
programs written in TurboBASIC (Borland International). The results of
the calculations (e.g., H vs. u data) were stored on disk in ASCII format,
which allowed the data to be retrieved and plotted with a commercially
available plotting program (Grapher, Golden Software, Inc.).
Mass Spectrometry
A Finnigan MAT TSQ70 triple quadrupole mass spectrometer was used in
these studies. Electron energies of 70 and 100 eV were used for electron
ionization (El) and chemical ionization (Cl), respectively. The
appropriate interchangeable ion volumes were used to obtain El and Cl
spectra. In order to obtain true ion source pressures, indicated ion
source pressures were calibrated with a capacitance manometer. This was
done by connecting a capacitance manometer (MKS Instruments model 127AA)
to the Cl ion source via the side flange (normally used for GC/MS transfer
line) of the TSQ70. The capacitance manometer was used to calibrate the

16
ion source thermocouple gauge at various pressures of methane reagent gas.
Ion source pressures were determined as a function of flow rate of helium
and hydrogen carrier gases by connecting a mass flow controller (MKS
1159A) to the Cl ion source and recording the true pressures read from the
capacitance manometer. All ion source pressures reported are the absolute
pressures obtained from the calibrations. Ion source pressures for
chemical ionization were typically 0.2-0.5 torr. The ionizer temperature
was 150°C. The preamplifier gain was set to 108 V/A; the electron
multiplier was operated at 1000 V.
Gas Chromatography
A Varian model 3400 GC equipped with a split/splitless injector with
a 2 mm i.d. glass liner was used. Bonded phase fused silica open tubular
columns were used with an inner diameter of 0.25 mm and a 0.25 ¿im film
thickness (DB-5, J&W Scientific). Column lengths were typically 3 m. All
columns were inserted directly into the ion source. Column efficiencies
were measured with n-pentadecane (C15H32) with the column and interface at
100°C (k = 32) and an injection temperature of 200°C. For all other
applications temperature programming was used with the interface at 150°C.
Column temperatures were programmed at a rate of 25°C/min from 50°C to
150°C, after an initial hold period of 12 seconds.
A schematic of the gas chromatographic system, modified to allow for
either normal pressure-regulated operation or flow-controlled operation,
is shown in Figure 2.1. With valve 1 (VI) rotated to position A, the GC
can be operated in the normal configuration with inlet pressures above 1
atmosphere. With VI rotated to position B, flow regulation is used,
allowing inlet pressures above or below one atmosphere to be used. In

CA R R I
GAS
PRESSURE
REGULATOR
ME C HA NICAL
E R
Figure 2.1. Schematic diagram of gas chromatographic flow control system that allows inlet pressures above
or below one atmosphere to be used in the injection port.

18
addition, computer control of the flow controller allows the flow rate of
the carrier gas to be programmed with time if desired (see Chapter 3).
The mass flow controller (MFC) used was a MKS model 1159A with a type
246 power supply/digital readout. The output flow rate of the MFC, which
has a range of 0-100 mL/min for N2, is set by a 0-5 V control voltage from
the TSQ70. The TSQ70 has two spare digital-to-analog converters (DACs),
or user outputs, one of which was used to control the flow rate of the
MFC.
In order to achieve the sub-ambient inlet pressures, a 30 L/min
mechanical pump (Alcatel model 8A) was connected to the split line of the
injection port. A Bourdon tube pressure gauge (Omega Engineering model
30V/30) capable of indicating pressures above or below one atmosphere was
connected to the septum sweep of the GC injection port. Valve V2 is a
solenoid valve that can be opened or closed under computer control. The
inlet pressure in the vacuum inlet mode is determined by the flow rate of
carrier gas from the MFC and the adjustment of the needle valve (V3). The
occurrence of air leaks in the system was monitored by observing the
intensity of the N2+ (m/z 28) and 02+ (m/z 32) ions. It was necessary to
replace the plastic fittings and viton ferrules of the Varian 3400 carrier
gas lines with stainless steel fittings to minimize leakage of air into
the system. Leakage of air through the septum was minimized by replacing
the septum after frequent use (e.g., after 30 injections). Once these
steps were taken, no additional increase in the intensity of the air peaks
was observed when vacuum inlet operation was used instead of normal
atmospheric operation.
During a split injection, samples are injected directly into the low-
pressure injection port with valve V2 open; thus, most of the sample is

19
swept away by the mechanical pump. One of the problems of vacuum inlet
GC that has limited its use in the past is that only split injections
could be performed. This is certainly a limitation, especially when
analyzing samples with analytes present at trace levels. Splitless
injection is commonly used for trace analysis [23], With the flow control
system shown in Figure 2.1, the advantages of vacuum inlet GC, as well as
splitless operation, can be utilized. A splitless injection is made with
V2 initially closed, with the injection port slightly above atmospheric
pressure. After allowing ample time for the sample to enter the column,
the inlet pressure is then rapidly reduced by opening V2. Based on the
internal volume of the injection port (0.24 mL), at a typical flow rate
of 5 mL/min, the injection port is completely flushed with carrier gas in
approximately 3 s. Thus, leaving V2 closed for any period longer than 3
s should be sufficient time to insure that all the sample has entered the
column. Nevertheless, in all experiments utilizing splitless injections
reported here, V2 was left closed for 12 s or more.
Average Velocity Measurement
It was found that the average velocity of the carrier gas could not be
accurately measured by conventional means with the short columns that were
used. Traditionally, the average velocity is determined from the dead
time, which is the time required for the elution of unretained air or
methane. A 3 m x 0.25 mm i.d. column at 50°C and 1 atmosphere inlet
pressure of helium carrier gas has a gas velocity of 240 cm/s and a dead
time of 1.25 s. Dead times that are this short are nearly impossible to
measure with a reasonable degree of accuracy. An alternative method is
to measure the gas flow rate and calculate the gas velocity. This is not

20
entirely straightforward in GC/MS. One can obtain a measure of the column
flow rate by measuring the gas flow rate exiting the forepump of the
vacuum system, since all of the carrier gas exiting the column must exit
through this pump. However, the pulsations produced by the pump limit
the accuracy and precision of this measurement, especially at low flow
rates. A more accurate indication of the average gas velocity when short
columns are used can be obtained by measurement of the inlet pressure.
If the inlet pressure, P1-, is known, the average velocity under vacuum
outlet conditions can be calculated from the Poiseuille equation [14]
v = 3Pir2/32r7L (2.6)
where r is column radius, rj is the viscosity of the carrier gas at the
column temperature, and L is the column length. With the pressure gauge
that was used, inlet pressures ranging from -14.7 psig to 30 psig could
be measured with a rated accuracy of ± 3% [24].
Results and Discussion
Effects of the Pressure Drop
As was stated previously, lower average column pressures result in
increased gas-phase diffusion coefficients and increased optimum gas
velocities. Lower column pressures are obtained in GC/MS, since the
outlet of the column is at vacuum. Figure 2.2 shows Golay plots
(calculated from eq 2.2) demonstrating the effect on the plate height of
using vacuum outlet or atmospheric outlet with 0.25 mm i.d. open tubular
columns with lengths of 3 m and 30 m. As shown in the figure, the most
significant increase in the optimum velocity is obtained with the short

Plate Height
21
0.10
^0.08
E
u
^0.06
0.04
0.02
0.00
Average Velocity (cm/s)
Figure 2.2. Theoretical dependence of plate height on the average carrier
gas velocity for 3 m and 30 m 0.25 mm i.d open tubular columns under
vacuum outlet and atmospheric outlet conditions. (Conditions: thin-film
stationary phase; He carrier gas; k - 10; 100°C; D . - 0.27 cm2/s; =
2.28 x 10'* poise)

22
column operated with vacuum outlet. The speed of analysis is ultimately
dependent on the gas velocity and the column length. Under vacuum outlet
conditions, analysis times are reduced by a factor of 30, if a 3 m column
is used instead of a 30 m column, and both are operated at their optimum
gas velocities. The reduced analysis times are due to a reduction of the
column length by a factor of 10, as well as the increase in the optimum
gas velocity by a factor of 3 for the shorter column. In addition, since
chromatographic resolution is proportional to the square root of the
number of theoretical plates, the resolution lost by using a column that
is 10 times shorter is only a factor of the square root of 10 (i.e.,
3.16). As shown in Figure 2.2, the theoretical minimum plate heights
obtained with vacuum outlet operation are slightly greater than those
which are obtained with atmospheric outlet pressure. This can be
attributed to the gas decompression term, f, in eq 2.2, which results in
an increase of the minimum plate height by a factor of 9/8 (in the worst
case) when vacuum outlet is employed instead of atmospheric outlet
operation [14].
A better understanding of the effects of the outlet pressure on the
plate height for a short column can be obtained from an investigation of
the pressure drop. The pressure, p(x), at each point, x, along an open
tubular column can be calculated from the inlet pressure, P1-, the outlet
pressure, P and the column length, L [25]:
P(x) = [(P02-P,.2)x/L + P,.2]1/2 (2.7)
This equation was used to calculate the pressure at each point along a 3
m x 0.25 mm i.d. open tubular column with various outlet pressures. As

23
shown in Figure 2.3 (a), the pressures calculated for outlet pressures of
100 torr and 1 torr are virtually the same over most of the length of the
column. Figure 2.3 (b) shows the theoretical dependence of the plate
height on the average gas velocity for the same outlet pressures used in
Figure 2.3 (a). Note that the effects of the outlet pressure on the plate
heights calculated are nearly the same with outlet pressures of 100 torr
and 1 torr. It is apparent from these data that any GC detector that
could be operated at the reduced pressures would offer significant
advantages in terms of speed of analysis. In addition, if operation at
1 torr were not possible, operation with outlet pressures of 100 torr
should yield nearly the same chromatographic results (at least with the
length and diameter column used in this example). Another advantage of
the low outlet pressure operation is that chromatograms obtained on a
stand-alone GC (e.g., with flame ionization or electron capture detection)
could be reproduced on a GC/MS system. If this could be done, routine
chromatographic optimization and methods development could be performed
on the low outlet pressure GC for analyses to be performed later on the
GC/MS system. This would obviate the use of valuable instrument time on
a GC/MS system for chromatographic methods development. Investigations
in these areas are currently underway in our laboratory.
The effect of the outlet pressure on the optimum velocity is even more
pronounced for short wide-bore columns. Figure 2.4 shows the effect of
using vacuum outlet or atmospheric outlet on 3 m columns with inner
diameters of 0.53 mm and 0.25 mm. As shown in the figure, the use of
wide-bore columns when compared to narrow-bore columns of the same length
results in a more significant increase of the optimum velocity with vacuum
outlet operation. However, it is evident from Figure 2.4 that a lower

24
Po=760 torr
Po=400 torr
Po=100 torr
P0= 1 torr
Figure 2.3. (a) Calculated pressures along a 3 m x 0.25 mm i.d. open
tubular column with an inlet pressure of 860 torr of helium and various
outlet pressures. (b) Theoretical dependence of the plate height on the
average gas velocity for the same outlet pressures used in (a) . The
conditions were the same as those listed with Figure 2.2.

25
Figure 2.4. Theoretical dependence of the plate height on the average gas
velocity for 3 m x 0.25 mm i.d. and 3 m x 0.53 mm i.d. open tubular
columns with atmospheric or vacuum outlet operation. The conditions were
the same as those listed with Figure 2.2.

26
minimum plate height can be obtained with the narrow-bore column. Since
the maximum number of theoretical plates is equal to column length divided
by the minimum plate height, more theoretical plates can be obtained with
the narrow-bore column. If a constant number of theoretical plates are
required, analysis times will be shorter for the narrow-bore column. For
example, approximately 5600 plates can be obtained with the 3 m x 0.53 mm
i.d. column at an optimum velocity of 182 cm/s. The same number of
theoretical plates and optimum velocity can be obtained with a 1.4 m x
0.25 mm i.d. column. Based on the lengths of the two different diameter
columns used to generate 5600 plates, the analysis times should be faster
for the narrow-bore column by a factor of 2.1 (i.e., 3 divided by 1.4)
Sub-ambient Inlet Pressures
The optimum gas velocity for thin-film open tubular columns operated
under vacuum outlet conditions, uQpt ygc can be calculated from [14]
(P13(l+k)gD,|lH.„,),'z
(2L»Kllk2+6k+l))1/2
(2.8)
where Hmin is the minimum plate height obtained at u t. As can be seen
from eq 2.8, the optimum velocity is inversely proportional to the square
root of the column length; thus, a reduction of the column length results
in higher optimum velocities. In addition, the optimum inlet pressure
needed to obtain the optimum gas velocity is given by [14]
(72P,L^Dgf1)
(Hmin)1/2r
1/2
P.
i,opt,vac
(2.9)

27
Thus, short columns and/or columns with larger inner diameters require
lower inlet pressures. In addition, optimum inlet pressures are predicted
to be lower for low-viscosity carrier gases. This is demonstrated in
Figure 2.5 (a) which shows a plot of optimum velocity vs. column length
and Figure 2.5 (b) which shows a plot of optimum inlet pressure vs. column
length for 0.25 mm i.d. open tubular columns with both He and H2 carrier
gases. As shown in the figures, short columns with vacuum outlet require
sub-atmospheric inlet pressures, if optimum gas velocities and hence,
minimum plate heights, are to be obtained. Most GCs designed to be used
with open tubular capillary columns utilize pressure regulators that are
referenced to one atmosphere, and are incapable of operating with the
injection port at reduced pressures. Because of this limitation, the
advantages of short-column vacuum outlet GC have never been fully
exploited.
Sub-ambient inlet pressures are readily obtained with the flow-
regulated system used in this work. The chromatograms shown in Figure 2.6
demonstrate the increased resolution obtained on a 3 m x 0.25 mm i. d. open
tubular column when the injection port was operated at a reduced pressure.
The chromatogram in Figure 2.6 (a) was obtained with an inlet pressure of
940 torr and a gas velocity of 297 cm/s. For the chromatogram in Figure
2.6 (b), the inlet pressure was kept at 940 torr for the initial 12 s,
after which the pressure was reduced to 380 torr and a gas velocity of 120
cm/s. Note that the two components unresolved chromatographically in
Figure 2.6 (a) exhibit baseline resolution in Figure 2.6 (b). Moreover,
only 35 s of additional time was required for the chromatogram in Figure
2.6 (b).

28
,500 q
CO
E
^400^1
: l
; I
: l
O300Í»
CP
>
:' i
co
o 200
O
100:
CL
O
(a)
:* \
l
' \
\ \
Sub—atmospheric Inlet
0
| I I I !
5 10
1 • 1 I 1
15
"i" I I
20
i~i
25
Column Length (m)
Ho
Figure 2.5. Calculated optimum velocity (a) and optimum inlet pressure
(b) as a function of column length for 0.25 mm i.d. open tubular columns
with helium and hydrogen carrier gases illustrating need for sub-
atmospheric inlet pressures with short columns.

Figure 2.6. GC/MS chromatograms obtained with a 3 mx 0.25 mm i.d. open
tubular column at two different inlet pressures: (a) P( = 940 torr, v = 297
cm/s, (b) Pi = 380 torr, u = 120 cm/s. Individual traces are for the
molecular ions of acenaphthylene (m/z 152) and tetradecane (m/z 198), as
well as the reconstructed ion current (RIC) from m/z 35 to 300.

Relative Intensity Relative Intensity
30
700%.
Retention Time (minis)

31
It has been suggested that chromatographic analyses be performed at the
optimum practical gas velocity (OPGV) rather than the optimum velocity
[26,27]. The OPGV has been defined as the velocity that yields the
highest number of theoretical plates per unit time [27]. The number of
theoretical plates per unit time can be determined by dividing the number
of theoretical plates by the retention time (i.e., N/tR). Thus, if N/tR
is plotted as a function of the average gas velocity, the OPGV will be
represented by the maximum on the curve. Figure 2.7 shows examples of
these calculated curves demonstrating the determination of OPGVs for 3 m
and 30 m 0.25 mm i.d. columns operated with atmospheric and vacuum outlet.
For this comparison, the value of the partition ratio (k) was chosen to
be 30. Although this is far from the optimum partition ratio of 1.76
suggested by Guiochon [22], a partition ratio of 30 is a typical value
with the inherent low operating temperatures employed with short columns.
Since chromatographic resolution is proportional to the square root of the
number of theoretical plates, the percent resolution lost (^Riost) by
operation at any gas velocity other than the optimum is given by
%Rlost = 100d - (VNopt)V2) (2.10)
where NQpt is the number of theoretical plates obtained at the optimum
velocity and is the number of theoretical plates obtained at an average
velocity, u.
As shown by the results of this comparison summarized in Table 2.1,
vacuum outlet operation results in the shortest analysis times, regardless
of whether the columns are operated at their optimum velocities or optimum

32
Figure 2.7. Examples of calculated plots to determine optimum practical
gas velocities (OPGVs) for 3 m and 30 m 0.25 mm i.d. open tubular columns
operated with vacuum or atmospheric outlet. For each plot, the number of
theoretical plates generated per second has been normalized to a maximum
of 100%. The maxima of the curves indicate the OPGVs, while the stars
indicate the optimum velocities. The conditions were the same as those
of Figure 2.2, except k - 30.

Table 2.1. Comparison of calculated optimum conditions
operated with atmospheric and vacuum outlet.3
Column
Length
Outlet
%tb
0PGVc
t d
R,opt
k#0PGV
(m)
(cm/s)
(cm/s)
(min)
(min)
3
atm
42
121
3.7
1.3
3
vac
122
161
1.3
1.0
30
atm
29
46
53.4
33.7
30
vac
39
50
39.7
31.0
3 same conditions as those of Figure 2.2, except k - 30.
b calculated by iteration of eq 2.2
c obtained from maxima of curves of Figure 2.7
d analysis time obtained with uQpt
e analysis time obtained with OPGV
f %resolution lost by operation at OPGV, rather than uQpt
for 3 m and 30 m 0.25 mm i.d. open tubular columns
N b
opt
(s'1)
N c
l0PGV
nopgvAr
(s‘1)
IR.o«
12814
57.7
7352
94.3
24
11408
146.3
10004
166.7
6
124003
38.7
97244
48.1
9
114031
47.9
99944
53.7
6
OJ
OJ
calculated from eq 2.10

34
practical velocities. As stated previously, atmospheric outlet operation
results in a lower minimum plate height (typically 10% lower), and thus
more theoretical plates at the optimum velocity, than vacuum outlet
operation. For example, if operated at their optimum velocities, a 3 m
column with atmospheric outlet would yield 5.6% better resolution than a
3 m column with vacuum outlet. However, the analysis times with the 3 m
column would be 2.8 times shorter if vacuum outlet was used instead of
atmospheric outlet. Moreover, if operated at their optimum practical gas
velocities, columns with vacuum outlet would result in faster analyses and
more theoretical plates (and hence, better resolution) than columns with
atmospheric outlet. For example, analysis times are 0.3 min shorter on
the 3 m column with a 15% improvement in the resolution if vacuum outlet
at its OPGV is used instead of atmospheric outlet at its OPGV. It can be
seen from Table 2.1, that for any column length, vacuum outlet operation
yields the shortest analysis times and the highest number of theoretical
plates per unit time, regardless of whether optimum velocities or optimum
practical velocities are used.
There are other significant advantages of utilizing reduced inlet
pressures with short columns in GC/MS. It has previously been reported
that short columns result in narrower bands and higher sample
concentrations at the detector than long columns [3]; thus, at least from
a theoretical perspective, short columns should provide the best possible
detection limits. In practice, short columns operated at conventional
inlet pressures result in high flow rates of carrier gas into the ion
source. In fact, sub-ambient inlet pressures are a necessity if short
wide-bore columns are to be used at all in GC/MS, since conventional inlet
pressures result in analyzer pressures that are excessive for the normal

35
operation of the mass spectrometer (see Chapter 3). It is possible that
the advantage of low detection limits obtainable with short columns in
GC/MS might be lost if excessive flow rates (which can disrupt the
ionization process in the ion source) are used. Experience in this
laboratory has shown that the sensitivity obtainable with chemical
ionization (Cl) is not greatly affected by the high flow rates of a
different carrier gas (e.g., He or H2) (see Chapter 3). However, when
electron ionization (El) is used the sensitivity decreases dramatically
with increasing flow rates of carrier gas into the ion source, especially
when flow rates above ca. 10 mL/min are used. Thus, lower inlet pressures
are preferred for use with El and short columns. The increased
sensitivity of the vacuum inlet operational mode is demonstrated in Table
2.2, which shows the integrated peak areas of the molecular ion of n-
hexylbenzene obtained with different injection methods and inlet
pressures. As shown in the table, splitless followed by low-pressure
operation at 560 torr yielded the best sensitivity. The vacuum split
injection is inherently less sensitive, since less sample enters the
column and is detected. For the splitless injections, the peak areas were
significantly lower when the higher inlet pressures (and flow rates) were
employed. For example, the peak area obtained by splitless operation with
an inlet pressure of 560 torr was approximately two orders of magnitude
greater than that obtained at 2155 torr.
The effects of splitless operation on retention times was also studied.
Traditionally, both split and splitless operation are performed at
constant inlet pressure. Thus, retention times are the same regardless
of which mode is used, provided that the same inlet pressures are used.
In the splitless method described here, the initial inlet pressure during

36
Table 2.2. Comparison of peak areas for four different injection of n-
hexylbenzene obtained with different injection methods and inlet pressures
of He carrier gas.
Inj ection
Method
Inlet
Pressure
(torr)
Average
Velocity
(cm/s)a
Flow
Rate
(mL/min)b
Peak Area of
M+ (m/z 162)
splitc
560
176
2.6
2.56 x 106
splitlessd
560
176
2.6
2.09 x 107
splitless
940
297
5.0
7.95 x 106
splitless
2155
679
30.0
2.11 x 105
a average velocities calculated with eq 2.6
b indicated flow rates from flow controller
c split ratio =10:1
d inlet pressure = 940 torr for initial 12 s, final inlet pressure = 560
torr

37
injection is much higher than that during the remainder of the elution.
Since a change in the inlet pressure results in a change in the gas
velocity, it was expected that the retention times for split and splitless
operation would not be the same. In order to determine the effects of the
large pressure change, the retention times for split injections and
splitless injections for a mixture of alkylbenzenes were compared. The
results are shown in Table 2.3. The split injection was performed at a
constant inlet pressure of 560 torr. The splitless injections were
performed with an initial pressure of 940 torr and a final pressure of 560
torr. For splitless operation, the split valve was left closed for 12 s
in one case and for 30 s in another. As shown in Table 2.3, the large
change in inlet pressure does not greatly affect the retention times, even
when a splitless time of 30 s is used. This is most likely due to
trapping of the solutes at the head of the column at the initial column
temperature of 50°C.
Experimental vs. Theoretical Performance
Figure 2.8 shows a comparison of experimental and calculated (from eq
2.2) plate heights obtained with a 3.1 m x 0.25 mm i.d. open tubular
column operated with H2 carrier gas and the column outlet at vacuum. As
can be seen in the figure, the experimental performance at higher
velocities is not as good as predicted by theory. The differences in the
experimental and theoretically calculated data can be attributed to extra
column variances. Extra column contributions are the result of band
broadening due to instrumental time constants and, thus are not accounted
for in the Golay equation (eq 2.1). The extra column variances result in
the addition of a constant band width to the theoretical band width. This

38
Table 2.3. Comparison of retention times for components of an
alkylbenzene mixture utilizing vacuum inlet split and splitless injection.
Injection Method
Split3 Splitless, 12sb Splitless, 30 sb
Component Retention times (mints')
n-hexylbenzene
1:17
1:14
1:10
n- octylbenzene
2:16
2:14
2:14
n-decylbenzene
3:13
3:12
3:13
n-dodecylbenzene
4:08
4:08
4:09
3 inlet pressure = 560 torr
b inlet pressure - 940 torr for initial 12 s, final inlet pressure = 560
torr

39
Inlet Pressure (torr)
Average Velocity (cm/s)
Inlet Pressure (torr)
(b)
Figure 2.8. Comparison of theoretical and experimental data obtained for
a 3.1 m x 0.25 mm i.d. open tubular column at 100°C with hydrogen as a
carrier gas: (a) peak widths at 10% height and (b) plate heights. The
experimental data agree well with theory if an extra column variance (oec2
= 0.073s2) is included in the calculations.

40
is demonstrated in Figure 2.8 (a), which shows the expected (theoretical)
peak widths and the experimentally observed peak widths for n-pentadecane
on a 3.1 x 0.25 mm i.d. open tubular column. The extra column band
broadening results in an additional 0.4 s to the theoretical band width
at 10% peak height. Guiochon et al. have elaborately discussed the role
of extra column effects and have stressed their importance in high-speed
GC applications where short and/or small diameter columns are used [8].
They demonstrated that an additional term (Du2) can be added to the Golay
equation which can be used to describe the apparent plate height, Happ
with:
Happ “ B/u + Cv + Du2
(2.11)
D - uec2/(l+k)2L
(2.12)
where aec2 is the extra column variance. When eq 2.11 was used with an
extra column variance of 0.073 s2, the resulting curve was an excellent
description of the experimental data, as shown in Figure 2.8 (b) . As
shown in the figure and by eqs 2.11 and 2.12, the extra column
contributions become more significant as shorter column lengths or higher
velocities are used. It is believed that the instrumental variances
observed here arise from sample injection rather than detection. The n-
pentadecane used for the plate height determinations was manually injected
as a liquid into the hot injection port. There is expected to be a finite
time required for vaporization of the sample and entrance of the solute
onto the column, which could result in the observed peak broadening. The
residence times of samples eluting from the GC column into the ion source
is not expected to cause a significant amount of band broadening. In

41
addition, the data were collected utilizing a scan time of 0.05 s over the
m/z of interest, which resulted in at least 40 data points per GC peak.
Therefore, no significant time constant was introduced during the data
collection. Because of the extra column effects, the use of short columns
dictates careful consideration of injection technique and injector design
if the maximum performance is to be obtained.
Choice of the Carrier Gas
The importance of the choice of the carrier gas for minimizing analysis
times has previously been discussed by Giddings [6], Hydrogen, which has
the largest gas viscosity to diffusivity ratio, is predicted to yield the
fastest analyses. This can be seen from the experimentally determined
plate heights for H2 and He on a 3.1 mx 0.25 mm i.d. open tubular column,
as shown in Figure 2.9. Hydrogen is preferred, since the optimum velocity
is higher and the Golay curve is flatter than that of helium. The dashed
line parts of the curves of Figure 2.9 indicate where sub-ambient inlet
pressures were used. The use of H2 results in lower optimum inlet
pressures than He as predicted by eq 2.9 and shown in Figure 2.5 (b).
Even though H2 is predicted to yield the fastest chromatographic
analyses, the effects of its use as a carrier gas on resulting mass
spectra have not been previously studied. When H2 is used a carrier gas,
H3+ ions are produced in the ion source. The presence of these ions does
not appear to affect El spectra and in an El source do not cause a
significant amount of chemical ionization (i.e., no increase in the MH+/M+
ratio was observed). This is most likely due to the fact that pressures
in the El ion source are not high enough to cause a significant amount of
reaction to occur. However, the reaction of Hj+ ions with sample molecules

42
Figure 2.9. Comparison of experimentally determined dependence of the
plate height on average velocity for helium and hydrogen carrier gases
with a 3.1 m x 0.25 mm i.d. open tubular column at 100°C. The dashed
lines indicate where sub-atmospheric inlet pressures are required. The
curves were calculated with eq 2.11.

43
in a chemical ionization (Cl) source is possible due to the higher
operating pressures. It was also predicted that the H3+ ions would alter
the mass spectra normally obtained with methane Cl. On the contrary, the
presence of H2 carrier gas does not appear to affect methane Cl spectra.
As shown in Figure 2.10 (a) and (b), the Cl spectrum of 2,4
dimethylaniline obtained with 0.5 torr H2 and 0.2 torr CH^ (b) is the same
as that obtained with 0.2 torr CH^ and no H2 (a). In fact, as soon as
methane is introduced in the ion source with H3+ ions present, the Hj+ ions
disappear. This is apparently due to the fact that the hydrogen reagent
ions (Hj+) are used in the protonation of methane. This is not surprising,
since methane has a higher proton affinity than hydrogen [28], It is
expected that similar behavior would be observed with other commonly used
proton transfer Cl reagent gases (e.g., isobutane, ammonia, etc.) because
of the low proton affinity of hydrogen. In the absence of methane,
hydrogen can be used as both the carrier gas and the Cl reagent gas, as
shown in Figure 2.10 (c) . The spectra that are obtained with hydrogen Cl
exhibit more fragmentation than those obtained with methane Cl, which is
due to the greater exothermicity of the proton transfer reaction between
the H3+ reagent ions and the sample molecules. With hydrogen Cl, no adduct
ions are observed as is normally the case with methane Cl. The absence
of these adducts is often desirable, since their presence may complicate
interpretation of the mass spectra.
Conclusions
A better understanding of the influence of the pressure drop on the
chromatographic behavior of open tubular columns in GC/MS has allowed for
better utilization of the inherent advantages associated with short

Relative Abundance
44
Positive ion Cl spectra of 2,4-dimethylaniline (MW 121)
m/z
Figure 2.10. Effect of using hydrogen carrier gas on the chemical
ionization mass spectra of 2,4 dimethylaniline: (a) 0.2 torr methane and
no hydrogen present, (b) 0.2 torr methane with 0.5 torr hydrogen present,
(c) 0.5 torr hydrogen and no methane present.

45
columns. The large pressure drop associated with vacuum outlet GC should
not be considered a disadvantage. On the contrary, the low-pressure
outlet results in increased optimum gas velocities and decreased analysis
times. It has been shown that vacuum outlet operation yields the shortest
analysis times, regardless of whether optimum velocities or optimum
practical velocities are used. In addition, the chromatographic
resolution obtained when optimum practical gas velocities are used is
predicted to be higher for vacuum outlet than for atmospheric outlet
operation. As shorter column lengths are used with vacuum outlet
operation, lower inlet pressures are needed to obtain optimum performance.
Sub-ambient inlet pressures can be obtained with a relatively simple flow
control system as described here. The capability of splitless operation
has greatly extended the utility of vacuum inlet operation. The lower
inlet pressures with short columns are also shown to provide the best
sensitivity when electron ionization is used, since the flow rates are
lower. The use of hydrogen carrier gas results in even lower optimum
inlet pressures and higher optimum velocities than helium. Hydrogen can
be used as the chemical ionization reagent gas or it can be used in the
presence of other commonly used proton transfer chemical ionization
reagent gases without affecting the mass spectra.

CHAPTER 3
CONCEPTS FOR PORTABLE GAS CHROMATOGRAPHIC
INSTRUMENTATION IN GC/MS: FLOW RATE
PROGRAMMING AND DIRECT COLUMN HEATING
Introduction
The object of any chromatographic analysis is the separation and
identification of components in mixtures. However, chromatographic
mixture analysis is often complicated by the "general elution problem"
[29]. This predicament results both in poor resolution in the early part
of a chromatogram due to coeluting peaks, and in severe peak broadening
near the end of a chromatographic run. In gas chromatography, temperature
programming is often used to circumvent this problem. In temperature-
programmed GC (TPGC), the column temperature is increased (usually
linearly) during the course of the analysis. Since the temperature range
covered can be up to 300°C (and higher depending on the stationary phase),
TPGC offers a wide dynamic range. In analyses where temperature
programming is used, the rate at which samples can be analyzed is not only
dependent on the time required by the separation, but also on the time
required to equilibrate (cool) the column for the next injection. Since
the column temperature is changed by changing the temperature of a large
gas chromatograph oven, the rate at which a GC column can be heated or
cooled in TPGC is usually slow (typically 50°C/min or less depending on
the manufacturer of the GC oven). Moreover, the need to heat a GC oven by
conventional convection heating, only to cool it back down, requires
46

47
significant amounts of electrical power, making temperature programming
unattractive for portable instrumentation.
In this chapter, two approaches to solving these problems are
presented. The first approach involved investigation of flow rate
programming as a robust alternative to temperature programming. The
second approach examined was direct resistive heating of metal-clad
capillary GC columns, as opposed to oven (convection) heating. Direct
resistive heating is especially exciting, and should be widely applicable
in portable GC instrumentation for environmental or process monitoring.
The concepts developed in these studies have led to the development of a
portable GC probe, which appears to be particularly attractive for future
applications involving portable mass spectrometers or as a convenient
analytical tool for that could be used in any mass spectrometry
laboratory.
Flow Rate Programming
Flow programming of the carrier gas has previously been examined as
an alternative to temperature programming [25,29-35]. Use of this
technique was limited in the early years of application due to the high
inlet pressures required with packed columns. The high permeability of
short and/or wide-bore open tubular columns allows for high flow rates to
be achieved at relatively low inlet pressures [34], In addition, it has
more recently been shown that exponential flow rate programming under
isothermal conditions results in peak distributions that are very similar
to those obtained with linear temperature programming [29,33-35], One
advantage of programmed flow GC is that the sample throughput is high,
since it is not necessary to wait for the column to cool for the next

48
injection. Although resetting of the column flow rate is required, this
step is essentially instantaneous due to the high permeability of open
tubular columns [34]. This technique might prove attractive for a field
instrument that could not be rapidly heated and cooled, or in the
laboratory where high-throughput analyses are desired. Flow rate
programming is well suited for the analysis of thermally labile compounds,
since the column can be operated isothermally at relatively low
temperatures. It has also been pointed out that bleed of the liquid
stationary phase from the column is reduced since lower temperatures are
required [34]. This could prove to be an important factor in the
reduction of chemical noise at the detector, or could help to extend the
life of GC columns. One disadvantage of the method that is evident from
GC theory, is that chromatographic efficiency is reduced if the flow rate
is increased beyond the optimum. However, as was shown in Chapter 2,
optimum flow rates are higher when the column outlet is at vacuum as in
GC/MS. In addition, the rate of increase of the plate height is much less
for short columns than for long columns, when higher than optimum
velocities are used. Nevertheless, as will be shown in this chapter, the
maximum allowable flow rate is ultimately dependent on the pumping speed
and associated performance of the mass spectrometer.
Direct Resistive Heating of Al-clad Capillary Columns
Another alternative that avoids many of the limitations of oven
heating is to heat the column directly instead of by convection.
Recently, aluminum-clad fused silica open tubular (Al-clad FSOT) capillary
columns have become commercially available (SGE, Quadrex). The potential
advantages of directly-heated capillary GC columns have been suggested

49
[36], but never demonstrated. In Chapter 4 it will be shown that these
directly-heated columns can be used as simple transfer lines for
introducing samples via GC into the collision cell of a triple quadrupole
mass spectrometer. In this chapter, a method is described for heating
these columns directly by passing an electrical current through the thin
aluminum cladding coated on the outer surface of the column. A method of
sensing the temperature of the Al-clad capillary directly, by measuring
the column resistance instead of using external thermocouples or other
temperature sensors, is also presented.
The principles of resistive heating are relatively simple. When a
voltage is dropped across an Al-clad capillary, the column temperature
increases depending on the amount of power dissipated. The power
dissipated, P, is dependent on the current through the column, i, and the
voltage across the column, V.
P = iV
(3.1)
The current is dependent on the voltage drop and the electrical
resistance, R.
i = V/R (3.2)
The electrical resistance is dependent on the length, diameter, and
thickness of the Al coating on the outside of the capillary. The amount
of heat (or energy) required to increase the temperature of any substance
by an increment AT is given by [37]

50
Q = mCAT (3.3)
where Q is the amount of heat (or energy, J or W-s), m is the mass of the
material (g), C is the specific heat of the material (J/g°C), and AT is
the change in temperature of the material (°C). In addition, since the
heat is produced resistively, the amount of energy consumed over a given
time period At is given by
Q = PAt (3.4)
From eqs 3.3 and 3.4 it is apparent that objects of large mass require
more heat or more power over a given time period to reach a given
temperature. Al-clad FSOT GC columns are coated with a very thin aluminum
film, and have a very small thermal mass. The low thermal mass of these
columns allows them to be rapidly heated and cooled with much less
electrical power than is normally required with chromatograph ovens.
Since no large oven is needed, direct resistive heating is extremely
attractive for use in portable GC instrumentation.
The low thermal mass of the Al-clad columns does present some problems
if conventional methods are used to measure the column temperature.
Currently, there are no thermocouples or resistance thermometers available
that are small enough to accurately measure the temperature of these
columns. Even if small sensors are used, placing the sensor against the
column causes a local cold spot and leads to inaccurate temperature
measurement. An alternative method of measuring the column temperature
that avoids this problem is to use the column itself as the temperature
sensor. The resistance of any metallic conductor is linearly related to

51
its temperature over a wide temperature range and is given by [38]
Rt = R0(l + oT) (3.5)
where Ry is the resistance (0) at temperature T (°C), RQ is the resistance
at 0°C, and a is the temperature coefficient of resistivity of the metal
(°C*1). In fact, eq 3.5 is the basis of operation of Pt resistance
thermometers, which are widely used in scientific applications. The
resistance of a column is readily calculated during resistive heating if
the current through the column and the voltage across the column are
measured. Once a calibration of temperature versus resistance has been
performed, a determination of the column resistance can be used as a
direct measure of the column temperature.
GC Probe
The concepts discussed above have been applied in the development of
a compact GC probe, which resembles a conventional direct insertion probe,
and interfaces with a commercially available mass spectrometer. A GC
probe utilizing packed columns operated isothermally has previously been
described [39], The GC probe described here allows the temperature to be
rapidly cycled for high-throughput GC/MS analyses requiring very little
electrical power. Removal or insertion of the probe from the mass
spectrometer is through the normal probe inlet assembly. This facilitates
changing of the column or allows for rapid changing to other techniques
(e.g., solids probe, FAB, etc.) with minimal effort. Once the probe is
removed from the mass spectrometer, the column can be rinsed with solvent
without removing the column from the probe. This is attractive in cases

52
when particularly "dirty" samples are being analyzed, as may be the case
for environmental monitoring in the field.
Experimental Section
Experimental Conditions
Mass spectrometry. A Finnigan MAT TSQ70 triple quadrupole mass
spectrometer was used for this work. The instrument was tuned with FC43
(perfluorotributylamine) with the GC probe in place. The electron
multiplier was maintained at 800-1200 V with the preamplifier gain set at
Q
10 V/A. Electron energies of 70 eV and 100 eV were used for electron
ionization (El) and chemical ionization (Cl), respectively. Normal Q1
mass spectra were acquired for all MS experiments. Methane reagent gas
was used in all Cl experiments with ion source pressures of 0.2-0.5 torr.
The same removable ion volumes commonly used with the direct insertion
probe were used with the GC probe.
Flow rate programming. All experiments with flow rate programming
were performed on a Varian model 3400 GC equipped with a split/splitless
injector. Open tubular GC columns, approximately 3 m in length with an
internal diameter of 0.25 mm and a 0.25 /¿m film thickness (DB5, J&W
Scientific) were used. Injection temperatures were typically 220°C;
column temperatures are reported with each analysis.
GC probe. Wide-bore (0.53 mm i.d.) Al-clad FSOT columns with a non¬
polar 1.0 /¿m film BP-1 stationary phase (SGE) were used with the GC probe.
Narrow-bore columns (0.33 mm i.d., 0.5 /nn film, BP-5 stationary phase)
were evaluated in terms of heating requirements; however, for all
chromatograms obtained with the GC probe, wide-bore columns were used,
which allowed for a mechanically simple injection port.

53
Flow Control System
Flow rate programming. For many of the experiments performed (e.g.,
exponential flow rate programming and sub-ambient inlet pressure GC) it
was necessary to be able to control the column flow rate instead of the
column inlet pressure. Several methods for programming the column flow
rate have appeared in the literature [25,29-33]. For example, a very
simple method of flow programming was achieved by manually manipulating
a needle valve in the splitter line [33]. However, due to the imprecision
and inconvenience of this method more reliable methods were developed.
Others have achieved flow control via feedback from a pressure transducer
with a motor-controlled regulator valve [29,34,35]. This type of system
first requires a calibration step to determine the dependence of flow rate
on inlet pressure. After the flow function is determined and programmed
into the computer, the pressure transducer detects any difference between
the inlet pressure and the setpoint flow. If a difference is detected,
a DC motor adjusts the regulator until the correct flow is achieved. The
main disadvantage of this method is the slow response time, since several
seconds are required for the motor to reposition the valve. The response
time of the flow controller becomes even more important for short columns
since the analysis times are typically only a few minutes long. For this
reason it was decided that commercially available mass flow controllers
would be most suitable for these applications. These types of transducers
incorporate thermal mass flow sensors to determine mass flow rates of
gases. Sensors placed at both ends of a laminar flow tube detect
differences in heat transferred along the tube, and directly relate this
difference to the flow rate of the input gas. The measured flow rate is
compared to the setpoint value and is adjusted by an electromagnetically

54
controlled automatic valve. The transducer and controller valve are
contained in a single package measuring approximately 1/2 in. x 5 in. x
5 in. An external power supply/readout unit provides the appropriate
power, real-time readout of the measured flow, and capability for computer
interfacing. The advantages of this system are fast response (500 ms for
the transducer), excellent precision (0.2% of full scale), and high
accuracy (0.5% of full scale). The flow controller is interfaced to the
mass spectrometer electronics via one of two digital-to-analog converters
(DACs, known as user outputs) that are variable over the range ± 5 V DC
via the TSQ70 trackball or Instrument Control Language (ICL) procedure.
A control voltage range of 0-5 V is used to adjust the flow controller
flow rate over its full range.
One concern about the use of a mass flow controller was its ability
to regulate flow when the output was connected to the vacuum of the mass
spectrometer, as it might be in short-column or vacuum inlet GC. In order
to test the performance of the transducer in this configuration, the flow
controller was utilized to control the input of collision gas into the
collision cell of the TSQ70. In fact, this configuration worked so well
that a flow controller dedicated for control of collision gas pressure has
since been installed.
For exponential flow rate programming experiments, the flow control
system was essentially the same as that described in Chapter 2. The only
difference was that, instead of a MKS 1159A mass flow controller, a MKS
1259B mass flow controller was used, which has a flow control range of 0-
20 mL/min for nitrogen and 0-29.6 mL/min for helium. It was later found
that the flow rate range of this flow controller limited the experiments
that could be performed. For example, during flow programming

55
experiments, the split and sweep valves had to be left closed during the
entire GC run to obtain the widest range of useable flow rates, which
limited analyses to splitless operation. A flow controller with a wider
flow rate range (0-100 mL/min for nitrogen) was eventually installed to
alleviate this problem. The flow function for an exponential flow program
is controlled automatically with the DAC user output of the TSQ70 with an
Instrument Control Language (ICL) program. The flow rate at time t, Ft,
is given by
kt
Ft = F0 + e
(3.6)
where F0 is the initial flow rate and k is a constant. In a flow rate
programming experiment, the initial and final flow rates, as well as the
time for the delay and end of the programming period are selected and
entered into the computer program. The constant, k, is calculated in the
computer program such that the flow rate at any time during the analysis
is determined by eq 3.6.
GC probe. Since short wide-bore columns (typically 3 m in length)
were used in the GC probe, the injection port could not be operated at
conventional injection port pressures (i.e., atmospheric pressure and
above) without a carrier gas flow rate exceeding the normal operating
pressure of the mass spectrometer. Instead, the injection port was
maintained at lower pressures (typically 100-200 torr) with the flow
control system described in Chapter 2. This system utilized a MKS model
1159A flow controller with a range of 0-100 mL/min for nitrogen and 0-145
mL/min for helium. The use of sub-atmospheric inlet pressures did require
some consideration of injection technique, since it was possible for the
sample to be prematurely aspirated from the syringe by the vacuum in the

56
injection port. It was found that the best way to avoid this problem was
to use a gas-tight syringe and to draw the sample as well as a small
volume (e.g., 1 /iL) of air into the syringe barrel prior to injection.
The column flow rate of He carrier gas used with the sub-ambient
inlet pressures was typically 3.5 mL/min. Flow rates were determined by
measuring the flow output of the flow controller with a soap bubble meter
prior to connecting the carrier gas line to the injection port. Davies
has shown that, under the conditions of vacuum outlet, flow rates can be
calculated by [40,41]
Q = 64^L3/9tQ2 (3.7)
where Q is the flow rate (mL/s), t is the retention time of the unretained
air peak (s) , r¡ is the gas viscosity (poise) , and L is the column length
(m). However, when short columns are used, the air peak times are so
short that they are not easily measured. For example, a 3 m column
operated at a typical gas velocity of 300 cm/s has an air peak time of 1
s. As discussed in Chapter 2, the average gas velocity can be calculated
if the inlet pressure is known. However, due to the very simple design
of the injection port of the GC probe, a conventional pressure gauge could
not be used to measure the inlet pressure. Therefore, it was more
convenient to measure flow rates instead. Once the flow rates were
determined, the inlet pressures and gas velocities were readily
calculated. Given that the average gas velocity is
u - L/t0
(3.8)

57
Average velocities can be calculated if eq 3.8 is substituted into eq 3.7
yielding
v = (375Q/167r»jL)1/2 (3.9)
where Q is in mL/min, L is in m, r¡ is in poise, and u is in cm/s. The
inlet pressure (Pj, dynes/cm2) can be calculated from the Poisuelle
equation for vacuum outlet conditions [14]
Pj - 32ur7L/3r2 (3.10)
where r is the column radius in cm, and L is in cm. A 3 m wide-bore
column, operated at a typical flow rate of 3.5 mL/min of He at 25°C, has
a calculated gas velocity of 204 cm/s and an inlet pressure of 146 torr.
Direct Resistive Heating
Electronics. A programmable DC power supply was constructed in-house
and was used to control the voltage across the Al-clad GC columns. The
electronic components for the power supply are contained in a box which
is approximately 10 in. x 6 in. x 3.5 in. A schematic of the overall
system electronics is shown in Figure 3.1, with details of the DC power
supply shown in Figure 3.2. The output of the power supply (up to 30 V)
is controlled by an input control voltage of 0-5 V DC. A DAC from the
TSQ70 was used to generate a control voltage for the power supply. The
control voltage (from the potentiometer or DAC) is fed into a voltage
follower with variable gain. The output of the op-amp provides a bias to
the base of power transistor that determines the output of the supply.

USER
Figure 3.1. Schematic diagram of electronics used for resistive heating and temperature sensing of Al-clad
capillary columns.

6A, SB
I < a«ns)
Figure 3.2
Circuit diagram of programmable 30 V DC power supply designed for column heating.

60
The output of the power transistor is rated at 30 V and 30 A. However,
in the present configuration the output of the supply is limited by the
transformer, which is rated at approximately 6 A. Typically, the gain of
the voltage follower was set such that the power supply yielded the
maximum desired temperature at the maximum control voltage. This
versatility is important, since a change in the length or diameter of the
column changed the amount of power required for the column to reach a
particular temperature. No additional circuitry had to be designed to
measure the column voltages and currents. The voltage drop across the
column, as well as the voltage drop across a 0.261 ÍÍ current sensing
resistor were measured with an external devices sensor circuit already
present in the TSQ70 electronics. This circuit consists of five
differential amplifiers, an analog multiplexer, and an analog-to-digital
converter (ADC). The voltages to be determined were fed into two separate
differential amplifiers of the sensor circuit. This allows the voltages
to be displayed and/or plotted in real time with the computer-controlled
TSQ70 and the ICL. Programs were written in ICL that allowed the voltage
across the column to be increased linearly with time during the
acquisition of data. The linear voltage ramp was used to simulate the
linear temperature ramp commonly used in temperature programming.
However, it should be pointed out that any desired voltage ramp could be
created with this system (e.g., exponential, logarithmic, etc.), by a
simple modification of the ICL program.
Temperature sensing. The resistance of the column, obtained by
measurement of the voltage and current, was used as an indication of the
column temperature. Al-clad GC columns were "calibrated" by placing them
in a GC oven (Varian model 3400) and recording the resistances of the

61
columns as a function of GC oven temperature. A wide-bore Al-clad FSOT
column (0.53 mm i.d. x 3.14 m) , and two different lengths (4.77 m and 3.05
m) cut from the same narrow-bore (0.33 mm i.d.) Al-clad FSOT column were
calibrated using this method. The nominal resistances of these columns
were very low (ca. 0.9 ft/m for the narrow-bore column and 0.5 ft/m for the
wide-bore columns), and thus were too small to be accurately and precisely
measured with a typical digital multimeter (DMM). In order to obtain
accurate resistance measurements for the calibration, the DC power supply
was used to apply a small voltage drop (e.g., 50 mV) across the column.
The current through the column and the voltage drop across the column were
measured with two DMMs (Fluke model 75). The resistances were then
calculated from the voltages and currents measured. The voltages applied
during calibration were not high enough to cause a significant amount of
column heating.
GC Probe Design
The GC probe is schematically illustrated in Figure 3.3. The 1.27 cm
x 28 cm stainless steel probe shaft was adapted from a Finnigan 4500
series ion volume insertion/removal tool. The probe shaft is threaded and
screws into one of the two 4 in. square end-blocks (machined from phenolic
material). The injection port is positioned in a milled slot on the other
end-block and is held firmly in place by a set screw. The column is
wrapped around a 2.5 in. diameter Teflon spool. The spool is held between
the two end-blocks and contains a slot that allows the column to be
inserted through the spool and down the length of the probe shaft. The
column is covered with Nextel glass braid insulation (Omega Engineering)
and is coiled around the Teflon spool. The insulation serves to

PHENOLIC
END-BLOCK
C u
CONNECTOR
PROBE
SHAFT
V I TON
O-RING
A I -CLAD
COLUMN
1/16"
UNION
hH
1 INCH
BANANA
JACK
Figure 3.3.
Illustration of direct insertion GC probe.
CARRIER
GAS
INLET INJECTION
COLUMN
AL
ON
fO

63
electrically isolate each strand of the coiled column. A 1/16 in.
stainless steel union (Swagelok) with a graphitised vespel ferrule and a
Viton o-ring are used to form a vacuum-tight seal at the end of the probe
shaft. Approximately 2 cm of the column extends from the end of Swagelok
union. A copper connector slides over this last few cm of the column and
electrically connects the end of the column to the probe shaft.
Electrical leads from the DC power supply plug into the banana jacks
mounted on the probe assembly. It is very important to minimize the
contact resistance of the electrical connections to the column, since poor
connections result in power consumption and local heating at the point of
contact. In addition, large connectors have a large thermal mass and
dissipate heat near the contact. For these reasons, the electrical
connection to the head of the column was made with a small removable clip
lead. The probe shaft, which is used as a ground return, remains cool
during column heating, since the resistance of the probe is much less than
that of the columns used. The nominal resistance of the column in the
probe was always checked prior to applying power to insure good electrical
connections. The GC probe shaft is inserted through the probe inlet
assembly and the end of the probe fits firmly against the ion volume
contained in the ion source block.
A compact, low thermal mass injection port was fabricated from a 1/8
in. x 1/16 in. stainless steel tube reducing union (Swagelok). For the
introduction of carrier gas into the injection port, a hole was drilled
into the side of this fitting and a 1/16 in. o.d. stainless steel tube was
silver-soldered in place. The wide-bore column is inserted into the
smaller end of the union and is sealed with a nut and a graphitised vespel
ferrule. The top of the injection port contains an 1/8 in. nut and a

64
septum. A standard GC syringe (SGE), equipped with a 26-gauge needle, was
used to inject samples directly onto the wide-bore columns. The inside
of the injection port contains a stainless steel guide, which directs the
syringe needle to the column entrance. Upon injection, the syringe needle
enters the column and is inserted far enough that the sample is injected
past the point where the electrical connection is made. This minimizes
sample loss due to unvolatilized components. The injection port remains
cold when the column is heated, and thus does not have to be cooled
between injections.
Results and Discussion
Flow Rate Programming
Effect of carrier gas flow rates on mass spectrometer performance.
Exponential flow rate programming has been used in many instances with
conventional GC detectors [25,29-35]; however, the effects of the
resultant high (and varying) flow rates on the operation of the mass
spectrometer have not been studied. One problem that became apparent from
some initial experiments employing electron ionization (El) was the
reduction in the analyte signal (sensitivity) associated with high flow
rates of carrier gas into the mass spectrometer ion source. This was
briefly addressed in Chapter 2. Initially, it was unclear whether this
problem was linked to mass analysis, ion formation, or a combination of
both. It is possible that the increased pressure in the mass analyzer
(which causes a decrease in the mean free path of the ions) results in
scattering losses and a reduction of the number of ions reaching the
detector. Analyte signal may also be decreased due to inefficient ion
formation with El. This associated decrease in sensitivity with El has

65
also been observed in supercritical fluid chromatography/mass spectrometry
(SFC/MS) , where the pressure in the ion source is elevated due to the
presence of the constantly eluting mobile phase [42].
It was discovered that the problem of reduced analyte sensitivity with
high carrier gas flow rates could be overcome by employing chemical
ionization (Cl) instead of El. Due to the presence of a reagent gas
(e.g., methane) at much higher operating pressures (typically 0.1-1 torr
for Cl vs. 10'4 torr for El), ion formation with Cl appears unaffected by
the increase in carrier gas flow rates. Figures 3.4 and 3.5 show a
comparison of the performance of El and methane Cl respectively under
identical changes in column flow rates. For these experiments, the
chromatograms were obtained in the splitless mode with a 3.2 m, 0.25 mm
i.d., DB-5 capillary column operated isothermally at 80°C. The sample
injected was 30 ng each of C14 ’ C15’ and C16 n-alkanes. Figure 3.4 shows
the chromatograms of the major fragment ion (m/z 85) obtained with El.
For Figure 3.4 (a) the column flow rate was 8 mL/min and that for Figure
3.4 (b) was 20 mL/min. Both traces are plotted on the same time and
intensity scale for ease of comparison. The integrated peak areas for the
components at the two flow rates are given in Table 3.1. As shown in the
table, the m/z 85 peak areas for the three hydrocarbons in a particular
chromatogram are reproducible within the expected limits of error.
However, the areas of the peaks at the higher flow rate are approximately
a factor of four less than the peak areas obtained at the lower flow rate.
Although the resulting sensitivity for only one particular ion is
considered in this comparison, all of the sample ions were observed to
decrease by approximately the same amount. The major sample ions obtained
with methane Cl of n-alkanes are formed by hydride abstraction to yield

Electron Ionization
(b)
w
c
0>
+-•
c
a>
o
c/>
A
<
Os
os
Retention Time (min:s)
Figure 3.4. Mass chromatograms of m/z 85 from 70 eV El of n-alkane mixture with (a) 8 mL/min and (b) 20 mL/min
He carrier gas flow rates. The sensitivity with El is drastically reduced when the higher flow rates are used.

Chemical Ionization
miz 197 f = 8 mLlmin
Retention Time (min:s)
Figure 3.5. Mass chromatograms of major ions
and (b) 20 mL/min He carrier gas flow rates.
obtained with methane Cl of n-alkane mixture with (a) 8 mL/min
The sensitivity with Cl is apparently unaffected by the increase
in flow rate.

68
Table 3.1. Integrated peak areas for comparison of El and Cl at different
He carrier gas flow rates.
Flow Rate
Ionization
Peak
Hydrocarbon
m/z
(mL/min)
Method
Area
C14
85
8
El
220694
C15
85
8
El
213729
C16
85
8
El
227111
CK
85
20
El
53244
C15
85
20
El
53148
C16
85
20
El
52292
CK
197
8
Cl
1893613
C15
211
8
Cl
1827399
C16
225
8
Cl
1617471
C14
197
20
Cl
1758975
C15
211
20
Cl
1804526
C16
225
20
Cl
1679119

69
(M-H)+ ions. Figure 3.5 shows the mass chromatograms of these ions for
the two flow rates. The resultant integrated peak areas for these ions
are shown in Table 3.1. As shown in the table, the variation in the peak
areas when the flow was increased from 8 to 20 mL/min is insignificant and
within the limits of experimental error. These results indicate that the
chemical ionization process is less affected by the increased He carrier
gas flow rates. This may be due to the fact that sample molecules in the
Cl ion source exist in a large excess of reagent molecules, thus the
variation of He flow rates does not significantly affect sample
ionization. The increased sensitivity (larger signal intensities and peak
areas) obtained with Cl for this alkane mixture can be attributed to the
differences in the amount of fragmentation obtained with El and Cl. The
El spectra exhibited a large amount of fragmentation, and thus resulted
in an abundance of fragment ions. In contrast, abundant pseudo-molecular
(M-H+) ions were obtained with Cl with less fragmentation than El. Thus,
the integrated peak areas for one selected ion were larger for Cl than for
El.
Once it was determined that El, but not Cl, sensitivity was degraded
by high flow rates of He carrier gas, it was postulated that increasing
the electron energy might help to improve the sensitivity of El with the
high flow rates of carrier gas. The electron energy, which is the
potential difference between the filament and the source block, represents
the kinetic energy of the electrons impinging upon the sample molecules.
The electron energy is usually set higher in Cl than in El (100 eV instead
of 70 eV) to insure that the electrons have sufficient energy to penetrate
the source region containing ca. 1 torr of reagent gas. However,
experiments have shown that an increase or decrease in electron energy

70
and/or optimization of any of the other ion optical parameters does not
improve the performance of El when relatively high flow rates are used.
These data indicate that Cl is the ionization method that should be used
if high flow rates (>10 mL/min) of carrier gas are to be used. This is
not a severe limitation since Cl is often more sensitive than El and is,
thus, usually preferred for trace analysis. In addition, since Cl is a
"softer" ionization method (less fragmentation than El), molecular weight
information is often more readily obtained. If more fragmentation were
desired, it might be possible to employ charge exchange ionization instead
of El or Cl, as shown in Chapter 4, and as has been done in SFC/MS [42].
Comparison of temperature programming and flow rate programming. In
order to illustrate the similarities and differences of linear temperature
programming and exponential flow rate programming, a series of
chromatograms was obtained under each set of conditions. In these
experiments, a 3 m x 0.25 mm i.d. DB-5 capillary column was used. The
sample was a GC test mixture obtained from J&W Scientific that is often
used for evaluating the chromatographic performance of non-polar bonded-
phase columns. The sample contained 23 ng/juL of each of seven components,
which are listed along with their boiling points in the caption of Figure
3.6.
Figures 3.6 and 3.7 show the separation of this mixture with
exponential flow rate programming and temperature programming,
respectively. Figure 3.6 (a) shows the chromatogram obtained with
isorheic (constant flow rate) operation at 5 mL/min of He carrier gas and
an isothermal column temperature of 65°C. Figures 3.6 (b) and (c) show
the mixture separated isothermally at 65°C using exponential flow rate
programming. The rate of the exponential was adjusted such that the final

Figure 3.6. Reconstructed ion chromatograms (methane Cl) of GC test
mixture obtained on a 3 m column with isothermal operation at 65°C and
various flow conditions of He carrier gas: (a) isorheic at 5 mL/min; (b)
exponential flow program from 5 mL/min to 30 mL/min in 4 minutes; (c)
exponential flow program from 5 mL/min to 30 mL/min in 3 minutes. The
components of the mixture and their boiling points are: 1, 2-chlorophenol
(175°C); 2, n-undecane (196°C); 3, 2,4-dimethylaniline (214°C); 4, 1-
undecanol (243°C); 5, acenaphthylene (270°C); 6, tetradecane (232°C); 7,
pentadecane (271°C).

72
(a)
(b)
(c)

Figure 3.7. Reconstructed ion chromatograms of GC test mixture performed
with isorheic operation at 5 mL/min He carrier gas and different
temperature program rates: (a) 2°C/min; (b) 5°C/min; (c) 15°C/min. The
components of the mixture are identified in Figure 3.6.

74
(a)
(b)
(c)
Retention Time (min:s)
0:21

75
flow rate of 30 mL/min was reached after 4 minutes and 3 minutes for
Figures 3.6 (b) and (c), respectively. Figure 3.7 illustrates isorheic
separations of the mixture with temperature programming at rates of
2°C/min, 5°C/min, and 15°C/min. It can be seen from Figures 3.6 and 3.7
that exponential flow rate programming yields similar results to linear
temperature programming.
It is evident from this comparison that a change in flow rate does not
influence retention as much as a change in temperature. For example, a
temperature program rate of 2°C/min and a resulting temperature increase
of 8°C over a period of 4 minutes (Figure 3.7 (a)) yielded nearly the same
results as an exponential flow program where the flow rate was increased
sixfold from 5 mL/min to 30 mL/min in 4 minutes (Figure 3.6 (b)). The
differences of temperature and flow rate programming are readily
understood if the thermodynamics of the chromatographic process are
considered. The retention time, tR, of a solute is given by [43]
tR = L(l+k)/u (3.11)
where v is the average carrier gas velocity, L is the column length, and
k is the partition ratio; thus, the retention time is inversely
proportional to the average velocity. It is also evident from eq 3.9 that
under vacuum outlet conditions the average velocity is proportional to the
square root of the column flow rate; therefore, an increase in flow rate
by a factor of 6 (e.g., from 5 to 30 mL/min) would decrease the retention
time by only the square root of 6 (or 2.4). In contrast, a change in
column temperature affects the equilibrium of the solute between the
mobile phase and stationary phase, and thus results in a change in the

76
partition ratio, k. The effect of the column temperature on the partition
ratio is given by [43]
In k - -AG/RT - In B (3.12)
where AG is the change in free energy for the evaporation of the solute
from the liquid stationary phase, T is the absolute temperature, R is the
gas constant, and B is the volume ratio of the mobile phase to the liquid
stationary phase. Consider n-pentadecane, which has a retention time of
311 s on the 3 m x 0.25 mm i.d. column at 65°C. The associated values of
k, AG, and B for these conditions are 259, -7.9 kcal/mol, and 500,
respectively. In order to decrease the retention time of this compound
by a factor of 2.4 (the same factor afforded by a factor of 6 increase in
flow rate), the partition ratio factor (1+k) of eq 3.11 must also be
reduced by this amount (i.e., k must equal 108). From eq 3.12, the
temperature must be increased by only 27°C (i.e., to 92°C) to decrease the
partition ratio and the retention time by a factor of 2.4.
The calculations discussed above as well as the experimental data
demonstrate that a change in temperature has a greater effect on retention
than does a change in flow rate; therefore, the range of boiling points
spanned in a flow program will not be as large as that obtainable with
temperature programming. Nevertheless, the chromatograms obtained with
flow programming are certainly an improvement over isothermal operation
(Figure 3.6). The range of boiling points covered is still relatively
wide, as the components in the test mixture have boiling points ranging
from 175°C (2-chlorophenol) to 271°C (pentadecane). Perhaps the real
advantage of flow rate programming is the low elution temperatures. The

77
components of Figure 3.6 eluted as much as 200°C below their boiling
points. This benefit might be of particular value in the separation of
thermally labile components or if limited power were available for the
heating of a portable GC.
Direct Resistive Heating
Calibration of columns for temperature sensing. Data obtained for the
calibration of the column resistance vs. temperature are shown in Figure
3.8. The column resistances were normalized by dividing by the length of
the column used in the calibration. Length normalization was necessary,
since the same length of column was not always used in the GC probe. Once
the calibrations were performed, the temperature of a resistively-heated
column could be determined from the relationship
T - aR/L + b (3.13)
where T is temperature (°C), R is the resistance (Q) , L is the column
length (m) , and a and b are the slope and y-intercept of the linear
calibration curve, respectively. The slopes, y-intercepts, and standard
deviations from the linear least squares fits of the calibration data are
shown in Table 3.2. As shown in Figure 3.8 and Table 3.2, essentially the
same calibrations were obtained for the two different lengths of the
narrow-bore column. This is an indication of the uniform thickness of the
Al coating. Since these columns were cut from the same original column,
no generalizations can be made about the column-to-column uniformity. As
shown in Figure 3.8, the resistance of the wide-bore column is less than
that of the narrow-bore column. This is due to the fact that there is a

Temperature (C)
78
Figure 3.8. Calibrations of column temperature vs. normalized resistance
(fl/m) for two different diameter Al-clad capillary columns. The
temperature of a resistively-heated column could be determined from the
calibration line with knowledge of the column resistance and the length.

79
Table 3.2. Results of linear least squares fits of temperature
calibrations shown in Figure 3.8.
column
slope ± s.d.a
Y-intercept ± s.d.b
r
c
0.33 mm i.d. x 3
0.33 mm i.d. x 4
0.53 mm i.d. x 3
05 m 276.7 ± 0
77 m 280.4 ± 0
14 m 522.2 ± 1
7 -217.9 ± 0
9 -220.8 ± 0
7 -221.7 ± 0
4
0.99991
5
0.99987
5
0.99986
aslope ± standard deviation, in units of “C/iim'1
by-intercept ± standard deviation, in units of °C
Correlation coefficient for the linear least squares fit

80
greater mass of Al per unit length on the wide-bore column, which can be
attributed to the greater surface area and/or thicker Al cladding on the
wide-bore column. Note that the y-intercepts obtained were practically
the same regardless of the column diameter. This can be better understood
by a rearrangement of eq 3.5
T - tyoHo * 1/a (3.14)
which demonstrates that the y-intercept (1/a) is dependent only on the
temperature coefficient of resistivity (a), a fundamental property of the
metal. Due to the limited availability of these columns, it has not yet
been possible to compare calibrations from different columns of the same
diameter. If the column-to-column Al coating thicknesses were consistent,
then calibrations would not have to be repeated for each new column.
Rather, the calibrations would only need to be done for each new diameter
column that was used.
One advantage of this method of direct column temperature sensing,
since thermocouples or other temperature sensors are not needed, is the
simplicity of the electronic circuitry required. This could be an
important advantage in the design of portable instrumentation for
environmental or process monitoring. Another advantage that is
particularly important in this application is that measurement of the
column resistance is effectively a length-average temperature measurement.
The temperature measured at each point along the column may vary,
depending on the insulation and thickness of the Al coating at a
particular spot; thus, measurement of the column temperature at one of
these spots may not be an accurate representation of the average column

81
temperature. In addition, we have observed that temperature sensing by
conventional means (e.g., with a thermocouple placed in contact with the
column) causes the temperature of the column to drop near the point of
measurement and leads to inaccurate determinations. This direct
measurement of column temperature might be attractive for other
applications as well. For example, it is normally assumed that the
temperature of a column inside an oven directly follows the oven
temperature; this method could be used to directly measure the actual
temperature of an Al-clad GC column inside a chromatograph oven.
Temperature response characteristics. As mentioned previously, direct
resistive heating of Al-clad columns allows for extremely rapid cycling
of the column temperature due to their extremely low thermal mass. Figure
3.9 shows the temperature response of a 2.3 m x 0.53 mm i.d. Al-clad FSOT
column installed in the GC probe, after the voltage output of the
transistor of the power supply had been stepped from 0.5 V to 8.2 V and
then returned to 0.5 V after a period of 25 s. Note that the voltage
across the column (Figure 3.9 (a)) did not remain constant during the
heating period, but increased from 6.8 V to 7.5 V. Similarly, the column
voltage did not instantaneously return to its initial value of 0.45 V at
the end of the heating cycle. The slow response of the column voltage can
be attributed to an increase in the column resistance as the temperature
is increased. The output voltage of the transistor and the resistance of
the 0.261 0 resistor remain constant during the heating period; thus, as
the column resistance increases, a greater fraction of the transistor
output voltage is dropped across the column. This causes the column
voltage to slowly increase during heating and slowly decrease during
cooling.

Figure 3.9. Response characteristics for a resistively-heated 2.3 m x 0.53 mm i.d. Al-clad FSOT column during
a 25 s heating period: (a) applied voltage across the column, and (b) indicated temperature obtained from
column resistance measurement.

83
During the 25 s heating cycle, the column temperature increased from
43°C to 150°C. During the first second of the 25 s heating period, the
column temperature increased from 43°C to 83°C, which represents an
initial heating rate of 40°C/s or 2400°C/min. The heating and cooling
response of the column exhibits an exponential behavior, much like that
caused by impedance in an electrical circuit. However, the response times
for heating and cooling are not the same. In this example, the heating
response time of the column is defined as the time required to raise the
column temperature 90% of the way from 43°C to 150°C (i.e., to 139°C).
As shown in Figure 3.9, this response time was found to be approximately
11 s. The temperature increase from 43°C to 139°C in 11 s represents an
average rate of increase of 8.7°C/s or 524°C/min. For comparison, the
ballistic heating rate of a typical gas chromatograph oven from 50°C to
150°C is only about 50°C/min [44]. In addition, the maximum heating rate
for a typical on-column injector is only 150°C/min [44]. Obviously, the
method of direct resistive heating offers considerable improvement in
terms of rapidity of response when compared to oven heating.
In applications where extremely rapid temperature ramps are used (as
in the example shown above), the speed of repetitive analyses will most
likely be limited by the time required to re-equilibrate (cool) the column
temperature between injections. During the first second of cooling, the
column temperature dropped from 152°C to 112°C, which is an initial
cooling rate of 40°C/s or 2400°C/min. Note that the initial heating and
cooling rates are the same. The response time for cooling the column,
defined as the time required to cool the column 90% of the way back down
to 43°C from 150°C (i.e., to 54°C), was determined to be approximately 35
s. This represents an average rate of cooling of 2.7°C/s or 165°C/min.

84
For comparison, the temperature cool-down time for a typical gas
chromatograph oven over the same temperature range would be approximately
4 minutes (about 7 times longer) [44].
As shown in Figure 3.9, there is some thermal lag associated with the
heating and cooling these of columns. A small amount of thermal lag can
be attributed to the voltage regulation of the power supply. The power
supply regulates the voltage output of the transistor; thus, the voltage
across the column changes as the temperature (and hence the resistance)
of the column changes. Much of this thermal lag can be attributed to the
slow thermal response of the insulation that was used to electrically
isolate the column; thus, it is expected that these columns would exhibit
even faster thermal response if the glass braid insulation were
eliminated. This would be possible if the column could be wrapped around
a grooved spool, which would separate each strand of the column and
prevent electrical shorting. Nevertheless, even without any further
improvements, the temperature response characteristics are impressive when
compared to conventional GC ovens.
Another advantage of direct column heating is that very little
electrical power (or energy) is needed. For example, approximately 35 W
of electrical power applied for 25 s (0.875 kJ of energy) was required to
resistively heat the 2.3m column to 150°C. For comparison, a typical GC
oven utilizes a 2400 W heater (240 V AC, 10 A) [44], Assuming an average
ballistic heating rate of 50°C/min, 3 minutes would be required to heat
the oven from 50°C to 150°C. Approximately 432 kJ of energy would be
consumed, which is nearly 500 times more energy than that required by
direct resistive heating. In addition, a typical injector is heated
separately from the oven and requires an additional 100-300 W of power

85
[44] . When the entire column is heated resistively and on-column
injection is used, as is the case with the GC probe, no additional
injector heaters are required. The power required to resistively heat a
column to a particular temperature is dependent on the thermal mass of the
column (eq 3.3). For columns of the same diameter and Al coating
thickness, the power needed to reach a particular temperature is directly
proportional to the column length. For example, a 23 m column would
require ten times more power than the 2.3 m column, or 350 W, to reach
150°C.
One limitation of the present system is that the voltages sensed at
the lower column temperatures are too small to yield a high degree of
precision for the column resistance (and hence temperature) measurement.
This is the reason for the quantization noise (at low voltages and
temperatures) in the temperature profile of Figure 3.9. This limitation
could be remedied if an autoranging circuit was used to automatically
adjust the sensitivity of the resistance-sensing electronics.
GC Probe Performance
Figures 3.10 (a) and 3.10 (b) illustrate two different reconstructed
ion chromatograms that were obtained with the GC probe and two different
temperature ramps using a 3.19 m x 0.53 mm i.d. Al-clad FSOT column. A
0.5 jiL on-column injection was made in both cases. The concentration of
each component (identified in the figure) was 250 ng/^L in hexane. In
Figure 3.10 (a), the voltage across the column was programmed, after a
delay of 15 s, from 0 V (25°C) to 8.6 V in 2 minutes. The voltage was
held constant for another 1.5 minutes, resulting in a final temperature
of 143°C. In Figure 3.10 (b), the same initial voltage (temperature) was

Figure 3.10. Reconstructed ion chromatograms obtained with the GC probe
using a 3.19 m x 0.53 mm i.d. column and two different linear column
voltage programs: (a) final voltage of 8.6 V after 2 minutes, (b) final
voltage of 10.8 V after 1 minute. The components of the mixture are: 1,
2-chlorophenol; 2, 2,4-dimethylaniline; 3, n-undecane; 4, 1-undecanol; 5,
acenaphthylene; 6, tetradecane; 7, pentadecane.

87
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Retention Time (min)
(b)

88
used as in Figure 3.10 (a). The delay was 10 s and the final voltage was
10.8 V after 1 minute. The voltage was held constant at 10.8 V for an
additional 30 s, resulting in a final temperature of 168°C.
On average, the peak widths at 50% height are approximately 3 s in
Figure 3.10 (a) and 2 s in Figure 3.10 (b) . The earliest eluting peak (2-
chlorophenol) has a larger peak width in both cases. This can be
attributed to the deleterious effects of the solvent. During cold on-
column injection, components that are much less volatile than the solvent
are focused at the head of the column and exhibit very narrow band widths.
In contrast, the volatile, early eluting components (if not well separated
from the solvent front) can partition in the liquid solvent as well,
resulting in increased solute peak widths [45]. This problem can often
be avoided if smaller volumes of solvent are injected, or more reliably,
if cryofocusing is used to trap the solutes in a narrow band at the head
of the column [46]. The peak widths obtained here are extremely narrow
considering that the column that was used had a stationary phase film that
is normally considered thick (1.0 /¿m). In addition to increasing the
chromatographic resolution, the capability of obtaining narrow band widths
increases the sensitivity and lowers the limit of detection [3,14].
In another study, the reproducibility of retention times and peak
areas obtainable with the GC probe were measured. The retention times and
resulting integrated peak areas were measured for four on-column,
temperature-programmed injections of 150 ng tetradecane on a 3.3 m x 0.53
mm i.d. column. Tetradecane, which has a boiling point of 232°C, eluted
at 125°C with a retention time of 90.00 ± 0.81 s (relative standard
deviation (RSD) of 0.9%). The integrated peak area of the m/z 198
molecular ion of tetradecane was determined with a precision of 3.5% RSD,

89
which is certainly within the limits of error for injections performed
with a standard syringe.
Conclusions
Several important concepts for low-power and/or high-throughput
analysis in GC/MS have been addressed. It has been shown that exponential
flow rate programming can be used to approximate linear temperature
programming. When chemical ionization is used, flow rates up to 30 mL/min
can be used without detrimental effects on instrument sensitivity.
Although the range of boiling points separated with flow programming is
less than that which can be obtained with temperature programming, flow
programming does represent a significant improvement over isothermal
operation. The inherent lower elution temperatures of flow rate
programming might also be attractive for the analysis of thermally labile
solutes.
The direct resistive heating of Al-clad FSOT columns has also been
demonstrated. This direct method of column heating obviates the need for
a large chromatograph oven, and should be widely applicable in portable,
low-power gas chromatographic instrumentation. This was demonstrated with
the design of a compact GC probe for high-throughput analyses in GC/MS and
GC/MS/MS. Measurement of the column resistance was shown to be a simple
yet accurate method of directly sensing the temperature of low thermal
mass Al-clad columns. It was demonstrated that linear voltage programming
of the Al-clad columns can be used to approximate linear temperature
programming. Future application of proportional-integral-differential
(PID) feedback control of the column temperature and elimination of the
column insulation is expected to further improve the thermal response
characteristics of direct resistive heating of these columns.

CHAPTER 4
GAS CHROMATOGRAPHIC SAMPLE INTRODUCTION INTO
THE COLLISION CELL OF A TRIPLE QUADRUPOLE MASS
SPECTROMETER FOR MASS-SELECTION OF REACTANT
IONS FOR CHARGE EXCHANGE AND CHEMICAL IONIZATION
Introduction and Theory
The maximum selectivity of any analytical technique can be realized
if all of the variable parameters of the method are considered. These
variable parameters can be considered as resolution elements that affect
the informing power (i.e., the amount of information available) in the
analytical method [47]. In mass spectrometry, two resolution elements
which may be varied are mass analysis and ionization. The informing power
of the mass analysis can be augmented by increasing the mass range,
increasing the mass resolution, or increasing the number of stages of mass
analysis (i.e., tandem mass spectrometry). Since the ionization method
is a resolution element, it too will affect the informing power. A
certain degree of "resolution" (or selectivity) can be obtained with
ionization by employing techniques that allow ionization of only the
desired components of a mixture, thereby excluding the ionization (and
hence detection) of undesirable interferents. This selectivity of
ionization is not frequently exploited, but can be controlled if careful
consideration is made for the selection of reactant ions for charge
exchange (CE) ionization or chemical ionization (Cl) [48], Traditionally,
ion-molecule reactions for CE or Cl are performed in a high-pressure
(e.g., 1 torr) ion source [48-50], However, there are limitations to
90

91
performing these reactions in the ion source. When a reactant gas is
introduced into the ion source, it is rare that only a single m/z reactant
ion is formed; due to the presence of other undesirable reactant ions,
the ionization process may not be well-controlled. In fact, many
ionization processes including CE, Cl, (each with various reactant ions)
and even El may compete with the ionization technique of interest. This
mixed-mode ionization certainly limits the selectivity of the ionization
process. In addition, instrumental parameters such as ion source pressure
affect the relative abundances of the various reactant ions; hence,
reproducible CE or Cl spectra are often not easily obtained.
A novel approach that eliminates the mixed-mode ionization discussed
above is to mass-select the desired reactant ion before allowing it to
react with the sample. Mass-selection of reactant ions has previously
been used for studying ion-molecule reactions and reactive collisions.
A variety of tandem mass spectrometers have been used, including sector
instruments [51], a double quadrupole instrument [52], quadrupole ion
traps [53-55] and triple quadrupole mass spectrometers [56-62] . Crawford
and co-workers have used Kr as a charge exchange collision gas in a triple
quadrupole mass spectrometer (TQMS) for monitoring carbon monoxide in the
presence of hydrocarbons [63], However, to date there has been no
demonstration of the analytical utility of mass-selecting the reactant ion
for mixture analysis, or of the combination of mass-selected reactions
(MSRs) with GC. For this reason, a gas chromatograph has been interfaced
to the collision cell of a TQMS. In gas chromatography/mass-selected
reaction/mass spectrometry (GC/MSR/MS), reactant ions are selected with
the first quadrupole mass filter (Ql) and are allowed to react with
neutral sample molecules which elute from a GC column into the second

92
quadrupole collision cell (Q2). The third quadrupole is scanned for the
products of the ion-molecule reactions. In this chapter, the selectivity
and sensitivity of a TQMS system combining short-column GC with mass-
selected reactions are discussed. Also, the capability of obtaining both
molecular weight and structural information in the same chromatogram is
demonstrated. A highly useful method of rapidly determining ionization
energies with the TQMS is also demonstrated.
The selectivity of charge exchange (CE) and chemical ionization (Cl)
techniques arises from control of the energetics of the ion-molecule
reactions between reactant ions and sample molecules. A CE reaction may
occur if the ionization energy of the compound of interest, IE[M], is less
than the recombination energy of the reactant ion, RE[R+*].
R+* + M -» M+* + R
(4.1)
AH = IE [M] - RE [R+‘ ]
(4.2)
For monoatomic species, the recombination energy of an ion is the same as
the ionization energy of the neutral. However, polyatomic ions may
possess excess vibrational/rotational energy; therefore, this equality
cannot always be assumed. The amount of internal energy deposited into
M+* is determined by the exothermicity of the CE reaction.
Similarly, proton transfer Cl reactions may occur if the proton
affinity of the sample molecule, PA[M], is greater than the proton
affinity of the conjugate base, R, of the reactant ion [R+H+] , PA[R].
[ R+H ]+ + M -+ [ M+H ]+ + R
(4.3)
AH = PA[R] - PA[M]
(4.4)

93
The amount of internal energy deposited into [M+H]+ is equal to the
difference in the proton affinities of R and M [28], Hence, for both CE
and proton transfer Cl, the exothermicity of the ion-molecule reaction and
the resulting extent of fragmentation can be controlled by the selection
of the reactant ion. These ionization processes can be used to enhance
selectivity by choosing reactant ions that will only ionize targeted
compounds or by excluding the ionization of undesirable interferents. For
example, if the molecular ion of benzene (C6H6+", IE - 9.2 eV) is chosen as
a CE reactant, only compounds with ionization energies less than 9.2 eV
are ionized (typically only aromatic compounds). The energetics of ion-
molecule reactions can also be used to obtain the desired amount of
fragmentation. For example, if CE is used and molecular weight
information is desired, then reactant ions may be chosen that have REs
just above the IEs of the analytes. Alternatively, if structural
information is desired, a reactant ion may be chosen that has a RE much
larger than the IE of the sample molecules. The same logic can be applied
to proton transfer Cl, except that the difference in proton affinities
determines the selectivity and the degree of fragmentation.
Experimental Section
Mass Spectrometry
A Finnigan MAT TSQ70 triple quadrupole mass spectrometer was used in
these studies. All reactant gases, except for benzene and acetone, were
introduced into the ion source through the gas line normally used for
introduction of Cl gas. Benzene and acetone were introduced into the ion
source via a variable leak valve (Granville-Phillips), which was mounted

94
on a 1/2 in. o.d. stainless steel probe that could be inserted into the
ion source through the probe inlet assembly. Reactant gases were ionized
in the source with electron energies of 70-100 eV and emission currents
of 200 pA. The continuous dynode electron multiplier was operated at
1000-1200 V, and the preamplifer gain was set at 108 V/A. The mass
spectrometer vacuum cradle was maintained at 100°C to minimize sample
memory effects in the collision cell. Reactant gas pressures of less than
0.3 torr were used in all cases. For experiments utilizing argon and
benzene as a reactant gas mixture, the pressures of the two reactant gases
were adjusted to yield approximately the same signal intensity for the Ar+"
(m/z 40) and C6H6+* (m/z 78) reactant ions.
The TQMS was tuned in the normal fashion with perfluorotributylamine
(FC43) to optimize ion transmission and calibrate the mass assignment.
An additional tuning procedure was used to optimize the ion optics for
maximum transmission of the ion-molecule reaction products. This was
performed by introducing Ar into the source and n-butylbenzene into Q2.
The first quadrupole (Ql) was set to pass Ar+" and the resulting charge
exchange products of Ar+* with n-butylbenzene were monitored. The ion
optics were optimized for the appearance of the product ions. It was
discovered that only two parameters needed further optimization following
normal tuning. The first is the Q2 offset voltage, which controls the
energy of the reactant ions in Q2. The optimum Q2 ion energy was
typically in the range of 1-10 eV. Since the ion energy effectively
controls the residence time of the ions in Q2, it could be expected that
low ion energies would be favored. The second parameter requiring
additional optimization was the Q2 RF voltage. Since Q2 is an RF-only
quadrupole, it is not capable of simultaneously transmitting all ions of

95
different m/z. In fact, at a given RF voltage, an RF-only quadrupole has
a low mass cut-off, below which ions are not transmitted [64], In
addition, low m/z ions entering Q2 (e.g. , [He]+*) have a very narrow range
of RF voltages that allow these ions to be transmitted. The adjustment
of the Q2 RF voltage was critical, since both the reactant ions and
product ions had to remain stable in Q2 for product ions to be observed.
Gas Chromatography
A Varian 3400 gas chromatograph with a split/splitless injector was
used. A 2.1 m x 0.25 mm i.d. DB-5 (0.25 ^m film) or a 3 m x 0.18 mm i.d.
(0.4 pm film) fused silica open tubular (FSOT) column (J&W Scientific) was
used, depending on the analysis. Carrier gas flow rates were controlled
with a mass flow controller (MKS Model 1159A) and a flow control system
described previously in Chapter 2. This flow control system allows the
injection port to operated at sub-ambient pressures such that low carrier
gas flow rates (1-2 mL/min) can be used with short columns with vacuum
outlet. It was necessary to limit the flow rates to less than 2 mL/min
in the collision cell to avoid exceeding the normal operating pressure of
the mass analysis region of the TSQ70. The pressure in the analyzer
region, with <2 mL/min of He carrier gas flowing into the collision cell,
was typically 2 x 10"5 torr, as indicated by a Bayard-Alpert ionization
gauge. It should be noted that longer columns could be used (instead of
short columns with reduced inlet pressures) to restrict the column flow
rates, at the expense of longer analysis times. Split injections were
performed by injecting the sample directly into the low-pressure injection
port, which was connected to a mechanical pump via the splitter line.
During the splitless mode of operation, the samples were injected with the

96
split valve closed and the injection port slightly above atmospheric
pressure. The inlet pressure was then sharply reduced after allowing
approximately 10 seconds for the sample to enter the column. At the flow
rates used, the injection port was completely flushed with carrier gas in
approximately 3 seconds. Final inlet pressures of 420 torr and 710 torr
were used for the 0.25 mm i.d. and the 0.18 mm i.d. column, respectively.
Temperature programming was used for all analyses.
It was important to select a carrier gas that would not be ionized by
mass-selected reactant ions, since the ionized carrier gas could interfere
with the ion-molecule reactions of interest. Helium was chosen for this
work, since it has an ionization energy well above the recombination
energies of the reactant ions used. In addition, it was expected that the
small collision cross section of helium would reduce the probability of
collisionally-activated dissociation (CAD) of reactant or product ions in
the collision cell.
GC/MS Interface
Two different transfer lines were used for GC/MS interfaces. In both
cases the effluent from the GC column was introduced directly into the
collision cell of the TQMS. For one transfer line, aim section of the
polyimide-clad FSOT column was passed through aim length of 1/16 in.
o.d. stainless steel tubing. The second transfer line consisted of a 0.9
m x 0.32 mm i.d. BP-5 aluminum-clad FSOT column with a 0.5 /¿m film
thickness (SGE). The 2.1 m FSOT column was coupled to the Al-clad column
with a glass-lined zero-dead-volume fitting. Both transfer lines were
resistively heated by applying an AC voltage across the tubing. This was
accomplished by connecting the line voltage (120 V AC) across an

97
adjustable autotransformer (Variac), and connecting the output of the
autotransformer to a step-down transformer (rated at 34 V, 10 A, with 120
V input). The voltage developed across the secondary of the transformer
was placed across the transfer line. The temperature of the transfer line
was adjusted by varying the voltage output of the autotransformer.
Approximately 5-6 V AC were required to heat either transfer line to
175°C. The transfer lines were covered with glass braid insulation to
minimize temperature fluctuations. The transfer line temperatures were
monitored with a digital temperature sensor utilizing a low thermal mass
type K thermocouple (Omega Engineering). Small alligator clips were used
to make electrical connections to the transfer lines. The transfer line
being used was inserted through a feedthrough into the mass spectrometer
vacuum chamber. A viton ferrule and a 5 cm piece of teflon tubing was
used to electrically isolate the transfer line from the vacuum manifold
of the mass spectrometer. Ground for the transfer line was asserted
inside the vacuum chamber, which allowed all but the last 5 cm of the
column to be heated. The end of the column was inserted into the
collision chamber through the same opening used for introduction of
collision gas. The TSQ70 vacuum manifold has a removable glass top that
allows easy access to the collision chamber. The procedure of installing
the column outlet into Q2 requires about the same amount of time as normal
installation of a column into the ion source. The transfer lines are
mechanically simple and can be rapidly heated (or cooled) to the desired
temperature.

98
Results and Discussion
Ionization Energy Measurements
Knowledge of the ionization energies (IEs) of reactant gases and
samples is important if the energetics of the charge exchange process are
to be understood and taken advantage of analytically. Since IEs of
certain species may not always be available in the literature, a routine
method for the determination of IEs would be a boon for the application
of charge exchange ionization. A number of methods have been used to
determine ionization energies, including photoionization [65,66], electron
ionization [65,67], resonant photoionization [68,69], and Penning
ionization [70], However, these methods are commonly implemented on mass
spectrometers specifically designed or modified for this purpose. It was
found that ionization energies could be measured rapidly with the
computer-controlled TSQ70 in good agreement with literature values [71].
With the sample or reactant gas introduced into the ion source, ionization
or appearance energies were determined by increasing the electron energy
(the potential difference between the filament and source block) in small
increments (0.1-0.2 eV) and determining the approximate value at which a
measurable ion signal first appeared. With the computer-controlled TSQ70,
a plot of ion intensity vs. electron energy from 1-70 eV can be acquired
in less than 30 seconds.
Examples of appearance energy curves are shown in Figure 4.1 for the
molecular ion (m/z 134) and two fragment ions (m/z 91 and m/z 92) from n-
butylbenzene. Since there is some uncertainty associated with estimation
of the appearance energy (Figure 4.1), each of the ionization energies
determined experimentally is reported in Table 4.1 as the average of the

99
Electron Energy (eV)
Figure 4.1. Variation of the electron energy for the determination of the
appearance energy or ionization energy for three different ions (m/z 91,
[C^]*; m/z 92, [C7H8]+‘; and m/z 134, [C6H5C4H9]+') of n-butylbenzene. The
brackets labeled 'AE' indicate the approximate uncertainty in the
estimation of the appearance energy from a single curve.

100
Table 4.1. Comparison of experimentally determined ionization energies
with literature values.
Species
Ion
Experimental CeV)
Literature (eV)a
He
He+*
24.80
±
0.10
24.59
±
0.01
n2
N+
24.50
±
0.20
24.32
±
0.03
He
He2+*
20.00
±
0.10
NF
co2
(co2)2+‘
16.40
±
0.10
NF
Ar
Ar+*
16.10
±
0.10
15.76
±
0.03
n2
n2+*
15.70
±
0.30
15.58
±
0.01
n2
n4+*
14.70
±
0.30
14.58
±
0.01
Ar
Ar2+*
14.50
±
0.20
14.71
±
0.01
co2
co2+*
14.40
±
0.20
13.80
±
0.02
ch4
ch4+’
13.00
±
0.20
12.76
+
0.24
ch4
ch3+
ND
13.50
±
0.05
C8H?Nb
c8h7n+-
9.60
±
0.10
9.45
+
0.05
C1oV
ciohh*
9.00
±
0.20
8.69
±
0.01
a Literature values obtained from reference [71].
b phenylacetonitrile
c n-butylbenzene
NF - Data not found in the literature.
ND - Value not determined experimentally.

101
highest and lowest values of the observed range of possible appearance
energies obtained from a single curve. The amount of uncertainty
resulting from the estimation of the appearance energy from a single curve
(rather than the standard deviation of several curves) is also reported
with each of the determined values. This uncertainty, typically 0.2 eV,
can be attributed to the low ion intensities obtained when the electron
energy is near the appearance energy. The most accurate results were
obtained if the first ion extraction lens after the source was set to 0.0
V with respect to the source block. This kept the field from this lens,
as well as others, from perturbing the electron energy within the source
and causing erroneous measurements. As shown in Table 4.1, the
experimental ionization energies are in good agreement with literature
values (generally within 1 or 2%). In nearly every case, the
experimentally determined ionization energy was higher (typically by 0.2
eV) than that found in the literature. This error may be attributed in
part to the inabilty to determine the first small increase in ion signal.
Other errors may be introduced from the voltage offset in the digital-to-
analog and analog-to-digital converters used by the TSQ70 to set and read
back the electron energy.
Simultaneous Structural and Molecular Weight Information
Electron ionization (El) typically provides structural information;
however, in many cases low intensity molecular ions (or no molecular ions
at all) are obtained. Chemical ionization (Cl) is then used to obtain
pseudo-molecular ions (e.g., [M+H]+) for molecular weight confirmation.
Since obtaining Cl spectra in most mass spectrometers requires physical
changes in the ion source region, El and Cl spectra cannot be obtained

102
simultaneously; consequently, molecular weight and structural information
are not readily obtained in a single chromatographic run. With mass-
selected reactions, however, the energetics of the ionization process can
be tailored to the type of information desired. In addition, structural
information and molecular weight information can be simultaneously
acquired. This can be accomplished by sequentially selecting different
reactant ions from the same reactant gas, or by introducing a mixture of
reactant gases in the ion source and selecting the reactant ions during
alternating scans that have different recombination energies or proton
affinities. It is also possible to acquire alternating Cl and CE spectra
by selecting different reactant ions.
Figure 4.2 shows CE reconstructed ion chromatograms of a GC activity
mixture that were obtained by selecting the Ar+" and C6H6+* reactant ions
during alternating scans. The chromatogram was obtained with the 3 m x
0.18 mm i.d. column (1 m of which served as the transfer line) operated
with a 25:1 split. The concentration of each component in the sample was
250 ng/^L, and a 1/iL injection was made. The components of the mixture
are identified in the figure. This type of mixture is commonly used for
evaluating the chromatographic performance of capillary columns. It
contains a wide variety of compound types (alkanes, aromatics, alcohols,
and an amine) , which has made it useful for investigating the ion-molecule
reactions.
Ionization energies of aliphatic compounds are typically above 11 eV,
while those of aromatics are typically below 10 eV. As shown in the
Figure 4.2, Ar+’ (RE - 15.8 eV) ionizes all of the compounds of the
mixture, while the C6H6+* ion (RE = 9.2 eV) only ionizes the aromatic
components. Fenselau et al. have previously evaluated benzene as a charge

% Relative Intensity
103
Figure 4.2. GC/MSR/MS chromatogram of GC test mixture with (a) Ar+’ charge
exchange and (b) C6H6+" charge exchange. The reactant ions were mass-
selected with Q1 during alternate scans. The impurity in the mixture is
acenaphthene.
Absolute Intensity

104
exchange reactant [50], However, their results showed that species not
ionized by the C6H6+* reactant ion were ionized by El in the source.
Clearly, the ability to mass-select the desired reactant ion and carry out
these reactions in a region without the presence of ionizing electrons
yields the maximum selectivity obtainable with the ionization process.
Figure 4.3 shows a comparison of the spectra of 2,4 dimethylaniline
obtained from Ar+‘ and C6H6+* CE from the chromatogram in Figure 4.2, as
well as an El spectrum acquired separately for comparison. The CE spectra
illustrate the capability of simultaneously obtaining molecular weight and
structural information. Our experience has shown that the C6H6+* reactant
ion yields predominantly the molecular ion for aromatic compounds with
little or no fragmentation. Conversely, the Ar+’ CE spectra are more
energetic than 70 eV El spectra. This can be better understood by
considering the internal energy imparted upon ionization. The internal
energy deposited during CE ionization is well-defined (eq 4.2), whereas
the internal energy deposited during ionization by El is not well-defined
due to the wide range of energies imparted during El with 70 eV electrons
[72],
Charge exchange ionization produces odd-electron species as does El
[42,73]. It has previously been shown that CE spectra can be searched
against the NBS El library with acceptable results [73]. These concepts
were investigated using the GC/MSR/MS technique. An advantage of MSRs is
that two (or more) different CE reactant ions can be used, one resulting
in El-type spectra, and another yielding very little fragmentation. Thus,
the CE spectra could provide an added dimmension for simultaneously
providing structural and molecular weight information for the
classification of unknowns. Charge exchange spectra from Ar+*, C02+*

105
2,4 dimethylaniline MW 121
m/z
Figure 4.3. Mass spectra of 2,4 dimethylaniline: (a) 70 eV El, (b) Ar+*
CE, and (c) C6H6+* CE. The CE spectra were obtained from the chromatogram
in Figure 4.2. The El spectrum was acquired separately.
Absolute Intensity

106
(RE “13.8 eV) , and CH3+ (RE = 13.5 eV) with the components in the GC test
mixture were compared with El library spectra. The results of the library
searches obtained with the TSQ70 ICIS system from the three CE reactant
ions as well as from El are shown in Table 4.2. As shown in the table,
the success rate of the library search for the CE spectra were comparable
to those obtained with El. In general, among the charge exchange
reactants studied, the CH3+ CE spectra yielded the most reliable match with
the El library spectra. This might be expected, since the CH3+ charge
exchange reactant ion deposits the least amount of energy of the three
reactant ions studied and yielded the most abundant molecular ions. In
cases where the compound was not found, knowledge of the molecular weight
(obtainable by CE with a less energetic reactant ion) led to more accurate
classification. Once the molecular weight was known, the molecular weight
filter could be applied before the library search was carried out. In
addition, in cases where the compound was not correctly matched,
information about the compound class could be obtained from the best hits
of the CE spectra, as is commonly done with El spectra.
Selectivity of Ionization
The chromatograms in Figure 4.2 illustrate the selectivity of C6H6+*
CE for a relatively simple mixture. A more dramatic illustration of the
selectivity of the C6H6+* reactant ion is its use as a means of rapidly
screening for the aromatic components of a complex mixture without high-
resolution chromatographic separation. An example of these concepts is
presented in Figure 4.4, which shows reconstructed ion chromatograms
(RICs) of JP-4 jet fuel on a 2.1 m x 0.25 mm i.d FSOT column (1 m of this
column served as the transfer line). The top two traces of the figure

107
Table 4.2. Comparison of El library search results for CE with various
reactant ions and El of compounds in GC test mixture.
Ionization
Mode
and Reactant Ion
El
CE
CE
CE
Ar+*
co2+*
ch3+
ComDound
Library Search Match 4
MW filter
2 -chlorophenol
1
3
3
2
off
1
3
3
2
on
undecane
Na
N
N
N
off
5
5
2
1
on
2,4 dimethylaniline
5
3
N
4
off
5
3
3
3
on
1-undecano1
N
N
N
N
off
1
3
2
1
on
tetradecane
4
9
1
1
off
2
1
1
1
on
acenaphthylene
1
7
N
N
off
1
4
10
2
on
pentadecane
N
N
N
1
off
1
3
1
1
on
aN, Not found among
the top
ten
best fits
of the
library searc

% Relative Intensity
108
JP-4 Jet Fuel
Figure 4.4. GC/MSR/MS chromatogram of JP-4 jet fuel on a 2.1 m column.
Reconstructed ion chromatograms: (a) Ar+* CE and (b) C6H6 CE. Mass
chromatograms of the m/z 134 M+ of ten-carbon alkylbenzenes. (c) Ar CE
and (d) C6H6+* CE. The arrows indicate where spectra were obtained for
Figure 4.5.
Absolute Intensity

109
show the RICs resulting from Ar+* and C6H6+* CE, respectively. The bottom
two traces show mass chromatograms for the m/z 134 ion obtained via CE
from the two different reactant ions. The m/z 134 ion is the molecular
ion produced from a number of different isomers of alkylbenzenes
containing a total of ten carbons. Figure 4.5 shows Ar+* and C6H6+* CE
spectra that were obtained from the chromatogram of Figure 4.4. The
points at which these spectra were taken is indicated by the arrows in
Figure 4.4. The Ar+* CE spectrum of Figure 4.5 (a) is not easily
interpreted, since molecular ions and fragment ions from both aliphatic
and aromatic constituents of the jet fuel are produced. However, in the
C6H6+* CE spectrum of Figure 4.5 (b), only the molecular ion (m/z 134 and
the 13C peak at m/z 135) from the alkylbenzene(s) is produced.
Mass-selected ion-molecule reactions can be performed using unique,
highly specific reactant ions. Often, selection of reactant gases for
chemical ionization in the ion source is limited to low molecular weight
gases (e.g., methane, ammonia, etc.), as reactant gases of higher
molecular weight (e.g., 100 amu), present in a large excess compared to
the sample, can produce ions due to adducts, dimers, and/or fragment ions
that obscure the presence of sample ions. With mass-selected reactions,
virtually any reactant ion can be used, provided that the reactant ion can
be produced in the ion source and does not have the same m/z as the sample
under study. An example of a unique reactant ion for chemical ionization
is the protonated acetone dimer (m/z 117). A chromatogram of the GC test
mixture obtained by MSR with the protonated acetone dimer is shown in
Figure 4.6. The 2.1 m FSOT column was used with the 0.9 m FSOT Al-clad
transfer line. It was discovered that selection of the m/z 117 ion with
Q1 resulted in the production of the protonated acetone (m/z 59) in Q2.

% Relative Abundance
110
100-
80-
60-
40-
20-
3_.
40
"T
60
134 ..
(b) C6H6+ CE
80
100
irrr-T
120
-44-
*E + 06
-4
-3
-2
-Í
140
160
180
43
200
m/z
Figure 4.5. Spectra obtained from the chromatogram marked by arrows in
Figure 4.4: (a) Ar+* CE and (b) C6H6+* CE. Only aromatic species of the jet
fuel are ionized with benzene charge exchange, with no fragmentation.
Absolute Intensity

% Relative Intensity
111
Retention Time (min:s)
Figure 4.6. GC/MSR/MS chromatogram of GC test mixture obtained with mass-
selection of the protonated acetone dimer (m/z 117) reactant ion. The
impurity in the mixture is acenaphthene.
Absolute Intensity

112
The m/z 59 ion, which was the result of metastable decomposition of m/z
117 or CAD of m/z 117 with the helium carrier gas, had an intensity of 30%
relative to the m/z 117 ion. Only three of the seven components were
ionized by the acetone reactant ions. Spectra for each of the components
ionized are shown in Figure 4.7. The acetone reactant ions can react via
proton transfer to form [M+H]+ ions if the reaction is energetically
favorable. Acenaphthylene and 2,4 dimethylaniline were the only mixture
components protonated by the acetone reactant ions. Compounds may also
form adducts with protonated acetone to yield [M+59]+ ions or with the
protonated acetone dimer to produce [M+117]+ ions. For the components of
the GC test mixture, only compounds with hydrogen bonding sites reacted
to form the acetone adduct ions (e.g., 2,4 dimethylaniline, and 1-
undecanol). Note that 2-chlorophenol, which does have a hydrogen bonding
site, was not ionized by the acetone reagent ions. All of the spectra
obtained from acetone Cl exhibited no fragmentation, as shown in Figure
4.7.
Detection Capabilities of GC/MSR/MS
The ability of GC/MSR/MS to detect low levels of analyte was also
examined. Initially, it was not expected that products of ion-molecule
reactions in Q2 could be efficiently extracted and mass-analyzed. The
products of the ion-molecule reactions have little or no axial kinetic
energy when they are formed; thus, the collection efficiency of the
product ions is dependent on the extraction capabilities of the Q2 ion
exit lens. In fact, Enke et al. have found it necessary to fill and trap
ions in Q2 for a period of time and then pulse them out with the Q2 ion
exit lens in order to obtain adequate sensitivity [60]. Nevertheless, we

113
lOO-
SO-
60-
40-
20-
0s
(a) 2,4 dimethylaniline
MW 121
122[M+H] +
iso [M+59]+
r~
30
100
a>
o
c
a
â– D
C
3
-Q
<
ffl
>
ffl
O
DC
100-1 (b) 1-undecanol
MW 172
80-
60-
4 0-
20-
0-
I
130
200
230
231
30
-""I—
100
150
11 T '
200
[M+59]
XE + 06
-2.0
-1 . 5
-1 . 0
-0.3
—\=a. o
300
XE + 03
-8
[M+117]+
,rT'
230
288
300
100-
SO¬
SO-
4 0-
20-
0-
(c) acenaphthylene i=3 [M+H]
MW 152
TT
30
~ â–  I"1"
100
-’T
130
XE + 06
-3
-2
200
230
-0
300
m/z
Figure 4.7. Acetone Cl spectra obtained from chromatogram in Figure 4.6:
(a) 2,4 dimethylaniline, (b) 1-undecanol, (c) acenaphthylene. Only
components with hydrogen-bonding sites reacted to form acetone adducts.
Absolute Intensity

114
have found that detection limits without ion trapping techniques with
GC/MSR/MS are comparable or better than those obtained with GC/MS. For
these experiments, benzophenone was the analyte and C6H6+* was the CE
reactant ion. The same column and transfer line, described previously
with the acetone Cl studies, was used for these experiments. Similar to
the spectra of other aromatic compounds studied, benzophenone was ionized
by CE with the C6H6+* reactant ion to produce only the molecular ion (m/z
182). Figure 4.8 (a) shows the mass chromatogram for the m/z 182 ion of
100 pg of benzophenone for a full scan of Q3 (80-240 amu in 0.5 seconds).
Figure 4.8 (b) shows the chromatogram obtained from 15 pg of benzophenone
when Q3 was scanned from 181-183 amu in 0.5 seconds. The signal-to-noise
ratios (S/Ns) for Figures 4.8 (a) and 4.8 (b) are approximately 9:1 and
30:1, respectively. For comparison, the specifications for the full scan
sensitivity of the TSQ70 with GC/MS are quoted as 200 pg for El with a S/N
of 10:1 and 100 pg with methane Cl with a S/N of 25:1. Benzene CE
experiments were attempted in the ion source for comparison with the
GC/MSR/MS results. With benzene CE in the ion source and a full scan of
Ql, 600 pg of benzophenone could be detected with GC/MS. However, the
spectra obtained from carrying out the reactions in the ion source
resembled El spectra instead of benzene CE spectra (due to mixed-mode
ionization), and thus do not represent a valid comparison. The ability
to obtain the molecular ion (or pseudo-molecular ion) with little or no
fragmentation for an analyte provides the maximum sensitivity for trace
analyis. Since such 'soft ionization' is easily performed with mass-
selected reactions, the GC/MSR/MS approach provides the opportunity to
'tune' the ionization to obtain the best limits of detection.

Relative Intensity
115
Benzophenone C6H6+* CE
Retention Time (minis)
Figure 4.8. Benzene (C6H6+‘) CE mass chromatograms for (a) 100 pg of
benzophenone obtained with a full scan of Q3, and (b) 15 pg of
benzophenone obtained with selected reaction monitoring (SRM) of the m/z
182 ion.
Absolute Intensity

t
lie
Chromatographic Integrity
The chromatographic peak shape obtained from introducing the effluent
from the GC column into the collision cell is not quite as good as
normally obtained when the GC effluent is introduced into the ion source.
The tailing of the 2,4 dimethylaniline peak (Figure 4.2 and Figure 4.6)
is most likely due to active sites and insufficient heating of the
collision cell. Since the temperature of the collision cell was
controlled by radiant heating from the vacuum manifold heater, the maximum
temperature was limited to 100°C to avoid damage to the electron
multiplier. It is expected that better chromatographic peak shape could
be obtained if the collision cell were heated separately and to higher
temperatures than 100° C. The best peak shape was obtained when a
polyimide-clad FSOT column was used as a transfer line (Figure 4.2)
instead of the Al-clad column (Figure 4.6). Although the Al-clad column
is more easily heated resistively, the polyimide-clad column can be
inserted closer to the quadrupole rods of the collision cell without
danger of electrical arcing, minimizing sample adsorption on the stainless
steel walls of the collision cell. The introduction of the GC column into
the collision cell did not appear to reduce ion transmission or decrease
the performance of the instrument, and has not required cleaning of any
of the ion optics.
Conclusions
A new technique in which samples are introduced via GC into the
collision cell of a TQMS system has been described. The GC/MSR/MS
approach offers significant advantages over GC/MS for control of the
selectivity and energetics of the ionization process. Ionization energies

117
are readily obtained with the computer-controlled TQMS. Knowledge of the
ionization energies is extremely useful when selecting the proper reactant
ions for charge exchange experiments. With mass-selected reactions
molecular ions, pseudo-molecular ions, or characteristic fragment ions can
be obtained depending on the reactant ion and the amount of internal
energy deposited during ionization of the sample. The utility of
searching charge exchange spectra against El library spectra has also been
demonstrated. When combined with short-column open tubular GC, mass-
selected reactions can be used to rapidly screen for particular classes
of compounds. For example, the aromatic content of a jet fuel can be
investigated with C6H6+* charge exchange, without interference from
aliphatic constituents. The detection limits of GC/MSR/MS are in the
picograra range, which is comparable to those which can be obtained with
GC/MS. The simple, resistively-heated GC/MS transfer lines described here
should make these types of experiments readily accessible on any TQMS
system. With this technique, it should be possible to devise highly
specific ion-molecule reactions for mixture analysis.

CHAPTER 5
CHARACTERIZATION AND OPTIMIZATION OF ION TRANSMISSION IN
TRIPLE QUADRUPOLE MASS SPECTROMETRY
Introduction
Although the triple quadrupole mass spectrometer (TQMS) is currently
in widespread use, the effects of many of the instrumental parameters that
govern the performance of these complex ion optical devices have not been
well understood. Since the pioneering work of Yost et al. [74], published
studies of the nature of the ion optical parameters in triple quadrupole
mass spectrometry have been limited to the work of Dawson and coworkers
[75-78]. In addition, the TQMS used by Dawson is of unique ion optical
design (commercially available as the Sciex TAGA 6000) and does not
represent the majority of the TQMS systems in current use. For example,
in the Dawson TQMS, there are no interquadrupole focusing lenses and a
straight, non-enclosed quadrupole collision cell is utilized. In the
commercially-available TQMS used for this work, there are six
interquadrupole focusing lenses and a non-linear, enclosed collision
quadrupole is used. Perhaps the real impetus for studying these ion
optical effects is that the only available data concerning the ion
transmission behavior of instruments of this design has been limited to
a few presentations given by the instrument manufacturers at conferences
[79,80],
The performance of TQMS instruments in terms of sensitivity for
quantitative analysis hinges on the ability to obtain efficient ion
transmission. For qualitative analysis, relative ion abundances are often
118

119
used to determine molecular structure or in studies concerning the
energetics of fragmentation processes. Therefore, it is important to be
able to obtain optimum and uniform transmission of ions (i.e., no mass
discrimination) over the entire mass range of interest. Ion optical
parameters that result in mass discrimination are said to exhibit mass-
dependent behavior. It is important to understand the origins of these
mass dependencies and to correct for them whenever possible. Martinez has
discussed the importance of eliminating mass discrimination if
dynamically-correct MS/MS spectra are to be obtained [81]. Recently,
there has been a suggestion for the implementation of a standard MS/MS
spectral library [82]. This idea has been somewhat controversial, since
MS/MS spectra are highly dependent on the numerous operating parameters
and the ion optical design of the instrument used. This has led to a
recent round-robin investigation of a variety of instruments in different
laboratories to detfermine the feasibility of such a spectral library.
In this chapter, the ion transmission characteristics of a computer-
controlled TQMS are reported. In particular, the parameters which result
in mass-dependent behavior, as well as the origins and strategies for
minimizing these effects, have been studied. Characterization of ion
transmission in the MS/MS scan modes, including the effect of using high
(>50 eV) parent ion axial kinetic energies, is also presented.
Experimental Section
Instrumentation
The instrument that has been used for these characterization and
optimization studies is a Finnigan MAT TSQ70. This instrument has proven
to be ideal for studies of this type, since all instrumental parameters

120
are under microprocessor control. In fact, with older TQMS instruments,
many of the experiments that have been performed would either not be
possible or would have been too time-consuming to carry out. The TSQ70
is controlled by six different single board controllers, each of which has
its own microprocessor. User interaction with the instrument is through
a unique procedural language known as the Instrument Control Language, or
ICL. ICL procedures (or programs) can be written that allow any
instrument task to be performed under computer control. Another advantage
of the ICL is that it is simple to use but highly structured (much like
the commonly used BASIC computer language). As a result, the capability
of real-time decision making exists with the ICL, which allows the
instrument to react to incoming data.
Shown in Figure 5.1 is a diagram of the mass analyzer assembly of the
TSQ70. As shown in the figure, the ion optics consist of three
quadrupoles (Ql, Q2, and Q3) and eleven focusing lenses (L11-L42) mounted
on an optical rail. Nine of the eleven lenses (L11-L33) make up 3
different three-element lens sets, known as einzel lenses [83]. In an
einzel lens, the first and third elements are usually held at similar
potentials with the center lens at some more negative potential (for
positive ions). This results in a focused ion beam with similar kinetic
energy on either side of the einzel lens. Typical voltages for a three-
element einzel lens set of the TSQ70 (e.g., Lll, L12, L13) for positive
ions are -10 V, -100 V, and -10 V, respectively. There are two other
lenses (L41 and L42) which do not make up an einzel lens, but are pre¬
detector lenses that serve to focus the ions exiting Q3 onto the ± 5 kV
conversion dynodes; the secondary ions (or electrons) produced at the
conversion dynodes are detected by the electron multiplier (EM). All of

Figure 5.1. Schematic diagram of mass analyzer assembly of TSQ70 triple quadrupole mass spectrometer.
Wwwww^

122
the lenses except lens L42 optimize at negative voltages for positive ions
and at positive voltages for negative ions. Lens L42 has two square ion
entrance holes that are in line with the conversion dynodes, instead of
one round hole in the center of the lens. This lens behaves atypically
in that it optimizes at positive voltages for positive ions and negative
voltages for negative ions, due to its shape as well as its placement in
front of the ± 5 kV conversion dynodes.
The first and third quadrupoles (Q1 and Q3, respectively) are mass
filters and can be operated in the mass-selective (RF/DC) mode or can be
set to pass a wide range of masses (RF-only), depending on the scan mode
selected. The mass filters are used to obtain a mass spectrum or for the
mass-selection of ions for MS/MS experiments. Regardless of the scan mode
used, Q2 is not a mass filter, but simply serves as an ion transmission
and focusing device (RF-only) during single MS experiments or as the
collision (or reaction) region for MS/MS experiments. One of the unique
features of the ion optical design of the TSQ70 is that it utilizes a non¬
linear second quadrupole (Q2) with an axis that is bent at an angle of
approximately 15°. This design has been reported to offer improved
signal-to-noise ratios over the conventional linear second quadrupole,
particularly when used with fast atom bombardment (FAB) ionization [79],
In the conventional straight axis Q2 design, excited neutrals, photons,
or X-rays produced in the ion source can traverse the length of the
instrument axis and can strike surfaces in the vicinity of the detector.
This can result in the production of secondary ions or electrons,
producing a significant amount of noise. The bent design of Q2 reduces
the amount of neutrals (and thus secondary ions) reaching the detector and
improves signal-to-noise ratios. However, the bent geometry also results

123
in some interesting ion optical effects, which are addressed in this
chapter.
One of the unique capabilities of the TSQ70 is that the ion optical
parameters may be varied with mass within a single mass spectral scan such
that the transmission of ions of different m/z can be optimized without
compromise for a particular mass or range of masses. This turns out to
be particularly advantageous in avoiding any mass discrimination that may
be present in the ion optics. In order to optimize the lenses with mass,
the lens voltages at specific points along the mass range are optimized.
Lens potentials for the other masses are automatically interpolated
between the set values. The result is called a tune table, which is a
plot of lens voltage (or other ion optical parameter) vs. m/z. The
concepts of mass-dependent tuning and tune tables are demonstrated in
Figure 5.2. In Figure 5.2 (a), the voltage for lens L13 is held constant
over the entire mass range; this is a "flat" tune table. In contrast,
Figure 5.2 (b) depicts a typical tune table that would be obtained after
mass-dependent optimization of lens L13, in which optima at different
masses have been determined. As the mass spectrometer scans, the voltage
of lens L13 follows the curve such that optimum ion transmission is
obtained over the entire mass range of interest. These tune tables can
be created for any of the ion optical parameters of the instrument.
However, mass-dependent optimization is normally performed for only the
parameters found to be mass-dependent. All of the other tune tables are
left flat.
An additional advantage of the computer control is the speed with
which the ion optical parameters can be optimized. Optimization which

124
Figure 5.2. Examples of tune tables for lens L13 illustrating: (a) flat
tune table in which lens voltage is constant over entire mass range, and
(b) tune table in which lens voltage is varied with the mass scan to
obtain optimum transmission over the entire mass range of interest.

125
took hours to perform on manually-controlled instruments can be performed
under computer control in a matter of minutes. One innovation of this
instrument that allows for such rapid optimization is that a lens voltage
digital-to-analog converter (DAC) may be varied over its full range in the
same amount of time that is normally required to execute one mass spectral
scan. In this mode of operation, known as "DAC scan", one analyzer is set
to pass a chosen m/z and the voltage on a particular lens is varied (or
scanned) over the entire range. As a result, a plot of ion intensity vs.
lens voltage is obtained and the optimum voltage can be determined in less
than one second.
Procedures
For all of the lens characterization studies performed, a common mass
calibration compound (perfluorotributylamine or PFTBA) was used. PFTBA
yields a wide mass range of positive ions (m/z 31 - m/z 614) with electron
ionization and is conveniently leaked into the ion source. In cases where
the mass dependency of a particular lens was being investigated, all of
the other lenses were held at constant, near optimum voltages, and not
varied with mass. All lens voltages reported are referenced to the ion
source potential, which is at ground.
Argon was used instead of PFTBA for the experiments designed to study
the transmission of ions as a function of collision energy or RF voltage
during MS/MS scan modes. Argon was chosen since it could not undergo
collisionally-activated dissociation (CAD). The transmission of ions in
the mass-selected reaction mode (described in Chapter 4) was also studied.
In these experiments PFTBA was introduced into the center quadrupole
collision cell, while reactant ions were mass-selected by the first

126
quadrupole. Ion transmission curves were obtained by plotting the
intensity for selected m/z's over a wide range of lens potentials
(typically 200 V) or RF voltages. With the computer-controlled TSQ70 and
the ICL, a transmission curve for a number of different masses (typically
4) can be acquired in a matter of minutes.
Results and Discussion
Mass-Dependent Optimization
The presence of mass dependencies in the ion optics of single
quadrupole mass spectrometers has long been known [84]. However, the
effects of these mass dependencies on the transmission of ions and the
resulting mass spectra have never been easily elucidated. The use of
computer-controlled instrumentation like the TSQ70 has allowed for more
rapid characterization and better understanding of these phenomena.
Ion optics theory predicts that all ions accelerated from a specific
point in space through an electrostatic lens will follow the same
trajectory regardless of the masses of the ions [83]. Thus, mass
dependency does not come from the electrostatic lens per se, but rather
is the result of non-idealities introduced from other forces. It was the
object of this work to determine which lenses exhibited mass dependencies,
and by observing trends in the data, to elucidate the origins of these
mass dependencies. After determining which lenses were mass-dependent it
was then possible to adopt the proper strategies for minimizing these
effects. With the TQMS used here, the lenses which were found to be mass-
dependent could be optimized over the entire mass range. The capability
of mass-dependent tuning significantly improves the performance, as will
be discussed below.

127
A well-behaved (ideal) lens has very similar transmission curves for
ions of different m/z; the lens voltage that yields the maximum
transmission (i.e., the optimum lens voltage) is therefore the same for
ions of different m/z. In other words, no mass dependency is observed
with an ideal lens. Figure 5.3 shows examples of typical transmission
curves for two different lenses in the normal Q1 mass spectral (Q1MS)
mode. Figure 5.3 (a) shows the transmission curve for a well-behaved lens
(L31) (refer to Figure 5.1 for the placement of this lens and other lenses
in the system). These curves demonstrate typical transmission curves
obtained (for positive ions) for the first lens of an einzel lens set:
maximum transmission at some low (negative) voltage, with decreased
transmission at higher negative voltages due to defocusing of the ion
beam, and transmission rapidly dropping to zero at positive voltages. In
contrast, Figure 5.3 (b) shows the ion transmission curves for a mass-
dependent lens (L13). Note that in (b) of this figure there are discrete
optimum lens voltages for ions of different m/z; thus, setting the lens
voltage at any one of these voltages will reduce the transmission of the
other ions. One interesting discovery was that some of the lenses were
found to be mass-dependent only in certain scan modes. For example,
Figure 5.4 (a) shows the mass-dependent behavior for lens L42 in the
normal Q3 mass spectral (Q3MS) scan mode. However, the same lens was
found to be non-mass-dependent in the Q1MS scan mode, as shown in Figure
5.4 (b).
After a thorough study of the ion optics, some generalizations could
be made. In the single MS scan modes (i.e., Q1MS or Q3MS), the lenses in
proximity to the mass-analyzing quadrupole (i.e., the quadrupole carrying
both RF and DC voltages) were found to be mass-dependent. In particular,

Figure 5.3. Ion transmission as a function of lens potential (in volts)
for two different lenses in the Q1MS scan mode: (a) non-mass-dependent
lens L31, (b) mass-dependent lens L13.

129
Ion Intensity vs. L31 Voltage Q1MS
o l-
-200
L13 Voltage (V)
10

Figure 5.4. Ion transmission as a function of lens L42 potential (in
volts) illustrating: (a) mass-dependent behavior in the Q3MS scan mode,
and (b) ideal behavior in the Q1MS scan mode.

131
Ion Intensity vs. L42 Voltage Q3MS
Ion Intensity vs. L42 Voltage Q1MS

132
for Q1MS, lenses L12, L13, L21, and L22 are mass-dependent; while for
Q3MS, the mass-dependent lenses are L32, L33, L41, and L42. This appears
to be the case regardless of the ionization method used. For instance,
positive chemical ionization (PCI) and electron capture negative Cl
(ECNCI) were studied and the same lenses were found to be mass-dependent.
Since El and Cl require different ion volumes (the Cl ion volume has a
smaller ion exit aperture), the optimum lens voltages were found to be
different for El, PCI and ECNCI; therefore, the best results are obtained
(i.e., maximum sensitivity) if the lens voltages are optimized for each
ionization method that is to be used.
Experiments were performed in order to compare the results obtained
with non-mass-dependent and mass-dependent tuning for the Q1MS scan mode.
For both mass-dependent and non-mass-dependent optimization, initial
voltages of -10 V, -100 V, and -10 V were used for all of the three-
element einzel lens sets; initial voltages of -10 V and +50 V were used
for the pre-detector lenses, LAI and L42, respectively. For non-mass-
dependent tuning, all of the lenses were then optimized only at m/z 219.
The DAC scan method was used to determine the lens voltage that yielded
the maximum ion signal for m/z 219 for each lens, starting with lens Lll
and ending with lens L42. For mass-dependent optimization, the lenses
which were found to be non-mass-dependent in the Q1MS scan mode (Lll, L23,
L31, L32, L41, and L42) were optimized with DAC scan only at m/z 219.
Then, the lenses which were previously determined to be mass-dependent in
the Q1MS scan mode (L12, L13, L21, and L22) were each optimized (one m/z
at a time) at m/z 69, m/z 219, m/z 414, and m/z 502. An example of a
typical tune table that was obtained with mass-dependent optimization is
shown in Figure 5.2 (b) . A significant improvement in sensitivity was

133
observed when mass-dependent optimization was performed, as shown by the
Q1MS electron ionization (El) spectra of PFTBA in Figure 5.5. Spectrum
(a) of Figure 5.5 was obtained after the non-mass-dependent tuning method
was used, while the spectrum in Figure 5.5 (b) was acquired after mass-
dependent optimization had been performed. Both spectra are normalized
to the full scale intensity of spectrum (b), and both are enlarged by a
factor of 10 beyond m/z 325. Note that the ion intensity is increased by
30% at m/z 69 and by 42% at m/z 502 when mass-dependent optimization was
performed. Optimization at only m/z 219 caused this ion to be the base
peak in (a), while m/z 69 was the base peak in (b). The ion intensities
for m/z 219 are essentially the same in both cases. Clearly, mass-
dependent optimization is preferred, since optimization at a single mass
will cause the sensitivity to be compromised at other masses.
There has been some concern that mass-dependent optimization would not
yield the correct ion intensity ratios for qualitative El spectra. This
is of particular concern, since El spectra are used for comparison with
standard library spectra. Indeed, the correct (and natural) ion
abundances are obtained only when there is no mass discrimination in the
ion optics. Since mass-dependent tuning provides maximum transmission for
each m/z, and therefore effectively removes any discriminating character
in the ion optics, the natural abundances of the ions are obtained.
Moreover, ion intensity ratios are affected by other parameters such as
ion source temperature and mass analyzer transmission characteristics;
therefore, any attempt to defocus a lens to obtain the 'correct' ratios
is not recommended. Although these studies have been performed on a
specific TQMS instrument, it is expected that the same mass-dependencies
would be observed with other instruments of similar ion optical design.

% Relative Abundance
(a)
100-
80-
60-
40-
20-
0-
219
1x10
69
131
i
414 5C
2
264
464
1
614
, M , j ,
J >1
..i .i .•
II — • -
. 1.1 .. i
L . - 1 -
. . 1
rv
100
200
' ' â–  i ' ' 1
300
^ 1
400
' ' 1 1 - ' '—
500
—r
600
i
700
(b)
69
I x10
502
4336000
m/z
Figure 5.5. El mass spectra of PFTBA: (a) lenses were optimized for transmission of only m/z 219 (i.
mass-dependent tuning), (b) mass-dependent lens optimization was used.
e., non-
Absolute Intensity

135
Preliminary results obtained by a coworker on a Finnigan MAT TSQ45 system
in this laboratory have suggested that this is the case.
Quadrupole Mass Filters
Operation. The basic operating principles of quadrupole mass
analyzers have been presented in a number of excellent review articles
[76,85-87]. The detailed characteristics of performance of these devices
are discussed in a book edited by Dawson [84]. In order to explain some
of the characteristic behavior of the ion optics, some basic theory of
operation of quadrupole mass filters is reviewed here.
In a quadrupole mass filter, RF and DC voltages are applied to a set
of four hyperbolic rods (one pair of rods being of opposite polarity to
the other pair) . These potentials are all referenced to a DC voltage
corresponding to the quadrupole offset voltage which defines the axial ion
kinetic energy in the quadrupole. The motion of ions within quadrupoles
is described mathematically by a second-order differential equation known
as the Mathieu equation [84],
d2u
+ (au ' 2% cos 2Ou - 0 (5.1)
d£2
The solutions of this equation define the position of an ion at any time
within the device, where u is a position-dependent parameter (representing
the x or y displacement of an ion) and ( is a time-dependent variable.
Ion trajectories within a quadrupole are either stable or unstable
depending on the values of the Mathieu parameters 'a' and 'q'. The
Mathieu parameters are given by

136
4VRFe
mr0V
a
u
8VDCe
mr 2w2
o
(5.2)
(5.3)
where VRf is the zero-to-peak RF voltage of angular frequency w, VDC is the
DC voltage, e is the charge and m is the mass of the ion, and rQ is the
inscribed radius of the quadrupole. An ion is said to have stable
trajectories if its excursions in the x or y directions do not exceed the
defined boundaries of the applied field. Conversely, unstable
trajectories represent conditions in which the displacement of the ion
increases without bound until it is eventually ejected and is not
transmitted through the device. Consequently, a stability diagram can be
drawn (shown in Figure 5.6) where 'a' and 'q' are plotted on the y and x
axes, respectively. Ions that have Mathieu parameters within the
triangular envelope of the stability diagram have stable trajectories;
those outside of the area are unstable. When operating as a mass filter,
RF and DC voltages are applied such that only a narrow range of masses
(usually one) have stable trajectories at any one instant in time, and
are transmitted to the detector. This is achieved by using a scan line
that slices through the very tip of the stability diagram. The range of
masses which will be stable is determined by the DC/RF ratio (i.e., the
slope of the scan line). In order to obtain a scan over a range of m/z's,
the RF and DC voltages are increased linearly with time (at constant
DC/RF) so that ions of increasing m/z are brought within the narrow window
of stability. Mass resolution is increased (with a reduction of the

STABILITY DIAGRAM FOR BOTH X a Y
Figure 5.6. Stability diagram for a quadrupole mass
filter (from reference [90]).
137

138
number of ions transmitted) by moving the scan line closer to the tip of
the stability region.
Origins of mass dependencies. As stated previously, only the lenses
near mass filters exhibited mass-dependent behavior. Lenses next to RF-
only quadrupoles were not found to be mass-dependent. The mass
dependencies in the lenses that are observed with this TQMS can most
likely be attributed to the fringing fields present at both ends of the
mass filters. In the TSQ70, the fringing fields at both ends of the
quadrupole mass filter may penetrate the immediate lens regions, thereby
affecting the efficiency with which ions may be transmitted. It has
previously been shown that fringing fields can cause ions to have unstable
trajectories as they attempt to enter or exit the quadrupole, which can
significantly reduce the number of ions transmitted [76], A number of
alternative ion optic designs have been employed in attempt to circumvent
the fringing field problem. One TQMS has been designed without focusing
lenses between adjacent quadrupoles [77,78]. Instead, the rod sets are
closely coupled such that the ions never leave one quadrupole field before
entering the next. The effects of fringe fields in single quadrupole
instruments have been shown to be reduced if the ions are allowed to
experience the RF field before experiencing the DC field. This has been
accomplished with either a short RF-only quadrupole [88] or dielectric
tube [83,89] placed at the entrance of the mass filter.
Alternatively, with the computer-controlled TQMS used in this work,
the effects of the fringing fields can be minimized with mass-dependent
optimization of the lenses. Since the lens voltages may be changed as a
function of mass, all ions may be transmitted with equal efficiency. Ion
transmission in quadrupole mass spectrometers has been shown to be a

139
function of the initial position and velocity of the ions as they are
injected into the mass filter [76,84], The initial conditions that result
in transmission define the ion acceptance of the mass filter. Theoretical
calculations have demonstrated that the optimum acceptance for an RF/DC
mass filter is obtained if the ions spend approximately two RF cycles in
the fringe field, assuming that the ion source/lens system emittance is
well-matched to the mass filter acceptance [76,84], If ions of different
mass are injected into the quadrupole at constant energy, all ions would
not spend the optimum time in the fringe field, which would result in mass
discrimination. For this reason, an optimum ion velocity for different
masses, instead of an optimum kinetic energy, is expected in cases where
fringing fields are present. The velocity (v) of an ion is given by
v = (2E/m)1/2 (5.4)
where E is the kinetic energy and m is the ion mass. Thus, as the mass
is increased, the kinetic energy must increase accordingly if the optimum
ion velocity is to be maintained. Indeed, most quadrupole mass
spectrometers include a programmed ion energy which permits the quadrupole
offset voltage (and hence the ion kinetic energy through the mass filter)
to be increased (most often linearly) with mass, in order to increase
transmission of high-mass ions [76,90], Unfortunately, increasing the
ion velocity through the quadrupole has the undesired effect of decreasing
the number of RF cycles the ion experiences and hence the resolution [76] .
Programming the lens potentials with mass, in contrast, can increase the
ion velocity of higher mass ions through the fringing fields without
affecting their velocity within the quadrupole. The calculation of an

140
optimum ion velocity is in agreement with the experimental results
obtained for mass-dependent lenses. In all cases, mass-dependent lenses
required higher absolute lens potentials and hence higher kinetic energies
for optimum transmission of higher mass ions. Table 5.1 shows the optimum
lens potentials and the calculated optimum ion velocities for ions of
different masses obtained with mass-dependent lens L13 in the Q1MS mode.
As shown in the table, the optimum lens voltage increases, but the optimum
ion velocity remains essentially constant. With a relatively low Q1
offset (ca. -2 V), one can roughly estimate the ions' average velocity
between L13 and Q1 as half the velocity calculated at the L13 potential;
hence, the optimum average velocity is -1.7 x 103 m/s. Given an
approximate distance of -0.5 mm between L13 and Ql, this corresponds to
a transit time of -0.85 ns, or approximately 1 cycle of the 1.2 MHz RF.
This is in good agreement with Dawson's theoretical optimum of 2 RF cycles
[76], considering the rough estimates that were made for the average ion
velocity and the distance between L13 and Ql. The low-mass ions (m/z 69
and m/z 131) have slightly higher optimum velocities, which is not fully
understood. However, this may be attributed to an additional energy
spread induced by the fringing fields. It is expected that computer
simulation of the ion trajectories with a program such as SIMION [91] may
eventually help to elucidate this behavior.
Ion Transmission Characteristics of the Center Ouadrupole
Low mass cut-off. In triple quadrupole mass spectrometry the center
quadrupole operates in the RF-only mode. It is a popular misconception
that an RF-only quadrupole is capable of passing all masses
simultaneously. However, transmission of ions through an RF-only

141
Table 5.1. Optimum lens potentials and ion velocities for various masses
obtained with mass-dependent lens L13 in the Q1MS mode.
m/z
Optimum Lens
Potential
(V)
Optimum Ion
Velocity
(m/s)
69
-7.5
4.6 x 103
131
-10.4
3.9 x 103
219
-14.0
3.5 x 103
414
-23.0
3.3 x 103
502
-29.5
3.4 x 103
614
-36.5
3.4 x 103

142
quadrupole is subject to the same constraints as an RF/DC quadrupole. The
difference is that since the applied DC voltage is zero, a = 0, and the
scan line follows the x-axis (q-axis) of the stability diagram in Figure
5.6 [80,92]. Thus, for an ion to be stable it must have a value of 'q'
between 0 and 0.908. Since there are a range of q's over which a
particular mass is stable, it follows from eq 5.2 that there is also a
range of RF voltages over which a particular mass is stable. This means
that proper choice of the RF level is critical to avoid low mass cut-off.
Low mass cut-off occurs when the RF voltage has been set high enough such
that ions below a certain m/z have q's greater than 0.908, and are not
transmitted. In addition, since q is constrained to values between 0 and
0.908 for stable trajectories, it can be seen from eq 5.2 that the range
of RF voltages allowing stable ion trajectories decreases linearly with
the mass of the ion.
Figure 5.7 is a demonstration of the principles discussed above. In
this experiment, ion intensities for the daughters from the collisionally-
activated dissociation (CAD) of m/z 219 from PFTBA are plotted against Q2
RF voltage. As shown in the figure, there is an optimum RF voltage for
the transmission of each ion of different m/z; thus, the Q2 RF voltage is
a mass-dependent parameter as well. Also note that the transmission
curves become more narrow with decreasing m/z as discussed above. This
brings forth a fundamental limitation of the second quadrupole. In single
MS experiments (i.e., Q1MS or Q3MS), only one m/z is passed through Q2 at
any one instant in time. Thus, the Q2 RF voltage can be optimized and
programmed with mass to obtain the maximum transmission of ions of each
different m/z. However, during MS/MS scans, Q2 is required to
simultaneously transmit parent ions, as well as daughter ions of different

Relative
Ion Intensity
100%
o
«-Increasing RF Voltage
Figure 5.7. Transmission of parent ion (m/z 219) and resulting daughter ions (m/z 69, 131) of PFTBA with Q2
RF voltage illustrating mass-dependent optimum RF voltages. The RF voltage corresponding to a q-value of 0.2
for the parent ion is indicated by the arrow.
143

144
m/z. Since different m/z ions have different optimum RF voltages, maximum
transmission of all ions during MS/MS may not always be possible. For
this reason, the RF voltage must be optimized in a manner to accomplish
the goals of the particular experiment of interest. For example, if good
qualitative daughter spectra are desired, then the RF voltage is optimized
for a low m/z daughter ion such that all of the resulting daughter ions
could be observed. For quantitative work, where only one or a few
daughter ions are monitored, the RF voltage is optimized such that maximum
sensitivity is obtained for the particular parent ion-to-daughter ion
transition under study.
In the TSQ70, the Q2 RF level is automatically set by the level chosen
for the parent ion in an MS/MS experiment. The default setting of the Q2
RF level corresponds to a q-value of 0.2 for the parent ion. Rarely does
this default setting correspond to the optimum for the observation of
daughter ions. In fact, as shown in Figure 5.7, setting the RF level for
the optimum for the parent ion m/z 219 (q = 0.21) severely limits the
transmission of daughter ions of lower mass. The lowest m/z ion that will
have a stable trajectory in the center quadrupole, (m/z) cut-0ff > i-s
determined by the m/z of the parent ion, (m/z)p, and the q-value of the
parent ion, qp, as shown by eq 5.5.
9p(m/z)p
(m/z> cut-off (5.5)
0.908
If the q-value of the parent is chosen to be 0.2, then eq 5.5 reduces to
(ra/z> cut-off = 0.220(m/z)p
(5.6)

145
As can be seen in Table 5.2, with increasing m/z of the parent ion, the
lower mass limit may prevent useful daughter ions from being observed.
This is demonstrated in Figure 5.8 which shows the intensities of the ions
obtained with CAD of the m/z 502 ion of PFTBA as a function of Q2 RF
voltage. The RF voltage corresponding to a q-value of 0.2 for the parent
ion (m/z 502) is indicated in the figure. If a daughter spectrum was
acquired with the default q-value of 0.2, then the lowest m/z ion that
could be observed would be m/z 110 (Table 5.2); thus, the m/z 69 ion would
not be present in the daughter spectrum. As shown in Figure 5.8,
operation at a q-value of 0.13 for the parent ion would include all of the
daughter ions and would yield more complete structural information. This
is not to say that all daughter experiments should be operated at a q-
value of 0.13, rather, intelligent choice of q should be made based on
the experimental conditions being used. By plotting the ion transmission
curves as a function of RF voltage, the choice of an appropriate RF level
certainly becomes more apparent.
The appearance of abundant fine structure (or beat patterns) in plots
of parent ion transmission versus Q2 ion kinetic energy or Q2 RF voltage
have also been reported [74,80,92]. An example of this behavior (although
perhaps not as dramatic as that observed by others [80,92]), can be seen
by observing the transmission of the parent ion (m/z 502) in Figure 5.8.
These beat patterns are due to the transverse periodic (sinusoidal)
trajectories of the parent ion beam and to the small size of the exit
aperture of the collision cell (usually required to attain suitable
pressures for CAD). If the trajectories of the parent ions correspond to
an integral number of half-cycles, the parent ions will exit Q2 close to

146
Table 5.2. Lower m/z limit of the daughter (DG) ion possible with the RF-
voltage applied to Q2 to yield a q-value of 0.2 for the parent ion.
m/z Parent Ion
69
31
219
264
414
502
614
652
2000
4000
Lowest m/z DG Ion
15
29
48
58
91
110
135
143
440
880

Relative Ion Intensity
«-Increasing RF Voltage
Figure 5.8. Ion transmission as a function of the Q2 RF voltage for the daughter ions from the CAD of m/z 502
of PFTBA. Note that with the default parent ion q of 0.2, the m/z 69 ion would not be observed in the daughter
spectrum. Instead, operation at a parent ion q of 0.13 would yield the complete daughter spectrum.
147

148
the axis, and thus a maximum in parent ion transmission is observed.
However, if the parent ion beam exits Q2 during the midpoint of a half-
cycle, some of the ion excursions exceed the size of the exit aperture of
the collision cell, and thus a minimum in transmission is observed. The
beat patterns are not normally observed for daughter ions, since they are
formed along the entire length of the collision cell [80]. However, this
behavior does make the comparison of parent-to-daughter ion ratios rather
unpredictable and in many cases useless, due to the non-uniform
transmission of the parent ion. For this reason, other types of collision
cells (e.g., hexapoles and octapoles) have recently been investigated,
both for the purposes of obtaining uniform ion transmission as well as
increasing the range of masses which can be simultaneously transmitted
through the collision cell [93-95].
Effects of ion energy. In MS/MS experiments one of the most important
parameters for controlling the degree of fragmentation during CAD is the
collision energy. The collision energy is varied directly by adjusting
the quadrupole offset voltage on Q2 (i.e., a quadrupole offset voltage of
10 V results in 10 eV ion kinetic energy in Q2). Because of the
importance of the collision energy for MS/MS, the effects of this
parameter on ion transmission through the second quadrupole were studied.
It has been shown that the optimum RF voltage is mass-dependent (Figure
5.7); thus, it was anticipated that the optimum RF voltage might be
kinetic energy-dependent as well. This is demonstrated in Figure 5.9,
which illustrates ion transmission as a function of the Q2 RF potential
for three different Q2 offset voltages. In this experiment, Q1 was set
to pass argon ions (Ar+, m/z 40), Q2 contained no collision gas (base
pressure -1 x 10‘6 torr) , and Q3 was scanned from m/z 37.5 to m/z 42.5.
As seen in the figure, the optimum RF level increases with increasing Q2

Transmission of Ar (m/z 40) vs. Q2 RF Voltage
Figure 5.9. Ion transmission of Ar+ (m/z 40) with Q2 RF voltage, illustrating that the optimum RF voltage is
dependent on the Q2 offset voltage (collision energy) that is used.
149

150
offset voltage. This behavior can most likely be attributed to the bent
geometry of Q2. For a non-linear quadrupole, it has been shown that ions
experience an outward force causing them to be displaced from the
quadrupole axis. The magnitude of the displacement, AX, is periodic and
is dependent on the radius of curvature of the quadrupole axis, R [79].
16E
(1-cos(q£/2%)
(5.7)
AX =
The ions are eventually lost and transmission decreases if AX becomes too
large. At higher ion kinetic energies, higher RF voltages (higher q-
values) are required to contain the ions through the bend of the
quadrupole. This relationship and the data that are presented above
demonstrate the necessity to optimize the Q2 RF voltage for the particular
Q2 offset voltage that is to be used.
Figure 5.9 indicates that the ion containment efficiency in Q2 at a
particular RF voltage is dependent on the ion axial energy. This energy
dependence may also explain in part why different mass daughter ions have
different optimum RF voltages (Figure 5.7). Upon CAD of the parent ion,
the kinetic energy of a newly formed daughter ion, EDG, is given by [78,80]
EDG = epHg/V
(5.8)
where Ep is the kinetic energy of the parent ion (which is equal to the
collision energy), m^ is the mass of the daughter ion, and mp is the mass
of the parent ion. Since different mass daughter ions have different

151
kinetic energies, it is not surprising that the optimum RF amplitude is
mass-dependent.
In CAD experiments, the masses of the daughter (product) ions are of
lower mass than the parent ion. This is in contrast to the mass-selected
reaction mode discussed previously in Chapter 4. In mass-selected
reactions (MSRs), the product ions are typically of higher mass than the
parent (reactant) ions. The range of RF voltages that result in stable
trajectories for low-mass ions is very narrow (eq 5.2). Since the
reactant ions (typically low mass) must be efficiently contained in Q2 for
the product ions to be observed, proper choice of RF level is critical.
This is illustrated in Figure 5.10, which shows ion transmission as a
function of Q2 RF voltage for the C2H5+ reactant ion and the resulting
product ions formed by reaction of this ion with PFTBA (m/z 502 and m/z
652) in Q2. Note that benzene from a previous experiment was also present
in Q2, which resulted in the formation of m/z 277 ([C4F9+C6H6-HF]+) and m/z
105 ( [C6H6+C2Hj]+) . As shown in the figure, product ions are observed only
when the reactant ion is transmitted. In addition, the optima for maximum
transmission for different m/z product ions occur at the same Q2 RF
voltage. This might be attributed to the fact that the product ions are
formed with little or no axial kinetic energy, since they were produced
by proton transfer from neutral molecules in Q2. The product ions do
eventually gain kinetic energy and are drawn out of Q2 by the ion exit
lens. If a constant ion extraction potential is used, the product ions
have essentially the same range of kinetic energies; thus, different mass
product ions would be expected to have the same optimum RF voltage. This
is in contrast to the results obtained for the daughter scan mode in which
ions of different mass have different kinetic energies and exhibited
different optimum Q2 RF voltages.

Relative
Product Ion Intensity
100%
0
100%
0
Figure 5.10. Ion transmission as a function of Q2 RF voltage for the C2H5+ (m/z 29) reactant ion and the
resulting product ions formed by reaction of this ion with PFTBA and benzene in Q2.
Reactant Ion Intensity

153
Ion Transmission at High Collision Energies
Many compounds of analytical concern (e.g., polycyclic aromatic
hydrocarbons) form very stable ions that are particularly difficult to
fragment during MS/MS. For example, daughter ions for the CAD of the
molecular ion of pyrene are not observed at all with collision energies
less than 100 eV [80] . Even at high collision energies and collision gas
pressures, the yield of daughter ions from these stable parent ions can
be very low. Therefore, proper optimization is crucial for efficient
detection of the low-abundance daughter ions. In addition, it has been
found that operation at collision energies above approximately 50 eV
requires some special considerations for adjustment of the ion optical
parameters. After tuning for the Q1MS and Q3MS scan modes, the lens
voltages obtained are used in the MS/MS scan modes. For MS/MS scans, all
ion optical parameters prior to Q2 (i.e., L11-L23) are obtained from those
optimized during Q1MS tuning. The ion optics following Q2 (i.e., L31-L42)
are taken from those optimized during Q3MS tuning. Most of the lenses
behave in the MS/MS modes as they do in the single MS scan modes; thus,
no special optimization for these lenses is required. However, it has
been found that lenses on either side of Q2 (L23 and L31) play a key role
in the transmission of ions during MS/MS, especially when collision
energies above 50 eV are used. This is shown in Figure 5.11 for the case
of L23 for the 100 eV CAD of m/z 219 from PFTBA. Ion transmission is
improved dramatically if the L23 voltage is made at least as negative as
the Q2 offset voltage. In fact, as shown by the inset of Figure 5.11
there is a linear relationship between the optimum L23 voltage and the Q2
offset voltage. It should be noted that the step increase in ion
transmission is nearly insignificant at smaller Q2 offset voltages (i.e.,

Relative
Ion Intensity
Figure 5.11. Ion transmission as a function of the lens L23 voltage for the 100 eV CAD of the m/z 219 fragment
ion of PFTBA. The inset of this figure illustrates the linear relationship between the optimum L23 voltage
and the Q2 offset voltage.
154

155
more positive than ca. -50 V). The causes for this apparent behavior are
discussed in more detail below.
Figure 5.12 shows the transmission of m/z 40 from Ar+ in the daughter
scan mode as a function of collision energy. Curve A in Figure 5.12 was
obtained using ion optical parameters that were found to be optimum with
a collision energy of 10 eV. As shown in the figure, ion transmission
rapidly diminishes if the Q2 offset is made more negative than
approximately -50 V. It was found that this loss in transmission that
occurs is due to two major factors. The first is the aforementioned
dependence of optimum Q2 RF voltage on collision energy. The second is
ion entrance and exit conditions that exist when the Q2 offset is made
more negative than ca. -50 V. Typically, the lenses are optimized at
relatively low Q2 offset voltages (e.g., -10 V). Thus, when MS/MS is
performed, the ion beam becomes defocused if the Q2 offset is made more
negative than -50 V. However, ion transmission through Q2 can be sig¬
nificantly improved by optimizing the Q2 entrance and exit lenses (L23 and
L31) for the collision energy being used. Trace B of Figure 5.12
illustrates the transmission curve for Ar+ that is improved significantly
after proper optimization of these lenses and the Q2 RF voltage had been
performed at each collision energy. The capability to obtain uniform ion
transmission over a range of kinetic energies is especially important when
studying the energetics of the CAD process. These data demonstrate that
the effects of the various instrumental parameters should be taken into
consideration if the collision energy is varied over a wide range,
particularly if energies above ca. 50 eV are used.

Transmission of Ar (m/z 40) vs. Q2 Offset Voltage
Figure 5.12. Ion transmission of Ar+ with Q2 offset voltage. For (a), the ion optical parameters used were
those found to be optimum for 10 eV collisions. For (b), lenses and the Q2 RF voltage were optimized at each
collision energy.
156

157
Conclusions
The ion transmission characteristics of a computer-controlled TQMS
have been studied for MS and MS/MS operational modes. Studies that have
traditionally been laborious and time-consuming with manually-controlled
instruments can be rapidly performed with the computer-controlled TSQ70.
The results of these studies have provided a better understanding of the
effects of the instrumental parameters.
It was found that ion optical lenses positioned close to quadrupole
mass filters exhibited mass-dependent transmission behavior if fixed lens
potentials were used. These mass dependencies were attributed to the
presence of fringing fields that exist near the quadrupole rods carrying
both RF and DC voltages. The mass dependencies can be eliminated if the
lens voltage is programmed with the mass scan, as can be done with the
TSQ70. As a result, optimum transmission can be obtained (at least for
single MS scans) over the entire mass range.
Ion transmission in MS/MS experiments was shown to be particularly
dependent on the characteristics of the second quadrupole. Ion
confinement in the second quadrupole at a particular RF voltage was shown
to be dependent on the ion kinetic energy. When CAD was performed in Q2,
transmission of daughter ions at a given RF voltage was found to be mass-
dependent. In contrast, product ions of mass-selected reactions in Q2
were found to have the same optimum RF voltage, regardless of the ion
mass. The effects of the ion energy on transmission was found to be more
pronounced at high collision energies (i.e., above 50 eV). Part of this
energy-dependent behavior can be attributed to the bent design of the
second quadrupole. Ions traversing Q2 at high axial energies require
higher RF amplitudes to be efficiently transmitted around the bend. The

158
lenses on either side of Q2 also have a dramatic effect on ion
transmission at high collision energies. All of these parameters must be
considered if results obtained over a wide range of collision energies are
to be compared.

CHAPTER 6
CONCLUSIONS AND FUTURE WORK
Conclusions
The research reported in this dissertation has demonstrated extensions
to the analytical capabilities of GC/MS, and GC/MS/MS, particularly in GC
sample introduction and ion transmission. These research efforts have led
to a better understanding of the fundamental concepts in both of these
areas. These types of studies are particularly significant considering
the growing popularity of MS/MS and the widespread availability of
commercial instrumentation for its use.
It was demonstrated that vacuum outlet operation in open tubular gas
chromatography results in a significant increase in the optimum carrier
gas velocity when compared to atmospheric outlet operation. The increase
was shown to be more prevalent with short and/or wide-bore open tubular
columns, both of which are attractive for use in GC/MS and GC/MS/MS due
to the inherent selectivity available with mass spectrometry. Theoretical
calculations have been an invaluable aid in understanding the effects of
low operating pressures on open tubular columns. The advantages of
operating the column inlet as well as the column outlet at vacuum have
been stressed. The use of sub-ambient inlet pressures with short columns
allows optimum chromatographic performance to be obtained. In addition,
lower inlet pressures result in lower carrier gas flow rates, allowing the
maximum sensitivity to be obtained when electron ionization is used.
Reduced inlet pressures and flow rates have allowed short wide-bore
159

160
columns to be used with the GC probe and have permitted short columns to
be interfaced to the collision cell of a TQMS without exceeding the normal
operating pressure of the instrument.
Alternatives to conventional oven heating and temperature programming
in gas chromatography were demonstrated with exponential flow rate
programming and direct resistive column heating. Direct resistive heating
of metal-clad open tubular columns appears to exhibit tremendous potential
for applications requiring portable instrumentation due to the low-power
requirements and the simplicity of the electronics required for
temperature control. Direct resistive temperature sensing was shown to
be an accurate method of determining the temperatures of the metal-clad
columns. It appears that this method would be applicable to the direct
measurement of column temperatures in chromatograph ovens as well.
Moreover, the rapid heat-up and cool-down rates of directly-heated columns
makes them attractive for performing high-throughput analysis in the
laboratory. These concepts were demonstrated with the compact GC probe
and associated electronics that were designed for use in GC/MS.
A new technique, GC/MSR/MS, was described, which involves mass-
selection of a single m/z reactant ion and reaction of this ion with the
effluent from a GC column which is introduced into the center quadrupole
collision cell of a triple quadrupole mass spectrometer (TQMS). One of
the advantages of mass - selected reactions (MSRs) is the enhanced
selectivity that is obtained by the selection of a single m/z reactant ion
for reaction with sample molecules via charge exchange, proton transfer,
or adduct formation chemical ionization. In addition, it was shown that
both molecular weight and structural information are readily obtained in
the same chromatogram by alternating between two (or more) reactant ions.

161
The combination of short open tubular GC columns with MSRs allows for
highly selective and rapid mixture analyses to be performed. The
GC/MSR/MS technique exhibits considerable promise, since the selectivity
of the technique is limited only by the choice of specific ion-molecule
reactions. The potential application of this technique to selective
negative ion reactions should also add to the attractiveness of the
method.
A thorough study of the ion transmission characteristics of a
computer-controlled TQMS has been performed. It was shown that
electrostatic lenses positioned on either side of quadrupole mass filters
exhibited mass-dependent behavior. The origins of the mass dependencies
were attributed to the fringing fields imposed by the mass filters. The
ion transmission behavior of the second quadrupole (Q2) was also studied.
The importance of selecting the proper Q2 RF voltage for obtaining both
qualitative and quantitative results was stressed. The inter-relation of
the MS/MS ion optical parameters was found to be very complex. The best
transmission efficiencies in MS/MS experiments were obtained when the
appropriate parameters were optimized at the collision energy that was
used. Finally, the importance of computer control for investigating the
ion optical behavior and correcting for undesirable effects (e.g., mass-
dependencies) is evident from these studies.
Suggestions for Future Work
Reduced Column Pressures with Conventional GC Detectors
The advantages of low column pressures are readily obtained with the
GC/MS combination; however, low-pressure operation of conventional GC
detectors has not been previously investigated. The commonly used flame

162
ionization detector (FID) and electron capture detector (ECD) could be
evaluated for low-pressure operation. As was shown in Chapter 2, high-
vacuum operation (e.g., <1 torr) may not be necessary. In fact, for a 3
m x 0.25 mm i.d. open tubular column, operation at an outlet pressure of
100 torr was predicted to yield nearly the same chromatographic results
as operation with an outlet pressure of 1 torr.
The effects of the low-pressures on the hydrogen flame of the FID
have not been previously studied, thus it is not known if the hydrogen
flame of the FID would remain lit at the low pressures. Perhaps the ECD
would be more easily evaluated due to its simplicity. The ECD employs a
Ji-emitting 63Ni foil which acts as a source of thermal electrons at
atmospheric pressure. However, operation of the ECD at the low pressures
might inhibit the thermalization of the Ji particles. These problems would
have to be addressed and the effects of the operating pressure on the
sensitivity of these detectors would have to be evaluated if vacuum
operation is to be a attractive alternative to conventional operation.
Directly-heated Open Tubular Columns
There are several improvements that could be made to the existing
technology in addition to other possibilities that have yet to be explored
with directly-heated capillary columns. It is expected that heating rates
would be significantly increased if the glass-braid insulation were
removed from the column. If this were to be done, a new column spool
would have to be designed that would prevent electrical shorting of
successive coils of the column. Improvements in the design of the power
supply and associated electronics might also be advantageous. For
example, application of proportional-integral-differential feedback

163
control of the column temperature and an autoranging circuit for
measurement of the column resistance is expected to significantly improve
the characteristics of direct resistive heating.
Smaller diameter columns could also be applied in the GC probe to
yield better chromatographic resolution for a given column length than the
wide-bore columns. This would not require a new design for the injection
port. Rather, a wide-bore column could be used as a pre-column and would
serve as the injection port for the narrow-bore column. In order for this
to be feasible, a low thermal mass column coupling would have to be
designed to electrically connect the two columns without creating a cold
spot. Another advantage of the smaller bore columns is that they can be
coiled to a smaller diameter, which would allow further reduction in the
size of the GC probe or other portable GC instrumentation.
It might also be possible to take advantage of the Peltier effect for
the rapid heating and cooling of metal-clad open tubular columns [38].
The feasibility of utilizing Peltier heating and/or cooling in open
tubular GC was speculated (but never demonstrated) at the University of
Florida in a discussion between S. 0. Colgate, R. A. Yost, and this
author. With the Peltier effect, thermoelectric heating or cooling occurs
if a voltage drop is placed across the junction of two dissimilar metals
(i.e., a thermocouple). Depending on the direction and magnitude of the
current, the junction can be made to heat or cool. Therefore, it should
be possible to coat fused silica columns with two different metals for the
purposes of thermoelectric temperature control. If the appropriate metals
were chosen for coating, the column itself could be used as a
thermocouple, allowing conventional commercially-available temperature
sensing electronics to be used for measuring the column temperature.

164
Perhaps the real challenge for this project would be devising a suitable
method of coating the desired metals on the columns. Recently, stainless
steel tubing with an inner surface of fused silica has become commercially
available (Restek Inc.). The manufacturers claim that the fabrication
process for this tubing involves depositing the fused silica inside the
stainless steel tubing with a chemical reaction, instead of depositing the
stainless steel on the outside of the fused silica. If this same process
were employed, it should be possible to fabricate virtually any type of
column with metallic coatings for thermoelectric temperature control.
Resonance Excitation and Ion Storage in 02
There are a number of different processes that could be investigated
that might enhance the characteristics of MS/MS in triple quadrupole mass
spectrometry. One process that warrants evaluation is resonance-enhanced
collisionally-activated dissociation (RECAD) in the center quadrupole
collision cell. Typically, the degree of fragmentation of the CAD process
in a TQMS is controlled by varying the parent ion axial kinetic energy in
Q2. It was shown in Chapter 5 that operation at high collision energies
usually requires special considerations for tuning. Nevertheless, in most
cases, operation at high axial collision energies results in reduced
transmission efficiencies. An alternative, and perhaps more efficient
method of collisional activation would be to increase the parent ion
oscillatory (instead of axial) kinetic energy via resonance excitation.
To perform RECAD, a supplementary RF voltage could be applied to the
quadrupole rods of Q2 at the fundamental resonant frequency of a
particular m/z ion. Application of the RF voltage at the correct
frequency and amplitude would increase the orbital velocity (and thus the

165
kinetic energy) of the ion of interest. Fragmentation would be induced
via collisions of the resonance-excited ion with neutral gas molecules.
The RECAD process is already being used in the ion trap mass spectrometer
(ITMS) and has proven to be a very efficient method of performing CAD
[96], Resonance excitation has also been used with an RF-only quadrupole
operated as a "notch filter" [97], In this mode, the supplementary RF
amplitude is set high enough such that the oscillations of a selected ion
become large and the ion is ejected from the quadrupole field. The notch
filter mode might prove to be useful for ejecting undesirable reactant
ions that are formed in Q2 during mass-selected ion-molecule reaction
experiments. For example, in Chapter 4 it was stated that even with mass-
selection of the protonated acetone dimer (m/z 117) with Q1, protonated
acetone (m/z 59) was formed (either by metastable decomposition or CAD)
in Q2. Application of resonance excitation might be used to eject the m/z
59 ion, and thereby further increasing the selectivity of mass-selected
reactions.
There are a number of considerations that would have to be addressed
if RECAD were to be successfully employed. One reason for the inherent
efficiency of RECAD in the ion trap is the long ion storage times
(hundreds of milliseconds) that are possible [96], In contrast, the
residence time of ions in the center quadrupole is on the order of
microseconds. The residence time, t, of an ion in Q2 is given by
t = L(m/2E)1/2 (6.1)
where L is the collision cell path length, m is the ion mass, and E is
the ion axial kinetic energy. Using this equation, an ion with a mass of

166
100 amu and 10 eV of axial energy has a residence time of 32 /is in the
collision cell of the TSQ70 (L - 14 cm). A residence time this short may
not be long enough to allow efficient RECAD to occur. It may be possible
to store ions in Q2 to increase their residence times. It has previously
been shown that the ion lenses on either side of the second quadrupole
collision cell can be used as "electrostatic mirrors" to store and pulse
the ions out of the collision cell [60-62,98]. Ion storage in Q2 might
also be attractive for increasing reaction times for ion-molecule
reactions. The TSQ70 is well-suited for these types of studies, since all
ion optical parameters are under computer control.

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BIOGRAPHICAL SKETCH
Mark Edward Hail, son of Carl W. Hail and Catherine C. Hail, was born
in Lexington, Kentucky. Mark lived in Ashland, Kentucky, until the age
of 12. The Hail family moved to Shelbyville, Kentucky, in 1975. He
graduated from Shelby County High School in 1980.
In the fall of 1980 Mark enrolled at the University of Kentucky (UK)
in Lexington. In the spring of 1983 he joined Lambda Chi Alpha fraternity
and during the next year served as president of the local chapter. He
spent the following two summers performing undergraduate research under
the direction of Dr. F. James Holler. Mark received two fellowships
during these two summers, including the Stephen Harris Cook Memorial and
an Ashland Oil fellowship. Under Dr. Holler's guidance, he developed a
strong interest in analytical chemistry and chemical instrumentation.
Mark graduated from UK in the spring of 1984.
In August 1984, Mark began his graduate work in analytical chemistry
under the direction of Dr. Richard A. Yost. On July 30, 1988, Mark
married Amy Marianne Duffy, a talented musician (flutist) whom he has
known since high school. Upon completion of his graduate studies, he will
begin work in the engineering division of Finnigan-MAT Corporation in San
Jose, California.
173

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. Winefordner'
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.
A.
Anna
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.
Samuel 0. Colgate
Associate Professor of Chemistry

I certify that I have read this study and that in ray 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.
Henri A. Van Rinsvelt
Professor of Physics
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.
May 1989
Dean, Graduate School

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