448 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry

Detectors: Mass Spectrometry

M. R. Clench and L. W. Tetler, School of Science and tion (EI) ion source. A schematic of a typical EI ion
Mathematics, Sheffield Hallam University, source is shown in Figure 1.
Sheffield, UK The Rlament, usually a simple coil of tungsten wire,
Copyright ^ 2000 Academic Press is heated. On heating it produces electrons, which are
then accelerated into the ion source chamber by ap-
plying a potential difference between the Rlament and
Introduction the chamber. This potential difference is usually in
the range 50}70 V, giving the electrons a kinetic en-
The mass spectrometer provides the most powerful ergy (the electron energy) of 50}70 eV (where
detector available for gas chromatography. It is sensi- 1 eV"1.602;10\19 J). Interaction of neutral
tive, selective and offers vastly superior qualitative sample molecules with the electrons causes ionization
information over conventional detectors such by removal of one electron:
as the Same ionization detector (FID) or electron-
capture detector (ECD). Modern instruments for gas M#e\PM#z#2e\
chromatography}mass spectrometry (GC-MS) are
small, reliable and much less expensive than formerly. This process creates the positively charged molecular
In many laboratories small ‘bench-top’ GC-MS in- ion of the sample molecule, i.e. a radical cation.
struments have virtually replaced ‘stand-alone’ gas However, the Rrst ionization energy of most organic
chromatographs for even routine applications. In this compounds is only of the order of 10 eV. Hence
short paper we shall attempt to describe some of the molecular ions formed in an EI ion source have excess
basic principles of mass spectrometry and how they internal energy and further fragmentation occurs in
are applied in GC-MS. In order to illustrate some of order to dissipate this energy. Fragmentation occurs
the techniques available the determination of nitrated via a variety of processes and leads to mass spectra
polycyclic aromatic hydrocarbons in diesel partic- containing a Rngerprint of the molecule. The pro-
ulates will be utilized as a case study. cesses of fragmentation are shown below:

M#zPF#
1 #R
z
Ion Formation
Mass spectrometers are used to analyse ionized and/or:
sample molecules. There are essentially four methods M#zPF#z
2 #N
in which a neutral sample molecule (M) can be con-
verted into an ionic species: (where F# 1 represents an even electron fragment ion,
Rz a neutral radical, F#z
2 an odd electron fragment
M#e\PM#z#2e\ (removal of an electron) ion, often called a radical ion, and N a neutral
species).
M#e\PM\z (addition of an electron)

M#X#P(M#H)# (addition of a positively

charged species,
usually a proton)

M!X#P(M!H)\ (removal of a positively

charged species,
usually a proton)

The ion sources used in GC-MS make use of each of

these four processes in order to form positive or
negative ions as appropriate.
Electron Ionization (EI) Figure 1 Schematic of an electron ionization (EI) ion source.
Ions formed by interaction of the sample molecules with electrons
The most important method for the production of emitted from the filament are extracted and focused into the mass
ions in GC-MS instruments uses the electron ioniza- analyser by the action of the repelller and the focusing lens.
 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry 449

Both types of initial fragment ions may also further Electron ionization is the most widely used ioniz-
fragment: ation technique for GC-MS. However, it has a num-
ber of limitations. The most important of these is
F# #
1 PF3 #N caused by the excess internal energy of the initially
and/or: formed molecular ions. For certain classes of com-
pounds, they all fragment in the ion source and hence
F#z #
2 PF4 #R
z
a molecular ion is not observed in the recorded mass
spectrum. This removes one of the key pieces of
Fragmentation will continue until the excess internal information from the mass spectrum, i.e. the relative
energy is dissipated. The appearance of EI mass molecular mass of the compound under investigation.
spectra is a function of the compound under invest- In order to overcome this, other ionization techniques
igation, the electron energy used and the ion source are available to the mass spectroscopist, the most
temperature. For this reason it is usual to record EI important of these being chemical ionization (CI).
mass spectra at an electron energy of 70 eV which
gives good sensitivity, interpretable fragmentation Chemical Ionization (CI)
and allows comparison to be made between spectra
Positive ion chemical ionization In positive ion CI
recorded on different instruments and with standard
an ion source slightly modiRed from that shown in
spectra stored in computerized libraries.
Figure 1 (by reduction of the size of the ion exit
An EI mass spectrum of 2-nitroSuorene, a nitrated
aperture) is Rlled with a reagent gas (e.g. methane,
polycyclic aromatic hydrocarbon, is shown in
isobutane or ammonia) to a pressure of about
Figure 2A. This mass spectrum illustrates some of the
0.1}1.0 mbar. At this pressure ion}molecule reac-
key features of EI spectra. A small molecular ion can
tions can occur between ions of the gas (created by EI
be seen at m/z 211 along with fragment ions corre-
processes) and neutral gas molecules. Taking as an
sponding to the loss of zOH and NO2 groups. The
example some of the processes that occur when meth-
pattern of fragment ions, i.e. their intensity and distri-
ane is used as a reagent gas:
bution is characteristic of 2-nitroSuorene and library
search, used where possible in combination with GC
retention time (obtained from a standard sample), nCH4#ne\PnCH#z # #
4 #nCH3 #nCH2 #nCH
#

allows the sample to be easily identiRed.

nCH4#nCH#z # z
4 PnCH5 #nCH3

nCH4#nCH# #
3 PnC2H5 #nH2

In a similar way when sample molecules are intro-

duced into the ion source, ion molecule reactions
between reagent gas ions and gaseous sample mol-
ecules can occur, to produce sample ions, i.e.

nM#nCH# #
5 Pn(M#H) #nCH4
proton transfer

nM#nC2H# #
5 Pn(M#H) #nC2H4
proton transfer

nM#nC2H#
5 Pn(M#C2H5)
#

electrophilic addition

The formation of sample ions via these reactions is

much less energetic than molecular ion formation via
electron ionization. Hence the mass spectra obtained
show less fragmentation than the corresponding EI
mass spectra. When methane or isobutane are used as
Figure 2 Comparison of (A) electron ionization, (B) positive
chemical ionization and (C) negative chemical ionization mass the reagent gas, proton transfer is the dominant reac-
spectra of 2-nitrofluorene. Note the higher degree of fragmenta- tion. Hence the relative molecular mass of the com-
tion in the EI mass spectrum. pound of interest can now be derived from the
450 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry

n(M#H)# protonated molecular species with addi- In a similar manner to the use of the electron-
tional conRrmation of the assignment being given by capture detector for gas chromatography, the use of
the presence of the (M#C2H5)# adduct ion. Where electron capture NCI GC-MS can introduce sensitiv-
ammonia is used as the reagent gas electrophilic addi- ity and speciRcity into an analysis. Whereas approx-
tion is often as important or the dominant process imately 100 pg of sample are required to record a
and in this case the (M#NH4)# adduct ion may be mass spectrum in EI mode, NCI spectra have been
used. recorded from a little as 500 fg (for appropriate elec-
Figure 2B shows the positive ion chemical ioniz- tron capturing compounds). This will be further illus-
ation mass spectrum of 2-nitroSuorene obtained us- trated below.
ing methane as reagent gas. Note the large (M#H)#
peak at m/z 212 and the reduced fragmentation com-
pared to the corresponding EI spectrum. Also visible The Separation of Ions and
is the adduct ion at m/z 240 corresponding to the Recording of Mass Spectra
(M#C2H5)# ion formed by the electrophilic addi-
There are many methods available for the separation
tion process discussed earlier.
of ions and recording of mass spectra. The ionization
methods described above have been incorporated into
Negative ion chemical ionization (NCI) Chemical
all of the current commercial types of mass spectro-
ionization is also a useful way of producing negative-
meter. In this section only brief descriptions of these
ly charged species for mass spectrometry. There are
are offered. For a more complete discussion see either
two important mechanisms for ion formation in NCI.
Chapman (1993) or Johnstone and Rose (1996).
The Rrst, which is analogous to the processes already
The key parameters to take into account in the
described for positive CI, is proton transfer:
selection of a particular type of mass spectrometer for
a GC-MS experiment are the masses of the com-
M#B\P(M!H)\#BH
pounds under consideration and the selectivity and
sensitivity required for the analysis. Where the largest
This type of reaction will occur when the relative
compounds to be encountered are likely to have
proton afRnity of the reagent gas anion (B\) is high. It
a relative molecular mass of less than 1000 any of the
is a relatively low energy process and leads to mass
types of mass spectrometer described below is useful.
spectra containing intense (M!H)\ ions and little
fragmentation.
The Quadrupole Mass Filter
However, a more important mechanism of ion
formation in NCI, and one that has been widely The quadrupole mass Rlter is the most widely em-
utilized in GC-MS, is via an electron capture process. ployed type of mass analyser in current use. It com-
If a compound containing one or more suitable elec- prises four metal rods accurately aligned around
tronegative groups is introduced into the ion source in a central axis. RF and DC voltages on the rods create
the presence of a high pressure (&1 mbar) of a buffer a complex electrostatic Reld within the area bound by
gas (e.g. methane) the following reaction can occur: them. Ions entering this region are acted on by the
electrostatic Reld and their motion through the rods
M#e\th#CH4P(CH4z M\z)HPM\z#CH4 can be likened to two superimposed sine waves. Un-
der these conditions the forces acting on most ions
In the above equation the thermal electrons (e-th) are cause the amplitude of the oscillations to increase and
produced from the electron ionization of the meth- accelerate them into the quadrupole rods. However,
ane. The neutral methane molecules also act to col- some ions are not accelerated into the rods and under-
lisionally stabilize the excited radical anion formed go trajectories that traverse the full length of the rods.
by associative resonance electron capture. This leads The parameters that govern the equations of motion
to the observation of a radical anion (M\z) in mass of ions in a quadrupole mass Rlter are the mass to
spectra recorded using this ionization method. charge ratio of the ions, the spacing between the rods,
Electron capture is a very low energy process and the frequency of the RF voltage and the magnitude of
the recorded mass spectra contain little or no frag- the RF and DC voltages. Hence the RF and DC
mentation. The NCI mass spectrum of 2-nitro- voltages may be selected such that ions of only one
Suorene is shown in Figure 2C. This compound con- m/z value have ‘stable’ trajectories. By varying the RF
tains an electronegative nitro group, and is ionized and DC voltages but keeping the ratio between them
via the electron capture process. An intense M\z ion the same a range of m/z values can be made to
can be seen at m/z 211 with no evidence of frag- undergo stable trajectories, be brought to focus on the
mentation. detector and a mass spectrum recorded.
 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry 451

The Ion Trap plications including dioxin analysis. For further de-
tails the interested reader is referred to Bruner (1993).
The ion trap operates in a similar manner to a quad-
rupole mass Rlter. It comprises a doughnut-shaped
ring electrode to which the RF voltage is applied Interfacing Mass Spectrometers with
and two end caps either earthed or with supple- Gas Chromatographs
mentary AC or DC voltages. Ions formed either in
the trap, or externally to the trap and transported into There are several methods available for interfacing
it, are initially stored within the trap. Mass separation gas chromatographs with mass spectrometers. These
is then achieved by increasing the RF voltage such include the use of jet separators for packed columns
that ions are ejected from the trap in ascending m/z and a variety of ways of interfacing capillary col-
order. umns. For packed columns the jet separator, a form
of momentum separator, is required to remove the
The Double Focusing Magnetic Sector majority of the carrier gas. A ‘solvent dump valve’ is
Mass Spectrometer also incorporated into these devices in order that the
injection solvent can be vented to waste rather than it
The double focusing magnetic sector mass spectro- passing into the mass spectrometer.
meter differs from those discussed so far in that the Although a number of interfaces for packed col-
mass analyser comprises two distinct components, an umn GC-MS have been described in the past, capil-
electromagnet and an electrostatic analyser. The lary columns are currently almost exclusively used for
magnet acts as a momentum analyser and affects GC-MS. The most widely used interface, in this case,
mass separation, while the addition of an electrostatic is the direct interface, where the column is passed
analyser corrects for some variations in the kinetic through a simple heated transfer line directly into an
energy of ions of the same m/z value and allows them EI or CI ion source. The low (1 mL min\1) carrier gas
to be brought to focus on the detector at the same Sow commonly used with capillary columns can
time. Hence the use of a double focusing arrangement readily be accommodated by the MS pumping system
as a mass analyser allows very high resolution to be in order to maintain a good vacuum. Figure 3 shows
achieved. In mass spectrometry resolution is deRned a complete instrument based around the use of a cap-
as the ability of the mass spectrometer to separate illary column, a simple direct interface and a quadru-
ions of very similar m/z value. pole mass spectrometer. For a more complete dis-
Resolution is important in mass spectrometry since cussion of the full range of GC-MS interfaces see
it may be used to introduce speciRcity into an experi- either Chapman (1993) or Johnstone and Rose
ment. An important application of high resolution (1996).
arises in the determination of polychlorinated diben-
zodioxins (PCDDs) and polychlorinated dibenzo-
furans (PCDFs) by GC-MS. These compounds are GC-MS Experiments
found ubiquitously in the environment and their de-
Full or Normal Scan
termination is important owing to concern about
their toxicity, mutagenicity and carcinogenicity. The The standard GC-MS mode of operation is the full or
only method that has been found to offer the appro- normal scan mode. On injection of the sample into
priate degree of sensitivity and speciRcity for this the GC, the mass spectrometer is set to repetitively
analysis is GC followed by high resolution MS detec- scan over a preset mass range. Typically this would
tion. High resolution is required since matrices which involve the mass spectrometer recording a mass spec-
accumulate PCDDs and PCDFs are also likely to trum over the scan range 35}500 Da once a second.
accumulate other polychlorinated aromatic hydro-
carbons, e.g. polychlorinated biphenyls. These com-
pounds, which may co-elute with the PCDDs and
PCDFs of interest, contain fragment ions in their EI
mass spectra which have the same integer m/z value
as the molecular ions of PCDDs and PCDFs. How-
ever, by monitoring the accurate mass value of the
PCDD and PCDF molecular ions (i.e. the exact mass
value of their elemental composition), at an adequate
resolution to separate them from likely interfering
ions, speciRcity is introduced. GC-MS is used exten- Figure 3 A quadrupole based GC-MS instrument employing
sively in environmental analysis for a range of ap- a direct interface for connection between the GC and the MS.
452 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry

The requirement for relatively fast acquisition rates is rated polycyclic aromatic hydrocarbons (nitro-PAH)
due to the fact that open tubular GC columns typi- in vegetation extracts. Nitro-PAH are absorbed on to
cally produce peaks of only about 10}15 s wide. vegetation from anthropogenic emissions, however
Hence, in order to acquire a representative number of their determination is made complex by the large
mass spectra from each peak, fast scan rates are amount of other compounds extracted from the veg-
required. A second consequence of these fast acquisi- etation by the sample preparation procedure.
tion rates is the requirement for a data system on all Figures 4A and B show a comparison between the
GC-MS instruments. Each mass spectrum can then be chromatogram obtained from an extract of bark from
stored in the data system for subsequent examina- a maple tree in an urban region using an ECD and the
tion/data processing. individual mass chromatograms obtained from the
Full scan data are used by the data system to same extract using GC-MS in NCI-SIM mode. The
generate a total ion chromatogram (TIC). This is
achieved by summing the intensity of the ions in each
mass spectrum to create a value for the total ion
intensity, as a number of ions or total number of
analogue to digital converter bits. This number is
then plotted against time/scan number to create
a chromatogram. One of the great strengths of GC-
MS using EI ionization is that the TIC generated by
this method is then directly comparable with a
chromatogram produced from the same sample using
Same ionization detection.

Selected Ion Monitoring (SIM)

Selected ion monitoring (SIM) is a technique widely
used for trace analysis. In this technique, rather than
the mass spectrometer being set to scan over a pre-
deRned mass range and record full mass spectra it is
set to monitor the intensity of speciRc m/z values.
SIM is used to introduce selectivity into an analysis
and improve sensitivity. Sensitivity is enhanced over
the full scan mode experiment since in the full scan
experiment a large proportion of the scan time is
spent recording areas of the spectrum where no ions
of interest occur. Ions are still being produced in the
ion source but are lost in the mass analyser as it brings
others into focus on the detector. In SIM, in a 1 s
duty cycle, only a few, i.e. 1}10, ions are selected.
Hence, the mass analyser transmits these ions for a
longer percentage of the time in which they are being
produced and therefore more of the ions of the par-
ticular m/z values of interest are recorded.
SIM may also used to introduce selectivity into the
experiment. This also has the effect of increasing
sensitivity by decreasing the amount of ‘chemical
noise’, i.e. real signal, but not from the compound of
interest, observed when peaks of interest elute. The
increase in selectively may also be achieved by the use
of a double focusing mass spectrometer and high Figure 4 A comparison of the chromatograms obtained from
resolution and this may be enhanced by, for example, the analysis of a complex extract containing nitrated polycyclic
aromatic hydrocarbons by gas chromatography (A) using an elec-
the use of negative chemical ionization.
tron-capture detector and (B) by GC-MS employing negative
An example of the increase in selectivity obtained chemical ionization and selected ion monitoring. Note the in-
by the use of SIM combined with NCI from our own crease in specificity afforded by the use of GC-MS under these
laboratory can be seen in the determination of nit- conditions.
 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry 453

quadrupole mass spectrometer used in this case was

set to monitor the M\z. ions obtained from 9 nitro-
PAH. The complex chromatogram shown in Fig-
ure 4A does not allow simple identiRcation of the
peaks of interest and the possibility of interferen-
ces/peak overlap leads to difRculties when attempting
quantiRcation. This can be observed for peak
5 (tr"34.1 min) in Figure 4A. This peak arises from
the presence of 2-nitroSuorene in the bark extract. As
can be seen, accurate and precise integration of this
peak is made difRcult by the presence of peaks with
very similar retention time. In contrast the peaks from
the nitro-PAH monitored by NCI-SIM can be seen
clearly in Figure 4B. Each chromatographic trace in
this Rgure represents the ion current observed from
monitoring the m/z value of the M\z ion of a series of
nitro-PAH. Peaks are readily integrated for quantiR-
cation with the 2-nitroSuorene peak (peak 5) appear-
ing well resolved on the m/z 211 trace.
One of many areas in which the use of resolution to
introduce speciRcity into SIM experiments is impor-
tant is the petroleum industry. Dibenzothiophenes
(DBT) have been suggested as marker compounds for
oil pollution. However, taking as an example diben-
zothiophene itself, this has the same nominal molecu-
lar mass (184 Da) as the C4-alkylated naphthalenes
which are also present in crude oil. Hence in order to
speciRcally measure dibenzothiophene in crude oil it
is necessary to monitor the accurate mass to charge
ratio (m/z 184.0347) of its molecular ion at high
resolution in a SIM experiment. Figure 5 compares Figure 5 GC-MS analysis of dibenzothiophene (DBT) in
the GC-MS-SIM analysis of dibenzothiophene in a crude oil using low resolution (top) and high resolution (bottom)
crude oil carried out using low and high resolution. selected ion monitoring. (Reproduced from Tibbets and Large
(1988) by kind permission of John Wiley and Sons.)
As can be clearly seen the C4-naphthalenes are not
observed in the high resolution data. The power of
such analyses can be seen in Figure 6 which shows prises two quadrupole mass Rlters and a multipole
a comparison of the GC-HRMS-SIM data obtained collision cell and is shown schematically in Figure 7.
from the analysis of methyl and C2 substituted diben- In principle the operation of such an instrument fol-
zothiophenes for three different crude oils obtained lows the following sequence: ion selection in the Rrst
from two North Sea oil Relds. The different crude oils quadrupole mass Rlter, ion dissociation in the colli-
can be clearly distinguished with such data. sion cell and separation of the products of ion dis-
sociation in the second quadrupole mass Rlter. For
illustration, Figure 2C, the NCI mass spectrum of
Other Techniques 2-nitroSuorene, only contains the M!z ion at m/z
211. Hence the only information contained in this
Mass Spectrometry^Mass Spectrometry (MS-MS)
mass spectrum is the relative molecular mass of the
Mass spectrometry}mass spectrometry, also known compound. In order to generate structural informa-
as tandem mass spectrometry, is the term used to tion a product ion scan could be carried out. To do
describe mass spectrometric methods employing in- this the Rrst quadrupole mass Rlter would be set to
struments that contain more than one mass analyser. transmit only m/z 211. This ion would then be sub-
Such instruments may be used to increase the amount jected to collisions with a gas held in the collision cell
of structural information obtained or to introduce (collisonally induced decomposition (CID)) and the
more speciRcity. resulting product ions recorded using the second
The simplest tandem mass spectrometer to consider quadrupole mass Rlter. The resulting product ion
is the triple quadrupole mass spectrometer. This com- spectrum then shows only ions that have arisen
454 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry

Figure 6 A comparison of the GC-HRMS-SIM fingerprints of methyldibenzothiophenes and C2-dibenzothiophenes in three crude oils
from two North Sea fields. (Reproduced from Tibbets and Large (1988) by kind permission of John Wiley and Sons.)

directly from the fragmentation of m/z 211 and con- increase the speciRcity of an analysis. After Rrst re-
tains structural information. It can be seen that this cording product ion mass spectra of the analyte(s) of
method is also useful for clearing up ambiguities in interest, one or more precursor/product ion relation-
the interpretation of EI spectra, since it allows precur- ships are chosen. The criteria for this are that the
sor/product ion relationships to be clearly deRned. product ions selected are intense and characteristic of
A second application of tandem mass spectrometry the speciRed analyte. Then in order to carry out the
often used in conjunction with GC-MS utilizes a tech- MRM experiment, the Rrst quadrupole mass analyser
nique called multiple reaction monitoring (MRM). is set up in SIM mode, to switch between the precur-
This technique, like high resolution SIM, is used to sor ions of interest. The collision cell is operated in
 II / CHROMATOGRAPHY: GAS / Detectors: Mass Spectrometry 455

clinical Reld in order to accurately determine isotope

ratios, e.g. the presence of Helicobacter pylori in the
gastric epithelium has been linked with gastritis, pep-
tic ulcers and gastric cancer. The presence of Heli-
cobacter pylori can be determined by measuring the
13
C/12C isotope ratio in exhaled breath following in-
gestion of isotopically labelled urea. Gerhards et al.
(1999) have examined the use of GC-MS in clinical
analysis and Platzner (1997) provides a full dis-
cussion of modern isotope ratio mass spectrometry.
Figure 7 A triple quadrupole mass spectrometer. On leaving
The future of mass spectrometry as the detector of
the GC column sample components are first ionized, then ions
selected by the first quadrupole mass filter are fragmented in the choice for gas chromatography looks secure. In this
collision cell for analysis by the second quadrupole mass filter. paper we have illustrated some of the beneRts of this
Such instruments may yield greater structural information than happy marriage of techniques. Modern GC-MS in-
single stage instruments and allow further selectivity to be intro- struments are compact, robust, sensitive, selective
duced into GC-MS experiments.
and give access to a range of information not possible
when using conventional detectors, hence their im-
the normal way, and the second quadrupole mass portance in the modern GC laboratory.
Rlter is set up to switch between the characteristic
product ions. See also: II/Chromatography: Gas: Detectors: Selec-
The output from such an experiment is chromato- tive. Chromatography: Liquid: Detectors: Mass Spectro-
graphic, producing one or more plots of signal inten- metry. Mass Spectrometry: Spectrometry - Mass Spec-
sity against time. Peaks are only observed in such trometry Ion Mobility. III/Clinical Diagnosis: Chromato-
chromatograms when an ionized compound yields an graphy. Geochemical Analysis: Gas Chromatography.
ion of the selected precursor ion m/z value, which
also subsequently fragments, under CID, to give
a product ion of the selected product ion m/z. The Further Reading
two stages of mass selection make this a highly selec- Bruner F (1993) Gas Chromatographic Environmental
tive technique and it has been proposed as a viable Analysis. New York: VCH Publishers.
alternative to high resolution SIM for a variety of Busch KL, Glish GL and McLuckey SA (1988) Mass Spec-
applications. trometry}Mass Spectrometry. New York: VCH Pub-
Recent advances in ion trap technology have meant lishers.
that similar modes of operation also available on Chapman JR (1993). Practical Organic Mass Spectrometry,
these compact, relatively low cost instruments. In this 2nd edn. Chichester, UK: John Wiley.
case ions other than the precursor ion of interest are Davis R and Frearson M (1987) Mass Spectrometry. Ana-
selectively ejected from the trap. The selected precur- lytical Chemistry by Open Learning Series. Chichester,
sor ion is then subjected to CID in the trap and UK: John Wiley.
Evershed R. (1993) In: Baugh PJ (ed.) Gas Chromatography
a product ion mass spectrum may be recorded by
} A Practical Approach. Oxford, UK: Oxford University
ejecting these ions. For a fuller discussion of tandem Press.
mass spectrometry see Busch et al. (1988). Gaskell SJ (ed.) (1986) Mass Spectrometry in Biomedical
Research. Chichester, UK: John Wiley.
Conclusions Gerhards P, Bons U and Sawazki J (1999) GC/MS in Clini-
cal Chemistry. Chichester, UK: John Wiley.
A developing Reld in GC-MS is the use of time of Johnstone RAW and Rose ME (1996) Mass Spectrometry
Sight mass analysers with short capillary columns for for Chemists and Biochemists 2nd edn. Cambridge, UK:
very rapid analyses. We have described time of Sight Cambridge University Press.
mass analysers and the advantages offered by their Lee TA (1998) A Beginners Guide to Mass Spectral Inter-
use for the acquisition of mass spectral data from pretation. Chichester, UK: John Wiley.
Oehme M (1998) Practical Introduction to GC-MS Analy-
narrow chromatographic/electrophoretic peaks in the
sis with Quadrupoles. Heidelberg: HuK thig Verlag.
companion paper to this one, on the use of mass Platzner I (1997) Modern Isotope Ratio Mass Spectro-
spectrometry as a detector for liquid chromatogra- metry. Chichester, UK: John Wiley.
phy. The same arguments apply for fast GC-MS ana- Smith RM and Busch KL (1999) Understanding Mass
lyses and a number of manufacturers have recently Spectra. A Basic Approach. New York: John Wiley.
launched instruments of this type. Specialist GC-MS Tibbets PJC and Large R (1988) In Crump GB (ed.) Petro-
instrumentation is also increasingly being use in the analysis ’87. Chichester, UK: John Wiley.