G
GAMMA-RAY SPECTROSCOPY
See RADIOCHEMICAL METHODS: Gamma-Ray Spectrometry
GAS CHROMATOGRAPHY
Contents
Overview
Principles
Column Technology
Gas Solid Chromatography
Multidimensional Techniques
High-Temperature Techniques
High-Speed Techniques
Instrumentation
Online Coupled LC GC
Pyrolysis
Detectors
Mass Spectrometry
Fourier Transform Infrared Spectroscopy
Physicochemical Measurements
Environmental Applications
Forensic Applications
Petrochemical Applications
Chiral Separations
established itself as a routine analytical technique in
Overview
most industrial and academic laboratories. From its
introduction and until the advent of high-perform-
ance liquid chromatography, it dominated separation
K Robards, Charles Sturt University, Wagga Wagga,
methods. This can be attributed to the capability for
NSW, Australia
high resolution, selectivity, and sensitivity.
& 2005, Elsevier Ltd. All Rights Reserved.
Separation in GC is achieved by partitioning of
gaseous solutes between a typically inert gaseous
mobile phase and a stationary liquid or solid phase
retained in a column. These variants are described as
Introduction
gas liquid chromatography (GLC) and gas solid
Gas chromatography (GC) is a dynamic method chromatography (GSC), respectively. With the ex-
of separation and detection of volatile compounds. ception of some specialized areas such as the anal-
It was first introduced in the 1950s and rapidly ysis for inorganic gases, it is GLC which is used.
2 GAS CHROMATOGRAPHY / Overview
2
3
Detector
gas(es)
4
Oven
Pressure Injection
Detector
6 Carrier
or flow port Column
(heated)
gas
controller (thermostatted)
7
Electrometer
5
1
Recording
8
9
device
Figure 2 Block diagram of a gas chromatograph.
is given in the following sections. The reader is also
Retention time referred to the more detailed discussion of detectors
and specific techniques such as pyrolysis GC, high-
Figure 1 A gas chromatogram showing the separation of a
speed GC, and GSC. Extensive resources for training,
nine-component mixture. Peaks due to individual compounds are
labeled 1 through 9. The unlabeled peak is attributed to the sam- method development, and support services are
ple solvent. The retention time for individual components can be
provided by a number of manufacturers of gas chro-
read from the chromatogram although it is generally provided in a
matographic instrumentation.
separate report.
Sample
Nevertheless, the instrumentation is virtually identi-
It follows from the preceding discussion that the ba-
cal for the two techniques.
sic requirement with respect to sample is that it has
In the most common approach (elution develop-
an appreciable vapor pressure at the column tempera-
ment), the sample is introduced into the chromato-
ture. As usually practiced, the sample must also be
graph via the sample inlet into a continuous flow of
thermally stable. This allows the sample components
mobile phase, which is referred to as the carrier gas.
to vaporize in, and move with, the gaseous mobile
The sample is vaporized in the inlet system and
phase. This requirement is not as severe a restriction
transported by the carrier gas to the thermostatted
as it appears since column temperatures as high as
column where separation occurs. The individual
4501C (3001C is more common) are used in GC.
components give rise to an electrical signal in the
Thus, GC can be applied to all permanent gases,
detector that may have provision for the inlet of ad-
most nonionized small- or medium-sized organic
ditional make-up gas. This is necessary to permit
molecules (typically up to C30), and many organo-
separate optimization of gas flow through the col-
metallic compounds but it cannot be used for macro-
umn and detector. After suitable amplification the
molecules or salts. In some instances, nonvolatile
detector signal is conducted to a recording device.
compounds can be converted into more volatile and
The detector output is produced as a chromatogram
stable derivatives before chromatography. In a typi-
(Figure 1). This is a plot of detector signal versus time
cal sample containing a mixture of volatile and
in which individual peaks represent the separated
nonvolatile components, care must be taken that the
components of the sample. Sample components can
nonvolatile solutes are not deposited in the system
be identified from their characteristic retention times.
where they can interfere with subsequent analyses.
With proper calibration, the amounts of the compo-
nents of a mixture can be measured accurately also.
Figure 2 shows the essential components of a gas
Mobile Phases
chromatograph as a block diagram. These parts can
be identified as the carrier gas, sample introduction Substances capable of interacting with the analyte
system, the column, the detector, and data acquisi- and influencing selectivity have been used as carrier
tion system comprising an electrometer and integra- gases in rare instances. However, the ideal mobile
tor/recording device. Although not apparent from the phase for GC is usually nonreactive toward the anal-
figure, there are three separately controlled heated yte(s), nonflammable, cheap, and environmentally
zones for the inlet, column, and detector in the typi- friendly since it is vented at the end of the instru-
cal instrument. An overview of some of these aspects ment. Hence, the choice of a mobile phase or carrier
GAS CHROMATOGRAPHY / Overview 3
Table 1 Physical properties (at 273 K and 101 kPa) of carrier
When the column connection has been completed,
gases used in gas chromatography
the system should be checked for gas leaks. Once any
leaks have been eliminated and the column purged
Gas Thermal Viscosity Density
7 3 1
conductivity (10 Pa s) (kg m ) with carrier gas, the volumetric flow rate (ml min )
1
(108 Wm 1 K )
or the theoretically more useful average linear gas
1
velocity (cm s ) can be measured.
Hydrogen 16.75 84 0.0899
Helium 14.07 186 0.1785
Nitrogen 2.39 166 1.2505
Sample Introduction Systems
Argon 1.67 212 1.7839
Neon 4.56 298 0.8999
The injection port or inlet system is the next major
component of the gas chromatograph. It must receive
the sample and deliver the correct amount of mate-
gas is determined by practical constraints of cost, rial to the column so as not to exceed the sample
availability, inertness, and detector compatibility
capacity of the column or the linear range of the
rather than its ability to effect a particular separa- detector in use. Several types of inlet and sample
tion. The usual mobile phase in GC is therefore a introduction techniques have been developed to accom-
noninteractive gas that does not influence selectivity. modate the diversity of sample types and particularly
However, the carrier gas can influence resolution the state of aggregation of the sample and the range
through its effect on column efficiency because of of columns. Specialized techniques include equili-
differences in solute diffusion rates for various gases. brium headspace sampling, purge and trap sampling,
Moreover, it can effect analysis time and plays a role pyrolysis GC, and multidimensional chromatogra-
in pressure-limiting situations because of differences phy in which the sample entering the column differs
in gas viscosities (see Table 1). from the composition of the original sample. How-
Taking these considerations into account, hy- ever, in the more usual case, the material entering the
drogen, helium, and nitrogen are the most popular column must have the same composition as the
carrier gases in GC. Carrier gases are usually sup- original sample. Additionally, the sample has to be
plied from a compressed gas cylinder. Gas purity is a
delivered to the column as a sharp band.
major consideration and, in general, the highest pu- The most common analysis involves injection of
rity gas should be used to reduce deterioration of the 1 3 ml of a liquid into a heated inlet. This is accom-
stationary phase and lessen detector noise. Moreover, plished by means of a microsyringe through a septum
it is usual to include oxygen and moisture traps in the made of elastomer or rubber, which seals the inlet
carrier gas lines. These traps are commercially avail- system as the syringe needle is withdrawn. Septa have
able, containing activated carbon (to remove organic a limited lifetime dependent on the mode of injection
impurities) or molecular sieves (for moisture and (automatic versus operator injection), the injector
oxygen). The traps must be monitored and periodi- temperature, and the septum quality. They are avail-
cally regenerated. When changing cylinders, it is im- able from a number of manufacturers and a good
portant to ensure that all fittings are free of dust and operating principle is to perform a separation with
dirt particles before connection to gas lines. a solvent blank, particularly with a new batch of
Typical compressed gas cylinders contain a pres- septa. Syringe injection is also applicable to gases
sure of 20 MPa whereas supply line pressures to the (50 1000 ml) but reproducibility is relatively poor
gas chromatograph are more commonly in the range and a sampling valve is more common. Syringes are
50 300 kPa. Thus, appropriate regulators and available from a number of manufacturers in various
controllers are used to step down and control the configurations; needle point style, length of needle,
pressure and flow rate to the column. With tradi- fixed or replaceable needle. Most needles are con-
tional instruments, the carrier gas is regulated by structed of stainless steel but specialty fused silica
either a pressure regulator or flow controller. The needles are available for on-column injection. An
choice between the two is dependent on the inlet important consideration in choosing a syringe is the
system and column type. In recent years, instrument correct needle length to ensure delivery of the sample
manufacturers have introduced completely electro- at the correct position in the injection zone.
nic programmable pressure-controlled gas chro- With packed columns, the sample solution is in-
matographs. troduced via the syringe into the sealed injection port
When a capillary column is installed in an instru- that is heated to a higher temperature than the col-
ment it should be checked for carrier gas flow before
umn in order to assist vaporization. Sample discrimi-
connecting the detector end of the column to avoid nation, which can be regarded as a measure of how
the possibility of heating a column with no flow. well the detected peak areas reflect the original
4 GAS CHROMATOGRAPHY / Overview
sample composition, is not a problem. On the other of less solute), reduced analysis time (to achieve
hand, the much smaller sample capacity and lower equivalent resolution), and greater chemical inertness
carrier gas flow-rates associated with capillary col- were gradually recognized. More recently, polymer-
umns magnify the extent of any problems and these clad flexible fused silica capillary columns with
are manifest as sample discrimination. Thus, more chemically bonded and/or cross-linked immobilized
attention has been given to detailed investigation of stationary phases have become commercially avail-
various injection techniques when using capillary able at reasonable cost and this has led to the current
columns. These include the use of a vaporizing in- popularity of capillary columns. These columns now
jector (i.e., heated injection port), cold syringe needle routinely provide high efficiency, inertness, and
injection, hot needle injection, and solvent flush reproducibility. Alternatively, some separation effi-
technique. The hot needle and solvent flush tech- ciency can be sacrificed by using shorter columns to
niques are about equally effective in reducing dis- achieve very rapid analyses.
crimination and are preferred over other methods. Capillary columns are available from several manu-
Traditional sample inlet systems were constructed facturers in a wide range of column internal dia-
of metal, thus providing metallic surfaces where meters (0.1 1.0 mm), column lengths (5 50 m), and
sample decomposition was possible during sample stationary phase film thicknesses (0.1 5.0 mm). Gen-
evaporation. Interchangeable glass liners in the inlet, erally, sample capacity increases but the efficiency
which are available in a range of configurations, are decreases as the internal diameter or film thickness
now standard in practically every sample injection increases. The larger bore capillary columns with in-
system involving evaporation of the injected sample. ternal diameters between 0.53 and 1.00 mm are
Capillary columns have a very low sample capacity, termed wide bore or megabore capillary columns and
and to avoid overloading the stationary phase spe- these have similar capacities, but greater efficiencies,
cialized injection systems have evolved. The more than packed columns (see Table 2).
important of these are split injection, splitless injec- The largest variation in properties between
tion, cold on-column injection, and programmed conventional packed columns and capillary columns
temperature vaporizer split/splitless injection. These is associated with the column permeability. For this
variants have evolved to meet the diverse needs of reason, capillary columns offer much less flow
sample type and analyte concentration. For instance, resistance and can be used in much longer lengths.
splitless injection is more suited to trace/ultratrace Ultimately, the comparison of different column types
analysis than is split injection. is between the efficient use of column head pressure.
Thus, a packed column containing 10 mm particles
can generate 50 000 theoretical plates per meter but
Columns 1
requires a head pressure of 20 MPa m , whereas a
70 m capillary column of 50 mm internal diameter
In GC where the mobile phase is noninteractive,
can provide over one million theoretical plates with a
the column alone determines the selectivity of the
column pressure drop of B2.2 MPa.
separation. From its inception, up to the 1980s,
The stationary phase distinguishes GSC from
almost all separations in GC were performed on
conventional packed columns despite the demonstra- GLC. In the former it is a solid adsorbent whereas
in GLC it is a liquid either coated on a solid support
tion by Golay in 1957 of much greater efficiency
(packed column) or deposited directly on the column
obtainable with capillary columns. However, the
walls. GSC preceded GLC but has never achieved
obvious advantages of capillary columns in terms of
the same prominence. Nonetheless, GSC has some
higher resolution, greater sensitivity (despite injection
Table 2 Comparison of packed and capillary columns
Parameter Column type
Packed Microbore capillary Capillary Megabore capillary
Internal diameter (mm) 1/4 in 100 mm 200 mm 530 mm
Length (m) 0.5 3 5 50 5 100 5 100
7
Permeability (10 cm2) 1 50 300 20 000
Film thickness (mm) 1 10 0.1 0.2 2 1 5
1
Carrier gas average linear velocity (cm s ) 2 4 20 30 20 35 20 40
1
Flow rate (ml min ) 50 60 0.2 0.5 0.2 2.0 3 5
Sample capacity (ng) 20 000 o5 20 500 1000 15 000
GAS CHROMATOGRAPHY / Overview 5
important application areas such as the separation of In practice, experience of similar separation prob-
inorganic gases and low molecular mass hydrocar- lems, literature data relating to the target separation,
bons for which GLC shows little selectivity. The and availability of the column phases are often the
main adsorbents for GSC are based on silica, char- factors that determine the choice of a particular
coal, alumina, or molecular sieves although the phase and column for a specific application.
development of new adsorbents is continuing. The ideal liquid phase has a low vapor pressure,
The liquids used as stationary phases in packed and high thermal and chemical stability, low viscosity,
capillary columns are closely related. Nevertheless, nonreactivity toward sample components, and a wide
liquid phases in capillary columns are usually cross- temperature operating range, extending from 801C
linked and bonded and may exhibit slight differences to 4501C. The phase must exhibit reasonable solvent
in selectivity to nominally equivalent packed column properties (i.e., dissolving power) for the solutes in
materials. The selection and comparison of stationary order to ensure symmetrical peaks. Stationary phases
phases is confusing for newcomers as some 300 phas- can be divided into nonpolar, polar, and specialty
es are available and in excess of 1000 have been de- phases. These differ in their ability to interact with
scribed in the literature. Nevertheless, a fairly limited solutes of different structure, i.e., their selectivity. The
set of packed columns will suffice in most laborato- nonpolar phases contain no functional groups capable
ries while an even more limited set of capillary col- of specific interaction (e.g., hydrogen bonding or di-
umns will satisfy the needs of most laboratories. pole interactions) with the sample. Here, interaction
Moreover, two forces have combined to contain the between solute and stationary phase is limited to
proliferation of phases. Firstly, the high efficiency of dispersive forces, and components therefore separate
capillary columns has reduced the necessity for many according to their volatility with the elution order fol-
selective liquid phases and, secondly, theoretical studies lowing the boiling points. Compounds that cannot be
have aided in phase selection. differentiated on the basis of their boiling points (i.e.,
There are several factors to consider in selecting a they have similar or equal boiling points) require a
stationary phase. General considerations include different stationary phase for separation. To obtain the
temperature limits of the stationary phase, column differentiation of solutes by forces other than disper-
efficiency, and lifetime and detector compatibility. sion, a polar phase containing groups capable of spe-
Since nonpolar phases generally give more efficient cific interactions with sample components is required.
columns that also exhibit superior lifetimes, it is wise The elution order now depends on a combination of
to use the least polar phase that provides satisfactory volatility and specific polar polar interactions. The
separation. Phases containing the specific element relative magnitude of the various interactions (dis-
corresponding with element-selective detectors (e.g., persive, dipole, hydrogen bonding, and acid/base)
cyanopropyl phases with an NPD detector; trifluoro- determines the selectivity of the phase toward parti-
propyl phases with an ECD detector) should be cular solutes. The selectivity and resolution of a sepa-
avoided where possible. These selective detectors will ration can be optimized by choosing a stationary
be substantially more sensitive to normal column phase that exploits the different interactions.
bleed with such phases. Nonpolar phases include a variety of hydrocar-
The most difficult factor to assess is the ability of a bons, such as squalane or Apolane C87, or mixtures
phase to effect the desired separation. From this per- of long-chain n-alkanes such as Apiezon L. Polymers
spective, the selection of a stationary phase and col- based on a silicon-oxygen-silicon backbone form the
umn is a daunting prospect. In theory, the selection is basis of the most widely used group of stationary
based upon maximizing the difference in selectivity phases. These linear polysiloxanes differ in their
between the solutes toward the phase. The separa- average molecular mass, thermal stability, and vis-
tion is increased by exploiting solute stationary cosity. The chemical difference lies in the substituent
phase interactions that retard the progress of some and degree of substitution on the silicon backbone.
solutes relative to others so as to increase their re- Polar phases have been prepared by substituting
tentions. The types of interactions to consider are: polar trifluoropropyl or cyano groups for the methyl
groups of the dimethylsilicones. By incorporating
*
different proportions of the polar groups, station-
London or dispersion forces which are weak and
nonspecific; ary phases with a wide range of polarities can be
*
dipole dipole interactions or dipole-induced di- produced. Other polar materials include polyethy-
pole interactions; and lene glycols or polyoxiranes with the structure
*
acid base interactions or proton transferring (or (CH2CH2 O)n .
sharing) tendencies of either the solute or station- Specialty phases have been developed for use with
ary phase. particular analytical techniques such as GC MS
6 GAS CHROMATOGRAPHY / Overview
where low bleed phases are essential, to meet the detector has definite advantages whereas a selective
needs of particular groups (e.g., United States detector may aid in the identification of an unknown
Environmental Protection Agency methods), or to compound or a given class of compounds. Selective
separate particular classes of solutes. Included in the detectors are particularly useful for the analysis of
latter are chiral phases and Carbowax phases modi- complex mixtures, where the selectivity may greatly
fied for separation of acids and bases. simplify the chromatogram through suppression of
the response of many potentially interfering com-
Column Temperature
pounds.
Detectors can also be classified as destructive or
Column temperature is an important variable that
nondestructive. With nondestructive detectors, the
must be controlled in GC. Thus, the column is
original chemical form of the analyte persists
housed in a thermostatted oven. For simple samples
throughout the detection process. This is an obvious
containing relatively few peaks, an appropriate col-
advantage when the analyte is required for further
umn temperature can be determined experimentally
analysis. In destructive detectors, the process of de-
to achieve the separation and isothermal analysis is
tection involves an irreversible chemical change in
suitable. Nonetheless, many samples contain com-
the analyte. A more useful classification distinguishes
ponents with a wide range of volatility and more
detectors on the basis of the transducer mechanism
volatile components are eluted rapidly with no reso-
as ionization, spectroscopic, etc.
lution when analyzed isothermally at a high tempera-
A consideration of the characteristics discussed
ture whilst the analysis time is unacceptably long and
above and the needs of a particular analytical prob-
later eluting peaks are very broad and may be lost as
lem will determine the most appropriate detector for
baseline drift when analyzed isothermally at a low
a given problem. A detector with a wide linear dyna-
temperature. For such samples, temperature pro-
mic range and low detection limit will be adopted for
gramming in which the column temperature is ramped
the determination of trace components in addition to
during the analysis is essential.
main components in a sample. On the other hand, the
use of a selective detector is convenient if the trace
Detectors
components belong to a particular class of substance
Online detection is an integral part of a gas chro- or possess some common functional group.
matograph. The detector monitors the column effluent Of the many available detectors, the most common
and produces an electric signal that is proportional to (Table 3) are thermal conductivity detector (TCD),
the amount of analyte being eluted. The output signal flame ionization detector (FID), electron-capture de-
is recorded as a continuous trace of signal intensity tector (ECD), alkali-flame ionization detector (AFID
against time. In principle, any physical or physico- or NPD), flame photometric detector (FPD), and
chemical property of the analyte that deviates from the mass selective detector. The TCD and FID are usually
properties of the carrier gas can serve as the basis for considered universal detectors as they respond to
detection. Thus, over 100 detectors for GC have been most analytes whereas the ECD, AFID, and FPD are
described but relatively few are in common use. the most useful selective detectors and give differen-
The operation and applicability of different detec- tial responses to analytes containing different func-
tors can be compared against several performance tional groups. Note that this does not imply that the
criteria. These criteria include the sensitivity, noise, magnitude of the response of the universal detectors
minimum detectable quantity or detection limit, de- is constant to all analytes. The mass selective detec-
tector time constant and response time, and the tor has the advantage of operation in either universal
selectivity of the response. For purposes of screen- or selective detection mode whilst an infrared detec-
ing a sample of unknown composition, a universal tor is a powerful tool for distinguishing isomers.
Table 3 Classification of the most common gas chromatographic detectors
Detector Response Optimal detection limit Destructive
TCD Organic and inorganic solutes 5 100 ng No
FID All organic solutes except formic acid and formaldehyde 10 100 pg Yes
ECD Halogenated and nitro compounds 0.05 1 pg No
AFID P- or N-containing solutes 0.1 10 pg Yes
FPD P- or S-containing solutes 10 100 pg Yes
Mass General all-purpose detector that is replacing FID in a number of Dependent on mode of Yes
selective situations operation
GAS CHROMATOGRAPHY / Principles 7
Dual Detection
The first detector commercially available for GC,
the TCD or katharometer, remains a consideration
The simultaneous use of two or more detectors,
for situations requiring universal detection. The TCD
whose outputs complement each other, can aid in
responds to any compound, irrespective of its struc-
compound identification by generating substance-
ture, whose thermal conductivity differs from that of
characteristic detector response ratios. In some
the carrier gas. Hence, it is the only choice for de-
instances, the detectors are operated sequentially or,
tection of compounds to which other more sensitive
alternatively, the column eluate is split and passed
detectors give a poor or negligible response. In par-
separately to the individual detectors. The combina-
ticular, it is the standard detector for determination
tion of a selective with a universal detector can
of inorganic gases such as the permanent gases, hy-
provide information on both the whole sample and,
drogen, oxygen, nitrogen, carbon dioxide, carbon
at the same time, greater quantitative sensitivity on
monoxide, carbon disulfide, and water.
specific components.
The FID is the standard workhorse detector in GC.
It consists of a stainless steel jet constructed so that
See also: Gas Chromatography: Principles; Column Tech-
carrier gas exiting the column flows through the jet,
nology; Instrumentation; Detectors; Mass Spectrometry.
mixes with hydrogen gas, and flows to a microburner
tip that is swept by a high flow of air for combustion.
Further Reading
Ions produced by the combustion are collected at a
pair of polarized electrodes, constituting a small
Cazes J and Scott RPW (2002) Chromatography Theory.
background current that is the signal. When solutes
Chromatographic Science Series, vol. 88. New York:
enter the detector, they are combusted and the signal
Dekker.
increases. The current produced is then amplified and
Gehrke CW, Wixom RL, and Bayer E (eds.) (2001) Chro-
passed to a recording device. Unlike the TCD, the matography a century of discovery 1900 2000. Journal
FID gives virtually no response to inorganic com- of Chromatography Library, vol. 64.
Grant DW and Grant RPW (1996) Capillary Gas Chro-
pounds. Most organic compounds, however, give
matography. Separation Science Series. New York: Wiley.
similar responses, which is approximately propor-
Grob RL and Barry EF (1995) Modern Practice of Gas
tional to the total mass of the carbon and hydrogen in
Chromatography, 3rd edn. New York: Wiley.
the analyte. A reduced response is usually observed
Handley AJ and Adlard ER (2001) Gas Chromatographic
with the first members of a homologous series and
Techniques and Applications. Sheffield: Academic Press/
compounds with a large proportion of oxygen.
Blackwell Science.
The popularity of the ECD can be attributed to the
Issaq HJ (ed.) (2002) A Century of Separation Science.
high sensitivity to organohalogen compounds, which
New York: Dekker.
include many compounds of environmental interest,
Jennings W, Mittlefehldt E, and Stremple P (1997) Ana-
including polychlorinated biphenyls and pesticides. It lytical Gas Chromatography, 2nd edn. San Diego, CA:
is the least selective of the so-called selective detec- Academic Press.
McNair HM and Miller JM (1998) Basic Gas Chro-
tors but has the highest sensitivity of any contempo-
matography. New York: Wiley-Interscience.
rary detector. The NPD or thermionic ionization or
Moldoveanu SC and David V (2002) Sample preparation
emission detector is a modified FID in which a con-
in chromatography. Journal of Chromatography Library,
stant supply of an alkali metal salt, such as rubidium
vol. 65.
chloride, is introduced into the flame. It is a detector
Niessen WMA (ed.) (2001) Current Practice of Gas Chro-
of choice for analysis of organophosphorus pesti-
matography Mass Spectrometry. Chromatographic Sci-
cides and pharmaceuticals. The FPD detects specific
ence Series, vol. 86. New York: Dekker.
luminescent emission originating from various excit-
Robards K, Haddad PR, and Jackson PE (1994) Principles
ed state species produced in a flame by sulfur- and
and Practice of Modern Chromatographic Methods.
phosphorus-containing compounds. London: Academic Press.
Principles
P J Marriott, RMIT University, Melbourne, VIC,
Introduction
Australia
Gas chromatography (GC) is the premier chemi-
& 2005, Elsevier Ltd. All Rights Reserved. cal separation method for volatile compounds. It
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