Gas Chromatography

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Chapter 32

• Gas Chromatography

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In gas chromatography, the components of a
vaporized sample are separated by being
distributed between a mobile gaseous phase and a
liquid or a solid stationary phase held in a
column. Two types of gas chromatography are
encountered In gas-liquid chromatography, the
mobile phase is a gas, and the stationary phase
is a liquid that is retained on the surface of an
inert solid by adsorption or chemical
bonding. In gas-solid chromatography, the
mobile phase is a gas, and the stationary phase
is a solid that retains the analytes by physical
adsorption. Gas-solid chromatography permits
the separation and determination of
low-molecular-mass gases, such as air components,
hydrogen sulfide, carbon monoxide, and nitrogen
oxides.
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32 A Instruments for gas-liquid chromatography
The basic components of a typical instrument for
performing gas chromatography are shown here.
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Carrier Gas System The mobile phase gas in gas
chromatography is called the carrier gas and must
be chemically inert. Helium is the most
common mobile phase, although argon, nitrogen,
and hydrogen are also used. Flow rates in gas
chromatographs were regulated by controlling the
gas inlet pressure. Newer chromatographs use
electronic pressure controllers both for packed
and for capillary columns.
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In a classical soap-bubble meter, a soap film is
formed in the path of the gas when a rubber bulb
containing an aqueous solution of soap or
detergent is squeezed. The time required for
this film to move between two graduations on the
buret is measured and converted to volumetric
flow rate.
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Sample Injection System For high column
efficiency, a suitably sized sample should be
introduced as a plug of vapor. Slow injection
or oversized samples cause band spreading and
poor resolution. Calibrated microsyringes are
used to inject liquid samples through a rubber or
silicone diaphragm, or septum, into a heated
sample port located at the head of the column.
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Column Configurations and Column Ovens The
columns in gas chromatography are of two general
types packed columns or capillary
columns. Chromatographic columns vary in length
from less than 2 m to 60 m or more. They are
constructed of stainless steel, glass, fused
silica, or Teflon.
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Column temperature is an important variable that
must be controlled to a few tenths of a degree
for precise work. The optimum column
temperature depends on the boiling point of the
sample and the degree of separation required.
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• Chromatographic Detectors
• The ideal detector for gas chromatography has the
  following characteristics
• Adequate sensitivity.
• Good stability and reproducibility.
• A linear response to solutes that extends over
  several orders of magnitude.
• A temperature range from room temperature to at
  least 400?C.
• A short response time that is independent of flow
  rate.
• High reliability and ease of use.
• Similarity in response toward all solutes or,
  alternatively, a highly predictable and
• selective response toward one or more
  classes of solutes.

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The flame ionization detector responds to the
number of carbon atoms entering the detector per
unit of time. It is a mass-sensitive rather than
a concentration-sensitive device. It is useful
for the analysis of most organic samples
including those that are contaminated with water
and the oxides of nitrogen and sulfur.
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The electron capture detector (ECD) has become
one of the most widely used detectors for
environmental samples because this detector
selectively responds to halogen-containing
organic compounds, such as pesticides and
polychlorinated biphenyls. Electron capture
detectors are highly sensitive and have the
advantage of not altering the sample
significantly. The linear response of the
detector is limited to about two orders of
magnitude. One of the most powerful detectors
for GC is the mass spectrometer. The
combination of gas chromatography and mass
spectrometry is known as GC/MS.
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A computer data system is needed to process the
large amount of data obtained by GC/MS mass
spectrometers. The data can be analyzed in
several ways. 1. The ion abundance in each
spectrum can be summed and plotted as a function
of time to give a total-ion chromatogram. 2.
One can also display the mass spectrum at a
particular time during the chromatogram to
identify the species eluting at that time. 3. A
single mass-to-charge (m/z) value can be selected
and monitored throughout the chromatographic
experiment, a technique known as selected-ion
monitoring. Mass spectra of selected ions
during a chromatographic experiment are known as
mass chromatograms.
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• Other important GC detectors include the
• thermionic detector - widely used for
  organo-phosphorous pesticides and pharmaceutical
  compounds.
• the electrolytic conductivity or Hall detector -
  compounds containing halogens, sulfur, or
  nitrogen are mixed with a reaction gas in a small
  reactor tube.
• The products are then dissolved in a liquid that
  produces a conductive solution. The change in
  conductivity as a result of the presence of the
  active compound is then measured.
• 3. the photoionization detector - often used for
  aromatic and other molecules that are easily
  photoionized.

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?32 B Gas chromatographic columns and stationary
phases Capillary columns are also called open
tubular columns because of the open flow path
through them. They are of the following types
1. Wall-coated open tubular (WCOT) are
capillary tubes coated with a thin layer of the
liquid stationary phase. 2. Support-coated open
tubular columns (SCOT) have an inner surface
lined with a thin film (lt30 ? m) of a solid
support material. 3. Fused-silica open tubular
(FSOT) columns are currently the most widely used
GC columns. 4. Capillary columns with 530 ?m
inside diameters, sometimes called megabore
columns, are also used.
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Modern packed columns are fabricated from glass
or metal tubing. They are typically 2 to 3 m
long and have inside diameters of 2 to 4
mm. Solid Support Materials The packing, or
solid support, in a packed column serves to hold
the liquid stationary phase in place so that as
large a surface area as possible is exposed to
the mobile phase. Packings for gas
chromatography were prepared from naturally
occurring diatomaceous earth, which consists of
the skeletons of thousands of species of
single-celled plants.
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Liquid Stationary Phases Desirable properties
for the immobilized liquid phase in a gas-liquid
chromatographic column include (1) low
volatility (ideally, the boiling point of the
liquid should be at least 100?C higher than the
maximum operating temperature for the column),
(2) thermal stability, (3) chemical
inertness, and (4) solvent characteristics such
that k and ? values for the solutes to be
resolved fall within a suitable
range. To have a reasonable residence time on
the column, an analyte must show some degree of
compatibility (solubility) with the stationary
phase especially with respect to the polarities
of the analyte and the immobilized liquid.
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32 C Applications of gas-liquid
chromatography It is applicable to species that
are appreciably volatile and thermally stable at
temperatures up to a few hundred degrees
Celsius.
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Qualitative Analysis Gas chromatography is
widely used to establish the purity of organic
compounds. Contaminants, if present, are
revealed by the appearance of additional peaks in
the chromatogram. The areas under these
extraneous peaks provide rough estimates of the
extent of contamination. The technique is also
useful for evaluating the effectiveness of
purification procedures. Although a chromatogram
may not lead to positive identification of the
species in a sample, it often provides sure
evidence of the absence of species.
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Quantitative Analysis Quantitative GC is based
on comparison of either the height or the area of
an analyte peak with that of one or more
standards. If conditions are properly
controlled, both of these parameters vary
linearly with concentration. Peak area is
independent of the broadening effects.
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Calibration with Standards A series of standard
solutions that approximate the composition of the
unknown is prepared. Chromatograms for the
standards are then obtained, and peak heights or
areas are plotted as a function of concentration
to obtain a working curve. A plot of the data
should yield a straight line passing through the
origin quantitative analyses are based on this
plot.
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The Internal Standard Method The highest
precision for quantitative GC is obtained using
internal standards because the uncertainties
introduced by sample injection, flow rate, and
variations in column conditions are
minimized. A carefully measured quantity of an
internal standard is introduced into each
standard and sample and the ratio of analyte peak
area (or height) to internal standard peak area
(or height) is used as the analytical parameter.
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Advances in GC High-Speed Gas Chromatography The
basic idea is that, for many separations of
interest, higher speed can be achieved at the
expense of some selectivity and resolution. The
principles of high-speed separations can be
demonstrated by where kn is the retention
factor for the last component of interest in the
chromatogram. The advantage is faster
separations by using short columns,
higher-than-usual carrier gas velocities, and
small retention factors. The disadvantage is
reduced resolving power caused by increased band
broadening and reduced peak capacity.
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Multidimensional Gas Chromatography In
multidimensional GC, two or more capillary
columns of differing selectivities are connected
in series. Multidimensional GC can take several
forms. In one implementation, called heart
cutting, a portion of the eluent from the first
column containing the species of interest is
switched to a second column for further
separation. In another methodology,
comprehensive two-dimensional GC or GC ? GC, the
effluent from the first column is continuously
switched to a second short column. Although
the resolving power of the second column is
necessarily limited, the fact that a column
precedes it produces high-resolution separations.
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Gas-solid chromatography is based on adsorption
of gaseous substances on solid surfaces.
Distribution coefficients are generally much
larger than those for gas-liquid Chromatography,
hence it is useful for separating species that
are not retained by gas-liquid columns. Gas-solid
chromatography is performed with both packed and
open tubular columns. For the latter, a thin
layer of the adsorbent is affixed to the inner
walls of the capillary. Such columns are
sometimes called porous layer open tubular
columns, or PLOT columns.
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