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sample loop system. Injection techniques somewhat similar to those used for liquid
samples are used for solids. In order to produce sharp chromatographic peaks with
minimum peak overlap, solid and liquid samples must be vaporized rapidly upon
injection by maintaining the injection port at a temperature greater than the boiling
point of the sample.
Solid-phase microextraction (SPME) is a relatively new method that allows trace
analytes to be introduced into the chromatographic system without the need for sol-
vents. The method, which was developed in the early 1990s at the University ofWater-
loo in Ontario, Canada, involves exposing a small segment of fused silica fiber coated
with a non-volatile polymeric material. The coated fiber is mounted in a syringe-like
device that can expose the fiber to the desired environment and also withdraw it for
protection during transfer to the gas chromatograph. The analyte of interest adsorbs on
the fiber coating and is thermally desorbed when introduced into the chromatographic
injection port. SPME has been commercialized by Supelco, Inc.™ and Varian.™
The column is the heart of the gas chromatograph, and separation of components
on packed columns depends more on the choice of liquid phase than on any other
factor. Typically the column is a glass or metal tube of 0.125 or 0.25 in. (6 or 13 mm)
in diameter and 4–6 ft (1–2 m) in length, packed with an inert diatomaceous earth
support coated with a nonvolatile liquid to 3–20% by weight. In open tubular or cap-
illary column technology, the support for the thin film of liquid phase is the wall of
the capillary itself. Support-coated open tubular columns are also sometimes used,
the sample capacity of the columns being increased by the presence of very loosely
packed support or by a roughening of the capillary walls.
The detector produces a response that is proportional to component that is sepa-
rated by column and is located at the end of the column. Different detectors may be
utilized dependant upon the analyte of interest and include a photoionization detector
(PID), flame ionization detector (FID), thermal conductivity detector (TCD), elec-
tron capture detector (ECD), flame photometric detector (FPD) or far UV absorbance
detector (FUV). Some of these detectors, such as the PID, have been commercial-
ized as hand-held units. These portable PID sensors use a small pump to suck vapor
through the ionization chamber. Water vapor is completely ignored, but it cannot
provide discrimination among different chemicals.
An amplifier, which could be considered part of the detector “package” receives an
output from a detector and amplifies it so that the signal can be detected by a recorder
or integrator. Subsequently, an integrator takes the signal from the amplifier and
produces an output (chromatogram) and peak height or area (used for quantification).
The height of the peak measured from the baseline to the peak maximum and the area
which is determined by integrating the area underneath the peak are proportional to
concentration. Generally integrators will provide both area and height values. At low
concentrations with packed columns, peak height may provide a better value.
Pros: The bench-top gas chromatograph can provide superior discrimination capabil-
ities (relative to other devices and sensors) with excellent precision, sensitivity, and
reproducibility.
Cons: Not portable. Expensive. Requires training to operate.