23.3 What is Gas Chromatography?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
23.3 What is Gas Chromatography?
Gas chromatography is the use of a carrier gas to convey the sample ( as a
vapour ) through a column consisting of an inert support and a stationary
phase that interacts with sample components, thus it is usually partition
chromatography. There are also a range of materials, especially for permanent
gas and light hydrocarbon analysis that utilise adsorption. The simplest
partition systems consisted of a steel tube filled with crushed brick that
had been coated with a hydrocarbon that had a high boiling point, eg
squalane. Today, the technique uses very narrow fused silica tubes ( 0.1 to
0.3mm ID ) that have sophisticated stationary phase films ( 0.1 to 5um )
bonded to the surface and also cross-linked to increase thermal stability.
The ability of the film to retard specific compounds is used to ascertain
the "polarity" of the column. If benzene elutes between normal alkanes
where it is expected by boiling point ( midway between n-hexane and
n-heptane ), then the column is "non-polar" eg squalane and methyl silicones.
If the benzene is retarded until it elutes after n-dodecane, then the column
is "polar" eg OV-275 ( dicyanoallyl silicone ) and 1,2,3-tris (2-cyanoethoxy)
propane. In general, polar columns are less tolerant of oxygen and reactive
sample components, but the ability to select different polarity columns to
obtain satisfactory peak resolution is what has made GC so popular.
The column is placed in an oven that has exceptional temperature control,
and the column can be slowly heated up to 350-450C ( sometimes starting at
-50C to enhance resolution of volatile compounds ) to provide separation of
wide-boiling range compounds. The carrier gas is usually hydrogen or helium,
and the eluting compounds can be detected several ways, including flames
( flame ionisation detector ), by changes in properties of the carrier
( thermal conductivity detector ), or by mass spectrometry. The availability
of "universal" detectors such as the FID and MS, makes GC a popular tool in
laboratories handling organic compounds. There are also columns that have a
layer of 5-10 um porous particulate material (such as molecular sieve or
alumina ) bonded to the inner walls ( PLOT = Porous layer open tubular ),
and these are used for the separation of permanent gases and light
hydrocarbons. GC is restricted to molecules ( or derivatives ) that
are sufficiently stable and volatile to pass through the GC intact at the
temperatures required for the separation. Specialist books on the production
of derivatives for GC are available [4,5].
There are several manufacturers of GC instruments whose catalogues and
brochures provide good introduction to the technique. (eg Hewlett Packard,
Perkin Elmer, Carlo Erba ). The catalogues of suppliers of chromatography
consumables also contain explanations of the criteria for selection of the
correct columns and conditions for analyses, and they provide an excellent
indication of the range of applications available. Well-known suppliers
include Alltech Associates, Supelco, Chrompack, J&W, and Restek. They also
sell most of the standard GC texts, as do the instrument manufacturers.
Popular GC texts include "Basic Gas Chromatography" [6], "High-Resolution
Gas Chromatography" [7], and "Open Tubular Column Gas Chromatography" [8].
There are Standard Retention Index Libraries available [9], however they
really only complement unambiguous identification by mass spec. or
dual-column analysis.
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