Instrumentation for Gas Chromatography - Forensic Applications of Gas Chromatography - page 16

Polyimide coating

Fused-silica

Stationary phase

(a) Schematic diagram of the physical construction of a typical GC column

Capillary column
(b) Photograph of a typical GC column coiled and mounted on a circular 'cage'

Support 'cage'

Figure 2.12 Capillary gas chromatography column: physical characteristics.

• Length of the capillary column
• Film thickness of the stationary phase

Each of these parameters can have a significant impact on the ability to sepa-
rate the compounds of interest. The basic anatomy of a capillary GC col-
umn is shown in Figure 2.12(a) as well as a photograph of a capillary column
coiled and mounted on a circular metal frame or 'cage' in Figure 2.12(b).
The stationary phase is chemically immobilised on the internal surface of
a fused silica tube. However, the brittle nature of the fused silica requires that
it be coated in a polymer (i.e., polyimide) that provides rigidity and flexibility
to the column as well as giving the GC column its overall brown colouration.

2.5.1 Stationary Phase Selection
Perhaps the most important of the four parameters is the choice of stationary
phase. The most commonly used stationary phases are based on polysiloxane
(Figure 2.13). Using the adage that 'like dissolves like', it would be appropri-
ate to try to match the stationary phase polarity with the polarities of the
compounds to be separated (e.g., for nonpolar compounds choose a nonpo-
lar stationary phase—that is, 100% polydimethylsiloxane. ( Note: Nonpolar