Instant Notes: Analytical Chemistry

pressures and hence their boiling points. Solutes are therefore generally eluted

in order of increasing boiling point, except where there are specific interactions

with the stationary phase. The gaseous mobile phase elutes the solutes from the

end of the column where they pass through a detector that responds to each

one. An elevated temperature, usually in the range 50–350∞C, is normally

employed to ensure that the solutes have adequate volatility and are therefore

eluted reasonably quickly.

There are two modes of gas chromatography:

∑ Gas-liquid chromatography(GLC), which employs a liquid stationary phase

in which solutes can dissolve, the sorption process being partition. Specific

interactions of solutes with the stationary phase may alter the order of

elution from that of increasing boiling points. GLC is by far and away the

more widely used mode of GC, the large number of alternative stationary

phases enabling many types of sample to be analyzed.

∑ Gas-solid chromatography(GSC) employs a solid, sometimes polymeric,

sorbent as the stationary phase, the sorption process being surface adsorp-

tion. GSC has limited specialist applications, being used mainly for

analyzing mixtures of gases or solvents with relatively low relative molecular

masses.

A schematic diagram of a gas chromatographis shown in Figure 1. It consists of

five major components:

138 Section D – Separation techniques

Microliter syringe

(or autosampler,

sample valve)

Gas flow control system

Carrier gas

Split flow

Sample

Injector

Septum

Makeup flow

Detector

Capillary column

Amplifier

Chromatogram

Column oven

Fig. 1. Schematic diagram of a capillary column gas chromatograph. Reproduced from D.W. Grant, Capillary Gas
Chromatography, 1996. © John Wiley & Sons Ltd. Reproduced with permission.