ST 18 shows the schematic make-up of a gas chromatograph.

In gas chromatography the separation takes place in a column. The mobile phase is a hot gas that flows through the column at a constant rate. The temperature of the column is adjusted so that all the components in the sample have a reasonable vapour pressure. In practice this means a column temperature that is ca. 50°C lower than the boiling point of the components to be separated. The stationary phase is either a solid or a liquid; we speak of solid – gas and liquid - gas chromatography respectively. Liquid – gas chromatography is almost always used; only when inert gases are being separated is solid - gas chromatography used. The separation relies on the difference in solubility of the components in the stationary liquid phase. The one component is more soluble than the other. The most soluble component stays in the liquid longest and exits from the column last. The least soluble exits first.

This technique is used for the separation of substances that are volatile but that do not decompose at the column temperature. The stationary phase may, of course, neither vaporise nor thermally decompose at the column temperature. The maximum temperature that the column can be set at is around 350°C. The stationary phases that can be usually used at this temperature are usually polymeric materials, especially polysiloxanes. In capillary columns the stationary phase consists of a film coated on the inner wall of the capillary tube. The length of the column varies from 10 metres to several tens of metres and the diameter varies between 0.5 and 1.0 mm.

The Illustration shows the various parts of the gas chromatograph. The components, dissolved in a volatile solvent, are injected into the injector (A). In the injector the mixture vaporises and is carried by the carrier gas (usually He or N₂) that is supplied by the gas cylinder (E), via a regulator (F), to the column (B). This column is situated in a programmable oven that regulates the temperature. The separation occurs on the column according to the same principle explained on the previous Illustration, ST 17. The separated components elute one by one into the detector (C).

One of the most common detection systems is the flame ionisation detector. In this system the components are burnt in the detector and form ions which generate a current.
This current is directly proportional to the concentration of the component and is recorded on either a recorder or a PC. This produces a so-called chromatogram. Today, the recorder has mostly been replaced by a PC which displays the peaks on the screen. The computer program can calculate the areas under the peaks, which are a measure of the quantity of each component present in the mixture.