In gas-liquid chromatography (g.l.c.) a long tube contains the chromatography material. The tube is usually coiled so that it takes up less space.

The column is fitted inside a thermostatically controlled oven. The operating temperature can be changed during the run if necessary.

The sample under analysis is carried through the column in a stream of inert gas, such as nitrogen.

The gas stream will carry the sample through and out of the column.

The different components in the sample are detected as they leave. A device called a flame ionisation detector is often used.

Signals from the detector are analysed and plotted by a recorder as a chromatogram.

Each component in the sample is represented by a peak in the chromatogram.

The sample under study is introduced into the column using a syringe.

Let's follow a g.l.c. run.

The carrier gas (the mobile phase) moves the sample through the column.

The stationary phase is a high boiling point liquid bonded to an inert solid in the column.

Different substances have different affinities for the stationary phase compared to the mobile phase. They pass the detector at different times.

The different times are called retention times.

This is a typical gas chromatogram of a premium grade petrol.

Each peak represents a different component of the petrol. The area under a peak represents the relative amount of that component.

Many factors influence the retention time, so known standard compounds are usually added to the sample before the run.