Gas /liquid chromatography

In gas-liquid chromatography, the stationary phase is a high-boiling liquid held on an inert solid support packed into a long column; the mobile phase is an inert carrier gas (e.g. nitrogen or helium).

The sample is vaporised and carried through the column by the gas. Each component partitions between the gas and the liquid — dissolving into the liquid and re-evaporating repeatedly. A component that is more soluble in the liquid (or less volatile) is held back longer; one that stays mostly in the gas moves through quickly. So the components emerge from the column at different times and are recorded as separate peaks by the detector.

A chromatogram is a plot of detector signal against time, with one peak per component.

Retention time — the time from injection until a component's peak appears — is characteristic of that component for a fixed column, temperature and flow rate. Comparing with known reference compounds run under identical conditions allows tentative identification (confirm by spiking: adding the suspected compound should make its peak grow).

Composition comes from peak areas, because the area under a peak is proportional to the amount of that component: $\%\,\text{component} = \frac{\text{peak area of component}}{\text{total area of all peaks}} \times 100$

Use areas, not heights — a broad short peak can represent as much material as a tall narrow one.

Limitations of retention time:

• Two different compounds can have the same retention time (co-elution), so a match is not proof.
• Retention time depends on the exact conditions (column, temperature, flow), so values are not transferable between instruments.

These are overcome by GC-MS (gas chromatography–mass spectrometry). The GC separates the mixture into individual components; each then passes into a mass spectrometer, which records its mass spectrum. The Mr and fragmentation pattern (matched against a database) give positive identification of each component — even unknowns. This is why GC-MS is used in forensics, drug testing and environmental analysis.