37.2 Gas / liquid chromatography

In gas chromatography the stationary phase is a high-boiling, non-polar liquid held on a solid support. The mobile phase is an unreactive carrier gas. Retention time is the time from injection to the detector signal.

Compounds repeatedly partition between the gas and stationary liquid. The instrument records when each compound emerges, not its Rf.

A compound with weak attraction to the stationary liquid spends more time in the carrier gas and reaches the detector sooner.

The carrier gas is not the sample and retention time is not a boiling point, although volatility can influence how long a compound remains in the column.

In a gas chromatogram, each resolved peak represents a component and its retention time helps identify it. The peak area is proportional to the amount detected, so relative areas estimate percentage composition when detector response factors are comparable.

Add the relevant peak areas, then divide each area by the total area and multiply by 100. Do not use peak height unless the method specifically validates it.

Peak areas 20, 30 and 50 give estimated compositions of 20%, 30% and 50%.

A larger retention time does not mean a larger percentage; time identifies the component, while area estimates amount.

Retention time is longer when a compound spends more time dissolved in or adsorbed by the stationary phase rather than travelling with the carrier gas.

Volatility and intermolecular interactions both matter. Compare retention times only under the same column, temperature program and carrier-gas conditions.

Two compounds with similar injection amounts can have different retention times because one has stronger attraction to the non-polar stationary liquid.

Retention time is not a universal identity constant: changing the column or temperature can shift it.