Core idea

· Gas/liquid chromatography (GLC) separates substances in a mixture because each component spends different amounts of time in the mobile phase and stationary phase.
· The sample is vaporised and carried through a column by an unreactive gas.
· Components that interact more strongly with the stationary phase move more slowly and have a longer retention time.
· Components that interact weakly with the stationary phase spend more time in the gas phase and have a shorter retention time.
· GLC is useful for separating and analysing volatile substances in mixtures.

This diagram shows the basic pathway of a sample through a gas-liquid chromatograph. The sample is injected, carried by an unreactive gas through the column, separated, detected and recorded as peaks. Source

Stationary phase and mobile phase

· Stationary phase = a high boiling point non-polar liquid held on a solid support.
· Mobile phase = an unreactive gas, often called the carrier gas.
· The mobile phase carries vaporised sample molecules through the column.
· The stationary phase does not move; it causes separation by interacting differently with different components.
· In CIE answers, always state both phases clearly: non-polar liquid stationary phase and unreactive gas mobile phase.

Retention time

· Retention time = the time from injection of the sample to the maximum height of the peak for that component.
· Each component in a mixture gives a peak on the chromatogram.
· A short retention time means the component passed through the column quickly.
· A long retention time means the component was retained in the column for longer.
· Retention times can help identify substances when compared with known standards under the same conditions.

This chromatogram shows separated components as peaks. The time position of each peak gives the retention time, while the area under each peak is used to estimate relative amount. Source

Explaining retention times

· A substance with stronger interaction with the stationary phase has a longer retention time.
· A substance with weaker interaction with the stationary phase has a shorter retention time.
· For a non-polar stationary phase, more non-polar components usually interact more strongly and may have longer retention times.
· Substances with higher boiling points often spend less time in the gas phase and may have longer retention times.
· A higher column temperature generally makes substances spend more time in the gas phase, so retention times usually become shorter.

These diagrams help show how the column provides contact between the moving carrier gas and the stationary phase. Different strengths of interaction with the stationary phase cause different components to leave the column at different times. Source

Interpreting chromatograms

· A chromatogram is a graph of detector response against retention time.
· Each peak represents a component leaving the column and reaching the detector.
· Peak position identifies the component by its retention time.
· Peak area shows the relative amount of that component in the mixture.
· Do not use peak height alone unless the question specifically tells you to; use area under the peak for composition.

Percentage composition

· Percentage composition is found from peak areas, not retention times.
· Formula: percentage of component = (area of that component’s peak ÷ total area of all peaks) × 100.
· If areas are already given, add all peak areas to find the total peak area.
· Larger peak area = greater proportion of that component in the mixture.
· Exam questions may give peak areas directly or ask you to estimate them from a chromatogram.

This labelled instrument diagram shows how the carrier gas, injector, column, oven and detector fit together. It is useful for linking the chromatogram peaks to the physical movement of components through the column. Source

Common exam traps

· Do not say the mobile phase is a liquid; in GLC it is an unreactive gas.
· Do not say the stationary phase is solid; in GLC it is a high boiling point non-polar liquid on a solid support.
· Do not confuse retention time with Rf value; Rf values are for TLC, not GLC.
· Do not use peak height instead of peak area for percentage composition unless instructed.
· Do not compare retention times unless conditions such as column, stationary phase and temperature are the same.