Chromatography: Thin-Layer and Gas/Liquid Chromatography

This guide covers subtopics 37.1, Thin-layer chromatography, and 37.2, Gas/liquid chromatography, from Topic 37, Analytical techniques, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content, grouped together as the syllabus’s two separation (rather than structure-elucidation) techniques — NMR Spectroscopy: Carbon-13 and Proton NMR covers the remaining two, structure-determining subtopics of Topic 37.

Before studying this

This resource assumes the general idea of interpreting an analytical technique’s output (rather than needing to know an instrument’s internal mechanism) from Analytical Techniques: IR and Mass Spectrometry. No further prior organic chemistry is assumed beyond recognising that different compounds are, chemically, different substances with different properties.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 37

37.1 Thin-layer chromatography — describing and understanding stationary phase (e.g. aluminium oxide on a solid support), mobile phase (a polar or non-polar solvent), Rf value, solvent front and baseline; interpreting Rf values; explaining differences in Rf values in terms of interaction with the stationary phase and relative solubility in the mobile phase.

37.2 Gas/liquid chromatography — describing and understanding stationary phase (a high-boiling-point non-polar liquid on a solid support), mobile phase (an unreactive gas), and retention time; interpreting gas/liquid chromatograms in terms of percentage composition of a mixture; explaining retention times in terms of interaction with the stationary phase.

Thin-layer chromatography (TLC)

TLC separates a mixture of dissolved compounds by how they distribute between two phases as they travel: a stationary phase (commonly aluminium oxide, or silica, coated as a thin layer on a solid support plate) that stays fixed, and a mobile phase (a polar or non-polar solvent, chosen to suit the compounds being separated) that moves up the plate by capillary action, carrying the dissolved sample with it.

Key terms: the baseline is where the sample is originally spotted, near the bottom of the plate; the solvent front is the highest point the solvent itself reaches as it travels up the plate; the Rf value (retention factor) for a spot is:

Rf = distance travelled by the spot / distance travelled by the solvent front

Rf is always between 0 and 1, and — for a given stationary phase, mobile phase and temperature — is a genuinely characteristic, reproducible property of a compound, useful for identifying it by comparison against known reference values or reference spots run alongside the sample.

Interpreting Rf values. A compound with a higher Rf value has travelled further relative to the solvent; one with a lower Rf value has travelled less far and stayed closer to the baseline.

Explaining differences in Rf values. Two competing interactions determine how far a compound travels: how strongly it’s attracted to (adsorbed onto) the stationary phase, and how soluble it is in the mobile phase.

• A compound that interacts strongly with the stationary phase (e.g. through polar interactions with aluminium oxide) is held back more, giving a lower Rf value.
• A compound that is more soluble in the mobile phase is carried further up the plate as the solvent moves, giving a higher Rf value.

A compound’s overall Rf value reflects the balance of these two competing effects — a more polar compound typically interacts more strongly with a polar stationary phase (lower Rf) but may also be more soluble in a polar mobile solvent (higher Rf), so the specific choice of stationary and mobile phase determines which compounds separate well from each other in a given TLC system.

Gas/liquid chromatography (GLC)

GLC separates a mixture of volatile compounds using a stationary phase (a high-boiling-point non-polar liquid, coated onto an inert solid support packed inside a long column) and a mobile phase (an unreactive carrier gas, such as nitrogen or helium, that doesn’t react with or dissolve the sample) that carries the vaporised sample through the column.

Retention time is the time taken for a particular component of the mixture to travel through the column and reach the detector at the far end, recorded as a distinct peak on the resulting chromatogram.

Interpreting a gas/liquid chromatogram. Each peak corresponds to one component of the mixture, appearing at its own characteristic retention time (useful for identification, by comparison with known reference retention times run under the same conditions). The area under each peak is proportional to the amount of that component present, so the percentage composition of a mixture is calculated directly from the relative peak areas:

percentage of component X = (area of X’s peak / total area of all peaks) × 100%

Worked example. A GLC trace of a three-component mixture shows peak areas of 12.0, 30.0 and 18.0 (arbitrary units) for components A, B and C. Calculate the percentage composition of B.

Total area = 12.0 + 30.0 + 18.0 = 60.0

Percentage of B = (30.0 / 60.0) × 100% = 50.0%

Explaining retention times. A component’s retention time depends on how strongly it interacts with (dissolves into, or is adsorbed by) the non-polar liquid stationary phase relative to how readily it stays in the gas phase and moves on with the carrier gas.

• A component that interacts more strongly with the stationary phase (for example, a less volatile, more non-polar compound that dissolves well into the non-polar stationary liquid) spends more time associated with the stationary phase and less time moving with the gas — giving a longer retention time.
• A component that interacts less strongly with the stationary phase moves through the column with the carrier gas more freely, giving a shorter retention time.

Common mistakes

Confusing which phase is which between TLC and GLC. In TLC, the stationary phase is a solid (or solid-supported) surface and the mobile phase is a liquid solvent; in GLC, the stationary phase is a liquid and the mobile phase is a gas — easy to mix up under exam pressure since both techniques use the same “stationary/mobile” vocabulary for genuinely different physical setups.

Forgetting that Rf is measured from the baseline, and as a ratio, not an absolute distance. Two TLC plates run for different total distances (but otherwise identical conditions) should still give the same Rf for the same compound, since Rf is a ratio — an absolute spot distance alone isn’t enough to identify a compound without dividing by the solvent front distance.

Calculating percentage composition from peak height instead of peak area. Area (not height) is proportional to the amount of a component present — a broader, shorter peak can represent the same amount of material as a narrower, taller one.

Assuming a longer retention time always means “more” of a compound. Retention time reflects how strongly a compound interacts with the stationary phase (related to its own chemical properties), not the quantity present — quantity is read from peak area, an entirely separate measurement.

Quick revision checklist

• TLC: stationary phase solid (e.g. Al₂O₃), mobile phase liquid solvent; Rf = spot distance / solvent front distance
• Lower Rf: interacts more strongly with the stationary phase, or less soluble in the mobile phase
• GLC: stationary phase a non-polar liquid, mobile phase a carrier gas; retention time = time to reach the detector
• Percentage composition from GLC: relative peak area, not peak height
• Longer retention time: stronger interaction with the (liquid) stationary phase, not necessarily more of that component present

Related resources

• Analytical Techniques: IR and Mass Spectrometry — the AS-level analytical techniques this topic’s approach builds on
• NMR Spectroscopy: Carbon-13 and Proton NMR — the remaining two subtopics of Topic 37
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.