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Learn9701Paper 4 · A Level Structured QuestionsAnalytical techniquesChromatography techniques
Analytical techniques · Subtopic 2 of 2

Chromatography Techniques

Chromatography separates mixtures using a stationary phase and a mobile phase. Components that interact more strongly with the stationary phase move more slowly. TLC: silica/alumina on a plate (stationary), solvent (mobile); adsorption; Rf=distance moved by spotdistance moved by solvent frontR_f=\frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}; colourless spots shown by UV, iodine or ninhydrin (amino acids). Gas chromatography (GC): liquid coating on a column (stationary), inert carrier gas (mobile); separation by partition and boiling point; retention time identifies compounds; peak area gives relative amount. GC–MS feeds each separated component into a mass spectrometer for identification. Column chromatography and HPLC separate larger quantities.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 2≈ 40 minRevision 1 · updated 2026-11-01
Section 1 of 14

Learning objectives

  • Describe stationary and mobile phases.
  • Calculate and use Rf values.
  • Interpret gas chromatograms using retention times and peak areas.
  • Describe GC–MS.
Pause and think

Two spots in a TLC run have the same Rf in one solvent. Does that prove they're the same compound?

Section 2 of 14

Core idea

Start hereSeparating the inseparable

Drug tests find tiny traces in urine by separating hundreds of compounds and identifying each one.

The big idea

Components share their time between a stationary and a mobile phase. Stronger attraction to the stationary phase means slower movement.

Choosing a technique

Each technique has a stationary and a mobile phase.

Worked exampleRf value

Spot moves 3.2 cm, solvent front 8.0 cm. Rf?

Goal
Rf.
Strategy
Spot ÷ front.
  1. Rf=3.28.0R_f=\frac{3.2}{8.0}
    Definition.
  2. Rf=0.40R_f=0.40
    Between 0 and 1.
Answer0.40.
✓ CheckLess than 1 ✓.

Summary

  • Rf = spot / front.
  • GC retention time; peak area = amount.
  • GC–MS identifies.
Key words
Retention time
Time from injection to detection in GC.
Stationary phase
The phase that does not move.
HistoryTsvet's colour writing. Mikhail Tsvet separated plant pigments in a column of chalk in 1903, naming the technique "chromatography" (colour writing).
Fun factOlympic drug testing. Anti-doping labs use GC–MS and LC–MS to detect banned substances at parts-per-billion levels in urine samples.
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