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A2 · Practice questions · Chromatography

Spots, peaks and percentages.

Six original Cambridge-style questions on chromatography.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Two phases, two numbers.

01
Calculation
[2 marks]

A mixture is separated by TLC. The solvent front moves 7.5 cm and one spot moves 2.7 cm. Calculate the Rf value of the spot.

  • Rf = distance moved by spot ÷ distance moved by solvent front. ✓
  • Rf = 2.7 ÷ 7.5 = 0.360. ✓
02
Explain
[3 marks]

State what is meant by the stationary phase and the mobile phase in gas-liquid chromatography, and explain why components leave the column at different times.

  • Stationary phase: a non-volatile liquid coated on a solid support inside the column. ✓
  • Mobile phase: an unreactive carrier gas such as helium or nitrogen. ✓
  • Components partition between gas and liquid; one more soluble in the liquid (or less volatile) spends longer in the stationary phase, so it has a longer retention time. ✓
03
Calculation
[3 marks]

A GLC chromatogram of a fuel sample shows three peaks with areas 2.4, 5.6 and 8.0 cm2. Calculate the percentage of each component, assuming equal detector response.

  • Total area = 2.4 + 5.6 + 8.0 = 16.0 cm2. ✓
  • Percentages: 2.4 ÷ 16.0 × 100 = 15.0%; 5.6 ÷ 16.0 × 100 = 35.0%. ✓
  • 8.0 ÷ 16.0 × 100 = 50.0%. ✓
04
Recall
[3 marks]

A student runs a TLC plate of a mixture of amino acids. The spots are colourless. Describe two ways of locating them.

  • Spray with ninhydrin and warm the plate: amino acids show as purple spots. ✓
  • Or use a plate with a fluorescent indicator and shine UV light on it: spots appear dark. ✓
  • Iodine vapour (brown spots) is a general alternative. ✓
05
Analysis
[3 marks]

A food dye gives two spots with Rf 0.250 and 0.700. Pure dyes X, Y and Z run on the same plate have Rf 0.700, 0.420 and 0.250. Deduce which dyes are present and explain why the result is not absolute proof.

  • The spots match X (0.700) and Z (0.250); Y (0.420) is absent. ✓
  • Matching Rf values in one solvent suggest identity because each compound has a characteristic Rf for that solvent and stationary phase. ✓
  • Two different compounds could share an Rf in one solvent, so a second solvent (or another technique) is needed to confirm. ✓
06
Explain
[2 marks]

Explain why the solvent level must be below the baseline and why the tank is covered while the plate develops.

  • If the solvent covers the baseline the sample dissolves into the bulk solvent instead of moving up the plate. ✓
  • Covering keeps the air saturated with solvent vapour, so the solvent does not evaporate from the plate and the front moves evenly. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.