AQA A-Level Chemistry: Chromatography (7405) – Revision Notes

These notes condense section 3.3.16 Chromatography of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams from June 2016 onwards. The section is “A-level only”. It is part of the organic chemistry content listed under A-level Paper 2, and Paper 3 can assess any content and any practical skill, including Required practical 12.

For full explanations and worked examples, read the study guide. To test yourself, go to the practice questions. The AQA A-Level Chemistry hub, the printable checklist and the free diagnostics help you plan the rest of your revision.

Key definitions

Term	Meaning
Chromatography	A method of separating and identifying the components of a mixture
Stationary phase	The phase that stays in place (the solid on the plate, in the column, or the solid or liquid-coated solid in a GC column)
Moving phase	The phase that flows past (liquid solvent in TLC and CC; gas in GC)
Baseline	Pencil line on a TLC plate where the samples are spotted
Solvent front	How far the solvent has risen when the plate is removed
Rf value	Distance moved by spot ÷ distance moved by solvent front, both from the baseline
Retention time	Time from injection into the GC column until the component reaches the detector
Standard	A pure, known substance run under the same conditions for comparison

The one principle behind every type

Separation depends on the balance between solubility in the moving phase and retention by the stationary phase.

• More soluble in the moving phase, less retained → moves faster → larger Rf, earlier from the column, shorter retention time.
• Strongly retained by the stationary phase → moves slower → smaller Rf, later from the column, longer retention time.
• Same balance → no separation (one spot or one peak).

The three types compared

	TLC	Column (CC)	Gas (GC)
Stationary phase	Solid coated on a plate	Solid packed in a column	Solid, or solid coated by a liquid, packed in a column
Moving phase	Solvent	Solvent	Gas, under pressure
Direction	Solvent moves up the plate	Solvent moves down the column	Gas passes through the column at high temperature
What you measure	Rf values	Order the components come out (fractions collected)	Retention times
Identify by	Comparing Rf with standards on the same plate	Testing the collected fractions	Comparing retention times with standards under the same conditions

Rf values

Rf = distance from baseline to centre of spot
     ----------------------------------------
     distance from baseline to solvent front

• No units. Always between 0 and 1.
• Depends on the stationary phase, the solvent and the conditions, so compare only with standards run alongside.
• A match is evidence, not proof: run again in a different solvent to strengthen it.

Worked reminder. Solvent front 7.00 cm, spot centre 3.15 cm: Rf = 3.15 / 7.00 = 0.45.

Method in steps: identifying substances by TLC

1. Spot the sample and each standard on the same baseline.
2. Run the plate and mark the solvent front.
3. Calculate the Rf value of every spot.
4. Match each sample spot to the standard with the same Rf value.
5. A sample spot with no match is a substance that is not among the standards.
6. If needed, repeat with a different solvent to strengthen the identification.

Gas chromatography and GC-MS

• A gas passes through the column under pressure at high temperature; the sample is vaporised and carried through.
• Strongly retained components have long retention times.
• Raising the column temperature shortens retention times, because components spend more time in the gas.
• GC-MS: components leave the GC column one at a time and pass into a mass spectrometer. The molecular ion gives Mr; high-resolution m/z gives the molecular formula (section 3.3.6.2, see the organic analysis notes).

Method in steps: GC-MS formula from a precise m/z

1. List the candidate formulae (they share the same whole-number Mr).
2. Calculate each precise mass from the given atomic masses.
3. Choose the formula whose precise mass matches the measured m/z.
4. Show every sum; a bare choice is not evidence.

Method in steps: Required practical 12 (TLC)

Apparatus and techniques: AT a, AT i (thin-layer or paper chromatography) and AT k (safe handling).

1. Handle the plate by its edges, wearing gloves.
2. Pencil baseline about 1 cm from the bottom.
3. Small, concentrated spots of sample and standards on the baseline; let them dry.
4. Solvent depth below the baseline.
5. Lid on the container (saturates the air with solvent vapour).
6. Remove near the top; mark the solvent front at once.
7. Dry; locate colourless spots (for example ultraviolet light, or a developing agent such as ninhydrin for amino acids, as section 3.3.13.2 states).
8. Measure to spot centres; calculate Rf; compare with standards.

Why each step matters

Step	Reason
Pencil, not ink	Graphite does not dissolve; ink would dissolve and separate
Solvent below baseline	Otherwise the samples dissolve into the solvent pool
Lid	Stops solvent evaporating from the plate, so the front rises evenly
Mark the front at once	The solvent evaporates and the front disappears
Small spots	Large spots spread and overlap
Standards on the same plate	Identical conditions, so Rf values are comparable

Safety: solvents are often flammable (no flames) and may be harmful (fume cupboard); never look into an ultraviolet lamp.

Must-know distinctions

Don’t confuse	Difference
Moving phase in TLC vs GC	Liquid solvent in TLC; gas in GC
TLC vs column direction	Solvent moves up a TLC plate, down a column
Rf vs retention time	Rf is a ratio of distances (TLC); retention time is a time (GC)
Large Rf vs long retention time	Large Rf = weakly retained; long retention time = strongly retained
Identifying vs separating	GC separates; MS then identifies each separated component
Spot distance vs plate length	Measure from the baseline, not from the plate edge

Quick self-test

1. A solvent front is 8.00 cm from the baseline and a spot centre is 2.80 cm from it. Calculate the Rf value.
2. A substance has Rf = 0.42. The solvent front moved 5.50 cm. How far did the spot move?
3. State the moving phase in gas chromatography.
4. In which type of chromatography does the solvent move down through the stationary phase?
5. Why is the TLC baseline drawn in pencil?
6. Why must the solvent level be below the baseline?
7. Define retention time.
8. What happens to GC retention times when the column temperature is raised?
9. A sample spot and a standard spot have the same Rf in one solvent. Is this proof they are the same substance? What further test helps?
10. A GC component gives a molecular ion at m/z = 86.0729. Is it C₄H₆O₂ or C₅H₁₀O? (H = 1.0078, C = 12.0000, O = 15.9949)
11. On a polar stationary phase with a less polar solvent, which travels further: a compound that can hydrogen bond or a weakly polar compound that cannot?
12. In column chromatography, which component of a mixture is collected in the first fraction?

Answers

1. Rf = 2.80 / 8.00 = 0.35.
2. 0.42 × 5.50 = 2.31 cm.
3. A gas (passed through under pressure).
4. Column chromatography.
5. Pencil does not dissolve in the solvent; ink would dissolve and separate.
6. So the samples are not washed off the baseline into the solvent.
7. The time from injection to the component reaching the detector.
8. They decrease: components spend more time in the gas phase.
9. No. Two substances can share an Rf value in one solvent. Run sample and standard together in a different solvent; a match again is stronger evidence.
10. C₅H₁₀O = 5(12.0000) + 10(1.0078) + 15.9949 = 86.0729; C₄H₆O₂ = 86.0366. It is C₅H₁₀O.
11. The weakly polar compound: it is retained less by the polar stationary phase and dissolves better in the less polar solvent.
12. The one least retained by the stationary phase and most soluble in the solvent, because it moves down the column fastest.

Where marks are usually lost

• Measuring from the bottom of the plate, or to the top of the spot, instead of baseline to spot centre.
• Dividing solvent front by spot distance, giving Rf greater than 1.
• Quoting Rf with units such as cm.
• Explaining separation with only one phase; you need both solubility in the moving phase and retention by the stationary phase.
• Saying a liquid moves through a GC column; the moving phase is a gas.
• Giving “to make it neat” as the reason for a lid; the reason is a saturated atmosphere that stops evaporation from the plate.
• Claiming a matching Rf or retention time proves identity.
• Comparing with a standard run on a different plate or a different GC column.
• In GC-MS, saying the mass spectrometer separates the mixture; GC separates, MS identifies.

Official syllabus

AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards (AQA). Section 3.3.16 Chromatography (A-level only), including Required practical 12.