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7.11Advanced Organic Chemistry

Chromatography — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 12

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Chromatography: TLC, Column, Gas Chromatography and Chromatograms — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Compare the main chromatography techniques and interpret retention factors, retention times and chromatograms.Chromatography is a technique that enables the separatio…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Stationary phase
  • Mobile phase

Part 1 of 6Principle of chromatography

Chromatography is a technique that enables the separation of mixtures and includes thin-layer chromatography, column chromatography and gas chromatography.

Principle of chromatography
All methods have a stationary phase (a solid, or a liquid on a solid) and a mobile phase (a liquid or gas) that moves through or over it.
Components separate because they have different relative affinities for the two phases, through adsorption or solubility: a component that is strongly adsorbed by the stationary phase moves slowly; one that is more soluble in the mobile phase moves quickly.

Part 2 of 6Thin-layer chromatography (TLC)

The stationary phase is a thin layer of silica (SiO2) or alumina (Al2O3) on a glass or plastic plate.

1
Method: draw a pencil baseline; spot samples and references on it; place the plate in a covered tank with solvent below the baseline; remove when the solvent front is near the top and mark the front immediately; dry and develop colourless spots with UV light (fluorescent plate) or iodine vapour (or ninhydrin for amino acids).
2
The retention factor is Rf = distance moved by the spot / distance moved by the solvent front.
3
Values are between 0 and 1. Rf values depend on the solvent, so compare with known compounds run on the same plate.
4
TLC is quick and is used to check the purity of a product, follow the progress of a reaction, or identify components.

Part 3 of 6Column chromatography

A glass column is packed with silica or alumina (the stationary phase) and the mixture is added at the top.

1
A solvent (eluent) is run through.
2
Compounds are collected as they leave the column at different times.
3
Compounds more strongly adsorbed to the stationary phase leave later.
4
It is used to separate and purify larger quantities (preparative scale) of mixtures.
5
The solvent is evaporated from each fraction to recover the pure compound.

Part 4 of 6Gas chromatography (GC)

A gas chromatograph has a long, thin column coated with a liquid or a solid stationary phase, in an oven.

1
The sample is injected and vaporised and carried through by an inert carrier gas (e.g. helium or nitrogen), the mobile phase.
2
Components separate by their solubility in the stationary phase and by boiling point, and are detected as they leave.
3
The retention time is the time taken from injection to detection of a compound.
4
Compounds with a high affinity for the stationary phase have long retention times.
5
The chromatogram shows peaks; peak area is proportional to the amount of each compound,
  • so the percentage composition can be found from the ratio of peak areas.
    • (when calibrated)

GC can be linked to a mass spectrometer (GC-MS): each separated component is passed to the mass spectrometer, and its mass spectrum is compared with a database to identify it. GC-MS is used in forensic analysis, drug testing, environmental analysis and food safety.

Part 5 of 6Interpreting chromatograms

Count the number of peaks or spots to find the number of components; compare retention times or Rf values with those of known standards; and use peak areas to find the relative amounts. A single peak or spot suggests a pure substance, but only if different solvents all give a single spot.

Part 6 of 6Worked examples

  1. Calculate Rf for a spot that moved 3.6 cm when the solvent front moved 7.5 cm. [1]1 mark

  2. Why must the baseline be drawn in pencil and above the solvent level? [2]2 marks

  3. Explain why different compounds have different retention times in GC. [2]2 marks

  4. In a GC trace, how can the percentage of a component be found? [2]2 marks

Key terms

TermMeaning
Stationary phasePhase that stays fixed in chromatography.
Mobile phasePhase that moves through the stationary phase.
Rf valueDistance moved by spot ÷ distance moved by solvent front.
Retention timeTime for a compound to pass through a GC column to the detector.
GC-MSGas chromatography combined with mass spectrometry.

Avoid these mistakes

1
Drawing the baseline in pen.
2
Forgetting to mark the solvent front before it dries.
3
Saying GC separates by boiling point only.
4
Reporting Rf as greater than 1.

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