Required practical · A-level chemistry
RP 12: Thin-layer chromatography
Hazards
⚠Developing solvents — flammable; lidded tank, no flames
⚠Ninhydrin spray — harmful; fume cupboard only
⚠UV lamp — never look directly at the source
Method
- Prepare the plateDraw the baseline in PENCIL about 1 cm from the bottom of the TLC plate, handling the plate only by its edges — fingerprints deposit grease and amino acids that show up later.
- Spot the samplesUse a fine capillary to place small, concentrated spots on the baseline, labelled in pencil beneath. Let each spot dry and re-spot two or three times to build up concentration without spreading.
- Develop in a closed tankPour solvent to BELOW the baseline, stand the plate in, and put the lid on so the atmosphere saturates with solvent vapour (otherwise solvent evaporates off the plate mid-run and the spots crawl). Remove the plate when the front is ~1 cm from the top.
- Mark the solvent front immediatelyPencil-mark the solvent front the second the plate comes out — it evaporates in moments and cannot be recovered.
- Visualise the spotsColourless compounds: view under UV light (fluorescent-backed plates show dark spots), or spray with ninhydrin and warm for amino acids (purple spots).
- Calculate RfRf = distance from baseline to the CENTRE of the spot ÷ distance from baseline to the solvent front. Compare against reference values, or better, against known standards run on the same plate.
Mistakes examiners see every year
| The mistake | Why it costs marks |
|---|---|
| Ink baseline | Ink is itself a dye mixture — it chromatographs all over your plate. Pencil only. |
| Solvent above the baseline | The samples dissolve off the plate into the tank instead of running up it. Solvent starts below the line. |
2 more examiner traps for this practical
Plus the step-by-step Lab Mode wizard with apparatus diagrams that highlight as you work.
Open it in ChemLabWhat to expect
Spots resolve up the plate with Rf between 0 and 1, constant for a given compound + solvent + stationary phase (AQA 3.3.15). Matching an unknown's Rf to a standard on the same plate identifies it; two spots with one Rf each means a two-component mixture.