Chromatography

AQA

A-Level

Chromatography relies on the differential distribution of substances between a mobile phase and a stationary phase to separate components within a mixture. Separation occurs because components travel at different speeds depending on their relative solubility in the solvent and affinity for the stationary medium. Candidates must demonstrate the ability to calculate Retardation Factor (Rf) values and use these ratios to identify unknown substances against reference standards. Distinction between pure substances and mixtures is assessed through the observation of single versus multiple spots on the resulting chromatogram.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating separation depends on the balance between solubility in the mobile phase and retention by the stationary phase
Award 1 mark for calculating Rf value = distance moved by spot / distance moved by solvent front
Credit responses that identify the stationary phase in TLC as solid silica or alumina acting by adsorption
Award 1 mark for explaining that in Gas Chromatography (GC), separation is based on relative solubility in the liquid stationary phase
Credit reference to using UV light or a developing agent (e.g., ninhydrin) to locate colourless spots on a TLC plate

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You identified the correct component, but failed to link its position to its affinity for the stationary phase."
"Remember, in TLC, the mechanism is adsorption onto the solid, not solubility in the solid."
"Your Rf calculation is correct, but ensure you measure from the baseline, not the bottom of the paper."
"Excellent link between polarity and retention time; consider how changing the solvent polarity would affect this result."

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating separation depends on the balance between solubility in the mobile phase and retention by the stationary phase
Award 1 mark for calculating Rf value = distance moved by spot / distance moved by solvent front
Credit responses that identify the stationary phase in TLC as solid silica or alumina acting by adsorption
Award 1 mark for explaining that in Gas Chromatography (GC), separation is based on relative solubility in the liquid stationary phase
Credit reference to using UV light or a developing agent (e.g., ninhydrin) to locate colourless spots on a TLC plate

Examiner Tips

Expert advice for maximising your marks

💡In 6-mark practical descriptions, explicitly mention drawing the baseline in pencil to prevent ink contamination affecting results
💡When comparing substances in GC, link shorter retention times to lower solubility in the stationary phase and higher volatility
💡Always check the polarity of the solvent and stationary phase; polar components retain longer on polar stationary phases due to stronger intermolecular forces
💡For GC-MS questions, clearly distinguish that GC separates the components while MS identifies them via m/z ratios

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the mechanism of separation: stating 'solubility' for TLC (which is adsorption) or 'adsorption' for GC (which is partition)
Measuring the solvent front distance from the bottom of the plate instead of from the pencil baseline (origin)
Failing to explain that the solvent level must be below the baseline to prevent spots from dissolving into the reservoir immediately
Stating that GC identifies substances solely by retention time without acknowledging the need for external calibration standards or MS data

Key Terminology

Essential terms to know

Likely Command Words

How questions on this topic are typically asked

State

Explain

Calculate

Deduce

Suggest

Describe

Practical Links

Related required practicals

•{"code":"Required Practical 12","title":"Separation of species by thin-layer chromatography","relevance":"Direct assessment of method, analysis, and error identification"}

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