A student is analysing a mixture of amino acids separated by paper chromatography. After the experiment, the student measures the distance travelled by each spot and the distance travelled by the solvent front. Explain how the student can use these measurements to identify the amino acids present in the mixture, and describe one limitation of this technique when the mixture contains a large number of components.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography can be used to separate and identify the components of a mixture. The distance each component travels relative to the solvent front gives a characteristic value that can be used for identification.
Model answer (5 marks)
The student calculates the Rf value for each spot by dividing the distance travelled by that spot by the distance travelled by the solvent front.
Because the Rf value is characteristic for a given substance under the same experimental conditions, the student can compare the calculated Rf values with known reference Rf values (or a reference chromatogram) for amino acids. Matching Rf values allow the student to identify which amino acids are present in the mixture.
A limitation of paper chromatography is that when the mixture contains many components, the spots can overlap or be too close together to distinguish, making identification difficult.
Because the Rf value is characteristic for a given substance under the same experimental conditions, the student can compare the calculated Rf values with known reference Rf values (or a reference chromatogram) for amino acids. Matching Rf values allow the student to identify which amino acids are present in the mixture.
A limitation of paper chromatography is that when the mixture contains many components, the spots can overlap or be too close together to distinguish, making identification difficult.
Examiner tips
- Show the Rf calculation clearly; use the formula Rf = distance spot / distance solvent front. Mention that Rf values are compared to reference values. Highlight the limitation of overlapping spots. Keep the answer concise and to the point.
Common mistakes
- Calculating Rf incorrectly (e.g. using solvent front distance as denominator for all spots). Failing to mention that Rf values are characteristic only under the same conditions. Ignoring the issue of overlapping spots when many components are present.
Mark scheme (5 marks)
- The Rf value is calculated by dividing the distance travelled by the spot by the distance travelled by the solvent front
- The Rf value is a characteristic/unique value for each substance (under the same conditions / with the same solvent)
- The calculated Rf values are compared to known/reference Rf values (or a reference chromatogram) to identify the amino acids
- A limitation is that spots may overlap / be too close together to distinguish when many components are present
- A second identification method (e.g. a different solvent / two-way chromatography / further testing) would be needed to resolve overlapping spots
Key terms in this question
solvent front · paper chromatography
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Analysis (chromatography, NMR) questions
- A pharmaceutical company uses paper chromatography to check the purity of a newl…
- A forensic scientist uses thin-layer chromatography (TLC) to compare an ink samp…
- A chemist uses thin-layer chromatography (TLC) to investigate a sample of an unk…
- A student uses paper chromatography to analyse a mixture of food colourings. The…