# 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.

> OCR A-Level Chemistry A (H432) — 6.3 Analysis (chromatography, NMR) · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. The Rf value is calculated by dividing the distance travelled by the spot by the distance travelled by the solvent front
2. The Rf value is a characteristic/unique value for each substance (under the same conditions / with the same solvent)
3. The calculated Rf values are compared to known/reference Rf values (or a reference chromatogram) to identify the amino acids
4. A limitation is that spots may overlap / be too close together to distinguish when many components are present
5. A second identification method (e.g. a different solvent / two-way chromatography / further testing) would be needed to resolve overlapping spots

## Key terms

- [solvent front](https://www.gradenine.co.uk/glossary/solvent-front)
- [paper chromatography](https://www.gradenine.co.uk/glossary/paper-chromatography)

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-analysing-a-mixture-204da1a6) · Published by Druglandscape Ltd.