# A forensic scientist uses thin-layer chromatography (TLC) to compare an ink sample found at a crime scene with inks from three suspect pens, labelled P, Q and R. The crime-scene ink produces two spots at distances of 3.2 cm and 5.6 cm from the baseline. The solvent front travels 8.0 cm from the baseline. Pen P produces one spot at 5.6 cm, pen Q produces spots at 3.2 cm and 5.6 cm, and pen R produces spots at 1.6 cm and 3.2 cm. Explain how the scientist can use these results to identify which pen was used at the crime scene, and justify that conclusion.

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

> Rf values are calculated as the distance travelled by a spot divided by the distance travelled by the solvent front. Two substances are identical if their Rf values match under the same conditions.

## Mark scheme (5 marks)

1. Rf value is calculated as distance moved by spot divided by distance moved by solvent front
2. Correctly calculates at least one Rf value for the crime-scene ink (0.40 or 0.70)
3. States that pen Q is the match because it produces spots at the same distances (3.2 cm and 5.6 cm) as the crime-scene ink
4. Explains that matching Rf values (or spot positions under the same conditions) indicate the same substance/component
5. Explains why P and R can be ruled out — P has only one spot / one matching component, and R has no spots at matching positions

## Key terms

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

## 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-forensic-scientist-uses-thin-layer-chromatography-57769ecc) · Published by Druglandscape Ltd.