# A forensic scientist is investigating a suspected forgery. Two ink samples are taken — one from a genuine document and one from the suspected forgery. The scientist uses paper chromatography to compare the dyes in each ink sample. Explain how the results of the chromatography experiment could be used to determine whether the two ink samples contain the same dyes, and explain what it means if a spot has a high Rf value compared to a spot with a low Rf value.

> OCR A-Level Chemistry B: Salters (H433) — CD Colour by design · Explain · 5 marks

> Paper chromatography can be used to separate and identify the dyes present in an ink sample. Each dye travels a different distance up the chromatography paper depending on its solubility in the solvent and its attraction to the paper.

## Mark scheme (5 marks)

1. Spots at the same Rf value (or same distance from the baseline) in both samples indicate the same dye is present
2. If the chromatograms show different numbers of spots or spots at different Rf values, the inks are different / do not contain the same dyes
3. Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front
4. A high Rf value means the dye is more soluble in the solvent (or has a greater affinity for the solvent / less attraction to the paper)
5. A low Rf value means the dye is less soluble in the solvent (or has a greater affinity for the paper / stationary phase), so it does not travel as far

## Key terms

- [chromatography](https://www.gradenine.co.uk/glossary/chromatography)
- [Rf value](https://www.gradenine.co.uk/glossary/rf-value)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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-is-investigating-a-b4054bc2) · Published by Druglandscape Ltd.