# A forensic scientist uses paper chromatography to analyse an ink sample taken from a document. The ink produces four separate spots on the paper. The scientist then tests a pure reference compound and it produces a single spot. Explain how the scientist can use the Rf value of each spot to identify the compounds present in the ink sample.

> Eduqas A-Level Chemistry — 1.7 Equilibria · Explain · 5 marks

> Paper chromatography separates substances according to how they distribute between the mobile phase and the stationary phase. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.

## Mark scheme (5 marks)

1. The Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent
2. An Rf value is calculated for each of the four spots in the ink sample
3. The Rf value of each spot is compared to the Rf value of known/reference compounds
4. A compound is identified when its Rf value matches that of a known reference compound (in the same solvent)
5. Because the ink produces four spots, it is a mixture of four different compounds (not a pure substance)

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-uses-paper-chromatography-2324d123) · Published by Druglandscape Ltd.