# A forensic scientist uses thin-layer chromatography (TLC) to separate compounds found in an unknown ink sample. Four spots are visible on the TLC plate after development. The scientist calculates the Rf value of each spot. Explain what Rf values are, how they are calculated, and how the scientist could use Rf values to identify the compounds present in the unknown ink.

> WJEC A-Level Chemistry (Wales) — 3.7 Spectroscopy and chromatography · Explain · 5 marks

> Thin-layer chromatography (TLC) is a technique used to separate and identify mixtures. Each compound travels a certain distance up the plate, and its Rf value can be compared to known reference values.

## Mark scheme (5 marks)

1. Rf value is the ratio of the distance travelled by the compound (substance/spot) to the distance travelled by the solvent (front)
2. Rf values are always between 0 and 1 (have no units)
3. Each pure compound has a characteristic/unique Rf value (in a given solvent)
4. The Rf values of the unknown spots are compared to Rf values of known/reference compounds (under the same conditions/same solvent)
5. A match between the Rf value of an unknown spot and a reference value (in the same solvent) allows the compound to be identified

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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-66570b79) · Published by Druglandscape Ltd.