# A forensic scientist is analysing a sample taken from a crime scene. The sample is suspected to contain a mixture of two organic compounds. Explain how gas chromatography (GC) could be used to identify whether the two compounds are present in the sample, and describe what the chromatogram would show if both compounds were present.

> Edexcel A-Level Chemistry (9CH0) — 19.1 NMR and chromatography · Explain · 5 marks

> Gas chromatography (GC) is an analytical technique used to separate and identify compounds in a mixture. Each compound travels through a stationary phase at a different rate depending on its interactions with that phase. The time taken for a compound to travel through the column and reach the detector is called the retention time.

## Mark scheme (5 marks)

1. The sample is vaporised and carried through the column by a carrier gas (mobile phase)
2. Different compounds have different retention times because they interact differently with the stationary phase
3. The retention time of each compound in the sample is compared to known retention times (reference standards) to identify the compounds
4. If both compounds are present, the chromatogram would show two separate peaks
5. Each peak appears at a retention time that matches the known retention time of one of the two suspected compounds

## Key terms

- [gas chromatography](https://www.gradenine.co.uk/glossary/gas-chromatography)
- [chromatogram](https://www.gradenine.co.uk/glossary/chromatogram)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](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-analysing-a-f8584140) · Published by Druglandscape Ltd.