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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (5 marks)

The sample is first vapourised and carried through the column by an inert carrier gas (mobile phase). As the vapourised molecules pass the stationary phase, they interact with it to different extents; the stronger the interaction the longer the molecule is retained. Consequently each compound has a characteristic retention time. The retention times obtained from the sample are compared with those of known reference standards of the two suspected compounds. If both compounds are present, the chromatogram will display two distinct peaks, each appearing at a retention time that matches the known retention time of one of the suspected compounds.

Examiner tips

  • Use the word ‘vapourised’ and ‘carrier gas’ to show understanding of the GC set‑up.
  • Explain that different interactions give different retention times and that comparison with standards is how identification is made.
  • Mention that two separate peaks would appear if both compounds are present.
  • Keep the answer concise – 5 marks are awarded for a clear, accurate explanation.

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 in this question

gas chromatography · chromatogram

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