A food scientist suspects that a batch of fruit juice has been adulterated with an unknown additive. Explain how gas chromatography-mass spectrometry (GC-MS) could be used to identify the unknown additive in the fruit juice sample.

Edexcel A-Level Chemistry (9CH0) — Topic 6 Modern analytical techniques · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

1. The fruit juice sample is injected into the gas chromatograph (GC). 2. Components are separated by their different retention times as they travel through the column. 3. Each separated component enters the mass spectrometer (MS). 4. The MS produces a fragmentation pattern or mass spectrum for each component. 5. The mass spectrum of the unknown additive is compared to a database or library of known spectra to identify it.

Examiner tips

  • Use the command word ‘Explain’ – give a clear, step‑by‑step description. Include the key stages: injection, separation, detection, and identification. Mention the role of the database for matching spectra.

Common mistakes

  • Failing to state that the sample is injected into the GC. Not describing the separation by retention time. Omitting that the MS gives a fragmentation pattern. Not mentioning the comparison with a spectral library.

Mark scheme (5 marks)

  1. The fruit juice sample is injected into the gas chromatograph / GC
  2. Different components travel through the column at different speeds / are separated by their different retention times
  3. The separated components are fed into the mass spectrometer
  4. The mass spectrometer produces a fragmentation pattern / mass spectrum for each component
  5. The mass spectrum of the unknown additive is compared to a database / library of known spectra to identify it

Key terms in this question

gas chromatography · mass spectrometry

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