A forensic scientist is analysing an unknown white powder found at a crime scene. She uses gas chromatography (GC) and mass spectrometry (MS) to identify the substances present. Describe how these two instrumental techniques are used together to identify the substances in the powder, and state two advantages of using instrumental methods compared with traditional chemical tests.

Eduqas A-Level Chemistry — 2.8 Instrumental analysis · Describe · 5 marks · View as Markdown

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

Forensic laboratories routinely use instrumental analysis to identify unknown substances. Gas chromatography separates mixtures, and mass spectrometry identifies individual components by producing a fragmentation pattern that acts like a 'fingerprint' for each substance.

Model answer (5 marks)

Gas chromatography separates the mixture into its individual components. Each component gives a distinct peak at a characteristic retention time in the chromatogram. The eluted components are then introduced into the mass spectrometer, where ionisation produces a fragmentation pattern – a mass spectrum – that is unique to each substance. By comparing the retention times and the mass spectra with reference data, the forensic scientist can identify the substances present in the unknown powder.

Two advantages of instrumental methods over traditional chemical tests are: 1) they are more sensitive, allowing detection of trace amounts of a substance; 2) they are more accurate and reliable, enabling analysis of very complex mixtures with greater confidence.

Examiner tips

  • Use the word ‘separates’ and ‘identifies’ to show the two steps; mention retention time and mass spectrum; list the two advantages exactly as in the scheme.
  • Show the flow: GC → separation → MS → identification.
  • Keep the answer concise – 5 marks only.

Common mistakes

  • Confusing the order of GC and MS; writing that MS separates the mixture instead of identifies it.
  • Failing to mention both retention time and mass spectrum as the identification criteria.
  • Not stating both advantages or using vague terms like ‘better’ instead of ‘more sensitive’ and ‘more accurate’.

Mark scheme (5 marks)

  1. Gas chromatography separates the mixture into its individual components
  2. Each component produces a different retention time / peak in the gas chromatogram
  3. Mass spectrometry identifies each separated component by producing a fragmentation pattern (mass spectrum) unique to that substance
  4. First advantage: instrumental methods are more sensitive / can detect very small amounts of a substance (trace amounts)
  5. Second advantage: instrumental methods are more accurate / give more reliable results / can analyse very complex mixtures

Key terms in this question

gas chromatography · mass spectrometry

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