A forensic scientist uses gas chromatography-mass spectrometry (GC-MS) to identify an unknown substance found at a crime scene. Explain how GC-MS works to identify the unknown substance, including the role of each technique.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Gas chromatography-mass spectrometry (GC-MS) is a modern analytical technique used to separate and identify substances in a mixture. It combines two separate processes to produce reliable identification of unknown compounds.
Model answer (5 marks)
Gas chromatography separates the mixture into its individual components.
The sample is injected into a GC column where the components travel at different speeds, giving distinct retention times.
At the end of the column the separated components enter the mass spectrometer.
The mass spectrometer ionises each component and records its mass spectrum, showing the fragmentation pattern.
The resulting mass spectrum is compared with a database or library of known spectra to identify the unknown compound.
The sample is injected into a GC column where the components travel at different speeds, giving distinct retention times.
At the end of the column the separated components enter the mass spectrometer.
The mass spectrometer ionises each component and records its mass spectrum, showing the fragmentation pattern.
The resulting mass spectrum is compared with a database or library of known spectra to identify the unknown compound.
Examiner tips
- Use the word ‘separates’ for GC and ‘identifies’ for MS. Include retention time and fragmentation pattern. Mention the database comparison as the final step. Keep each point short and to the point.
Common mistakes
- Confusing the order of GC and MS. Forgetting to mention retention times. Not stating that the spectrum is compared to a library.
Mark scheme (5 marks)
- Gas chromatography separates the components of the mixture
- Components travel through the column at different speeds / have different retention times
- The separated components are fed into / pass into the mass spectrometer
- The mass spectrometer identifies each component by producing a mass spectrum / fragmentation pattern
- The mass spectrum / fragmentation pattern is compared to a database / library of known substances to identify the unknown compound
Key terms in this question
gas chromatography · mass spectrometry · GC-MS
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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