A pharmaceutical company uses gas chromatography (GC) followed by mass spectrometry (MS) to analyse a new pain-relief tablet. Explain how GC-MS works to identify the components present in the tablet, and why this technique is preferred over traditional chemical tests for this purpose.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A pain-relief tablet is dissolved in a solvent and the resulting solution is injected into a GC-MS instrument. The instrument produces a chromatogram showing several peaks, and a mass spectrum is recorded for each peak.
Model answer (5 marks)
In gas chromatography the tablet extract is carried by an inert carrier gas through a column. The different components interact differently with the stationary phase, so they travel at different speeds and elute at different times – the retention times. Each separated component then enters the mass spectrometer where it is ionised and fragmented. The resulting mass spectrum, showing the molecular ion peak and characteristic fragmentation pattern, is compared with a database or library of known spectra to identify the substance.
GC‑MS is preferred because it can detect very small quantities of compounds (high sensitivity) and can analyse complex mixtures simultaneously, giving rapid, reliable identification that traditional chemical tests cannot provide.
GC‑MS is preferred because it can detect very small quantities of compounds (high sensitivity) and can analyse complex mixtures simultaneously, giving rapid, reliable identification that traditional chemical tests cannot provide.
Examiner tips
- Mention carrier gas and column separation, retention time, ionisation and mass spectrum, database comparison, and highlight sensitivity and mixture analysis.
- Use the exact terms ‘retention time’, ‘mass spectrum’, ‘molecular ion peak’, ‘fragmentation pattern’, ‘database/library’.
- Show the logical flow: separation → ionisation → detection → identification.
- Keep the answer concise, covering all five points.
Mark scheme (5 marks)
- In gas chromatography, the sample is carried through a column by a carrier gas
- Different components travel through the column at different speeds / are separated because they interact differently with the stationary phase, so they exit at different times (different retention times)
- Each separated component is then passed into the mass spectrometer, which identifies it by its mass spectrum / molecular ion peak / fragmentation pattern
- The mass spectrum of each component is compared to a database / library of known spectra to identify the substance
- GC-MS is preferred because it can identify very small quantities / traces of substances (high sensitivity) and can analyse complex mixtures simultaneously, which traditional chemical tests cannot do as reliably or quickly
Key terms in this question
gas chromatography · mass spectrometry
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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