A sports drink manufacturer wants to check that a new batch of drink contains the correct mixture of compounds. A sample of the drink is analysed using gas chromatography–mass spectrometry (GC-MS). Describe how GC-MS works and explain why it is particularly useful for analysing the sports drink sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. The sample is first vaporised and carried by an inert carrier gas through a chromatographic column.
2. Compounds in the sample interact differently with the column material, giving different retention times.
3. At the column outlet the eluent enters a mass spectrometer where the compounds are ionised and fragmented.
4. The mass spectrometer records a mass spectrum for each eluted compound, allowing identification by its characteristic mass pattern or molecular ion.
5. GC‑MS can detect very small (trace) amounts of compounds, is highly sensitive, and can separate and identify many components simultaneously, making it ideal for a complex mixture such as a sports drink.
2. Compounds in the sample interact differently with the column material, giving different retention times.
3. At the column outlet the eluent enters a mass spectrometer where the compounds are ionised and fragmented.
4. The mass spectrometer records a mass spectrum for each eluted compound, allowing identification by its characteristic mass pattern or molecular ion.
5. GC‑MS can detect very small (trace) amounts of compounds, is highly sensitive, and can separate and identify many components simultaneously, making it ideal for a complex mixture such as a sports drink.
Examiner tips
- Use the exact sequence: vaporisation → separation by retention time → ionisation → mass spectrum identification. Show that GC gives separation, MS gives identification and sensitivity. Mention trace detection and multi‑component analysis as key points.
Common mistakes
- Confusing GC with HPLC or omitting the mass spectrometer step. Failing to note the sensitivity and trace‑level detection. Using vague terms like "detects" without explaining how GC‑MS separates and identifies.
Mark scheme (5 marks)
- The sample is vaporised and carried through a column by a carrier gas
- Different compounds travel through the column at different speeds / have different retention times
- The mass spectrometer identifies each separated compound by its mass spectrum / molecular mass
- GC-MS can detect/identify very small amounts (traces) of compounds / is very sensitive
- GC-MS can separate and identify multiple compounds at the same time / in a mixture, making it suitable for a complex sample like a sports drink
Key terms in this question
gas chromatography · mass spectrometry
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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