A forensic scientist has found an unknown white powder at a crime scene. She uses two instrumental methods to identify the substance: mass spectrometry and infrared spectroscopy. Describe what information each technique provides and explain why instrumental methods are preferred over traditional chemical tests for forensic analysis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Mass spectrometry gives the relative molecular mass of the unknown substance by ionising the sample and measuring the mass‑to‑charge ratio of the ions produced.
Infrared spectroscopy identifies the functional groups or bonds present by measuring the wavelengths of light absorbed by the sample.
Instrumental methods are preferred because they are more sensitive, detecting trace amounts of a substance; they are more accurate, giving reliable, reproducible results; and they are faster, providing results more quickly than traditional chemical tests.
Infrared spectroscopy identifies the functional groups or bonds present by measuring the wavelengths of light absorbed by the sample.
Instrumental methods are preferred because they are more sensitive, detecting trace amounts of a substance; they are more accurate, giving reliable, reproducible results; and they are faster, providing results more quickly than traditional chemical tests.
Examiner tips
- Use the exact phrases from the mark scheme (e.g. ‘relative molecular mass’, ‘functional groups’).
- Show the three advantages in separate sentences to match the 5‑mark structure.
Common mistakes
- Confusing mass spectrometry with chromatography or mis‑labeling IR as NMR.
- Failing to mention all three advantages (sensitivity, accuracy, speed).
Mark scheme (5 marks)
- Mass spectrometry gives the relative molecular mass (or molecular mass) of the substance
- Infrared spectroscopy identifies the functional groups or bonds present in the substance
- Instrumental methods are more sensitive / can detect very small amounts (trace amounts) of a substance
- Instrumental methods are more accurate / give more reliable results than traditional chemical tests
- Instrumental methods are faster / give results more quickly than traditional chemical tests
Key terms in this question
mass spectrometry · infrared spectroscopy · instrumental methods
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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