A forensic scientist uses mass spectrometry and infrared spectroscopy to analyse an unknown liquid found at a crime scene. Explain how each technique provides different information that together helps to identify the unknown liquid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mass spectrometry gives a molecular ion peak at m/z = 46. Infrared spectroscopy shows a broad absorption in the range 2500–3300 cm⁻¹ and a strong absorption at approximately 1700 cm⁻¹.
Model answer (5 marks)
Mass spectrometry determines the relative molecular mass of a compound by measuring the m\/z of its ions. The molecular ion peak at m\/z = 46 shows that the unknown liquid has a relative molecular mass of 46.
Infrared spectroscopy identifies functional groups by the frequencies of IR radiation absorbed. A broad absorption between 2500–3300 cm⁻¹ indicates an O–H bond, typical of a carboxylic acid, while a strong absorption near 1700 cm⁻¹ indicates a C=O bond.
Thus, mass spectrometry provides the exact molecular mass, while infrared spectroscopy confirms the presence of specific functional groups. Together, they allow the forensic scientist to deduce the identity of the unknown liquid by matching both its mass and its functional groups to known substances.
Infrared spectroscopy identifies functional groups by the frequencies of IR radiation absorbed. A broad absorption between 2500–3300 cm⁻¹ indicates an O–H bond, typical of a carboxylic acid, while a strong absorption near 1700 cm⁻¹ indicates a C=O bond.
Thus, mass spectrometry provides the exact molecular mass, while infrared spectroscopy confirms the presence of specific functional groups. Together, they allow the forensic scientist to deduce the identity of the unknown liquid by matching both its mass and its functional groups to known substances.
Examiner tips
- Use the command word ‘Explain’ to describe what each technique does, not just state facts.
- Show the m\/z value and link it directly to the molecular mass.
- Mention the specific IR bands and the functional groups they indicate.
- Explain how the two pieces of information combine to give a complete identification.
Common mistakes
- Confusing the m\/z value with the exact mass of the molecule.
- Forgetting to mention the functional group that corresponds to the 1700 cm⁻¹ band.
- Not linking the two techniques together in the final sentence.
Mark scheme (5 marks)
- Mass spectrometry can be used to determine the relative molecular mass of the compound
- The molecular ion peak at m/z = 46 means the relative molecular mass of the compound is 46
- Infrared spectroscopy identifies functional groups / bonds present in the compound by showing which frequencies of infrared radiation are absorbed
- The broad absorption at 2500–3300 cm⁻¹ indicates an O–H bond from a carboxylic acid, and/or the absorption at ~1700 cm⁻¹ indicates a C=O bond
- Using both techniques together allows the compound to be identified, as mass spectrometry gives the Mr and infrared spectroscopy confirms the functional groups / bonds present
Key terms in this question
mass spectrometry · infrared spectroscopy
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →