A food scientist analyses a sample of a flavouring compound using mass spectrometry and infrared spectroscopy. The mass spectrum shows a molecular ion peak at m/z = 88. The infrared spectrum shows a broad absorption between 2500–3300 cm⁻¹ and a strong absorption at approximately 1710 cm⁻¹. Explain how these two techniques together allow the food scientist to identify the flavouring compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mass spectrometry gives the relative molecular mass of a compound by detecting the mass-to-charge ratio (m/z) of ions. Infrared spectroscopy identifies functional groups present in a molecule because different chemical bonds absorb infrared radiation at characteristic wavenumbers. A broad absorption between 2500–3300 cm⁻¹ is characteristic of the O–H bond in a carboxylic acid, and a strong absorption at approximately 1710 cm⁻¹ is characteristic of the C=O bond in a carboxylic acid.
Model answer (5 marks)
Mass spectrometry gives the relative molecular mass of the compound.
The molecular ion peak at m/z = 88 shows that the relative molecular mass (Mr) of the flavouring compound is 88.
Infrared spectroscopy identifies functional groups.
The broad absorption between 2500–3300 cm⁻¹ is characteristic of an O–H bond in a carboxylic acid, and the strong absorption at ~1710 cm⁻¹ is characteristic of the C=O bond in a carboxylic acid.
Combining the Mr of 88 with the presence of a carboxylic acid functional group allows the scientist to match the compound to a known flavouring with that formula and functional group, thereby identifying the compound.
The molecular ion peak at m/z = 88 shows that the relative molecular mass (Mr) of the flavouring compound is 88.
Infrared spectroscopy identifies functional groups.
The broad absorption between 2500–3300 cm⁻¹ is characteristic of an O–H bond in a carboxylic acid, and the strong absorption at ~1710 cm⁻¹ is characteristic of the C=O bond in a carboxylic acid.
Combining the Mr of 88 with the presence of a carboxylic acid functional group allows the scientist to match the compound to a known flavouring with that formula and functional group, thereby identifying the compound.
Examiner tips
- Use the command word ‘Explain’ – give a clear, logical sequence of how each technique contributes.
- Show the link between the m/z value and Mr, then link the IR bands to the carboxylic acid group, finally state the identification step.
Common mistakes
- Confusing m/z with the exact mass of the ion instead of Mr; "m/z = 88" means Mr = 88.
- Failing to mention both the O–H and C=O absorptions as evidence of a carboxylic acid.
- Not explicitly stating that the two data sets together enable identification.
Mark scheme (5 marks)
- Mass spectrometry gives the relative molecular mass (Mr) of the compound
- The molecular ion peak at m/z = 88 means the relative molecular mass of the compound is 88
- Infrared spectroscopy identifies the functional groups present in the compound
- The absorptions at 2500–3300 cm⁻¹ and 1710 cm⁻¹ indicate the compound contains a carboxylic acid functional group (O–H and C=O bonds)
- Using both pieces of data together (Mr of 88 and carboxylic acid functional group) allows the scientist to identify the specific compound
Key terms in this question
mass spectrometry · molecular ion peak · m/z · infrared spectroscopy
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →