# 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.

> Edexcel A-Level Chemistry (9CH0) — 7.1 Mass spectrometry and infrared · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Mass spectrometry can be used to determine the relative molecular mass of the compound
2. The molecular ion peak at m/z = 46 means the relative molecular mass of the compound is 46
3. Infrared spectroscopy identifies functional groups / bonds present in the compound by showing which frequencies of infrared radiation are absorbed
4. 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
5. 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

- [mass spectrometry](https://www.gradenine.co.uk/glossary/mass-spectrometry)
- [infrared spectroscopy](https://www.gradenine.co.uk/glossary/infrared-spectroscopy)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-uses-mass-spectrometry-71e82e27) · Published by Druglandscape Ltd.