Mass spectrometry and infrared spectroscopy are two modern analytical techniques used to identify unknown compounds. Explain how a chemist could use both mass spectrometry and infrared spectroscopy together to identify an unknown organic compound, including what information each technique provides.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A chemist has isolated an unknown organic compound from a reaction mixture and wishes to determine its identity using modern analytical techniques.
Model answer (5 marks)
Mass spectrometry first gives the molecular ion peak (M⁺), which yields the relative molecular mass of the unknown. The fragmentation pattern – the series of fragment ions – can be interpreted to identify characteristic groups or sub‑structures within the molecule.
Infrared spectroscopy then records the absorption of IR radiation by the unknown. Different bonds (C–H, C=O, N–H, etc.) absorb at characteristic wavenumbers, so the IR spectrum reveals the functional groups present.
By comparing the IR spectrum with a database of reference spectra the chemist can confirm the identity of the compound. Together, the two techniques provide both the molecular mass from MS and the functional‑group information from IR, giving a reliable, complete identification.
Infrared spectroscopy then records the absorption of IR radiation by the unknown. Different bonds (C–H, C=O, N–H, etc.) absorb at characteristic wavenumbers, so the IR spectrum reveals the functional groups present.
By comparing the IR spectrum with a database of reference spectra the chemist can confirm the identity of the compound. Together, the two techniques provide both the molecular mass from MS and the functional‑group information from IR, giving a reliable, complete identification.
Examiner tips
- Mention the M⁺ peak and fragmentation for MS; list key functional‑group bands for IR; state that database comparison confirms identity; show how the two data sets complement each other.
Common mistakes
- Confusing the MS peak with the IR peaks; not recognising that MS gives mass, IR gives functional groups; failing to mention database/library comparison; over‑generalising the IR bands without specific wavenumbers.
Mark scheme (5 marks)
- Mass spectrometry gives the molecular ion peak (M⁺), which gives the relative molecular mass of the compound.
- Fragmentation pattern (fragment ions / fragmentation peaks) can be used to identify parts of the molecule or suggest its structure.
- Infrared spectroscopy identifies functional groups present in the molecule because different bonds absorb infrared radiation at different (characteristic) wavenumbers / frequencies.
- The infrared spectrum of the unknown compound can be compared to a database / library of reference spectra to confirm the identity of the compound.
- Using both techniques together allows the chemist to determine both the molecular mass AND the functional groups present, giving a more complete / reliable identification of the compound.
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 →