BRAINCAKECombining IR and Mass Spectra
No single technique proves the structure of an unknown compound. Combining evidence from different techniques narrows the options until only one structure fits.
What each technique tells you
• Elemental analysis (percentage composition) gives the empirical formula.
• The mass spectrum gives Mr from the molecular ion peak, so the molecular formula; fragment peaks show parts of the structure.
• The IR spectrum shows which functional groups are present, such as C=O, O-H and C-O.
Strategy
1. Find the molecular formula (empirical formula and M+).
2. Use IR to identify the functional groups.
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Key terms in this lesson
- molecular ion
- The positive ion, M+, formed in a mass spectrometer when a molecule loses one electron; its mass gives the relative molecular mass.
- functional group
- A group of atoms in a molecule responsible for its characteristic chemical reactions.
More in Analytical Techniques
- Bond Vibrations and IR Absorption
- Characteristic IR Absorptions
- Interpreting IR Spectra
- IR in Breathalysers and Pollution Monitoring
- Mass Spectrometry of Organic Compounds
- The Molecular Ion Peak
- The M+1 Peak
- Fragmentation
All 10 lessons in Analytical Techniques · All OCR AS-level Chemistry topics