BRAINCAKEThe M+1 Peak
Look closely at a mass spectrum and you will usually see a very small peak one unit to the right of the molecular ion peak. This is the M+1 peak.
It is caused by molecules that contain one atom of carbon-13, 13C, instead of carbon-12. About 1.1% of all carbon atoms are carbon-13, so a small fraction of molecules are one mass unit heavier than the rest. For example, most ethane molecules have Mr = 30, but a few contain one 13C atom and have a mass of 31.
The more carbon atoms a molecule has, the greater the chance that one of them is 13C, so the M+1 peak becomes larger compared with the M+ peak.
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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.
- mass-to-charge ratio
- The ratio of the mass of an ion to its charge, given the symbol m/z, used to identify ions in a mass spectrum.
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
- Fragmentation
- Combining IR and Mass Spectra
All 10 lessons in Analytical Techniques · All OCR AS-level Chemistry topics