A level Chemistry · Edexcel 9CH0 · Topic 7A
Mass spectrometry
Key points
- The molecular ion M⁺ has the highest (ignoring small isotope peaks) and gives the relative molecular mass.
- Molecular ions break into fragments: one fragment is a positive ion (detected) and the other a neutral radical (not detected).
- Common fragments: 15 CH₃⁺, 29 C₂H₅⁺ or CHO⁺, 43 C₃H₇⁺ or CH₃CO⁺, 31 CH₂OH⁺ (primary alcohols).
- The base peak is the tallest peak (most stable or most common fragment).
- An M+1 peak arises from carbon-13; chlorine and bromine give M and M+2 peaks in ratios of 3:1 and 1:1.
- Use differences between peaks to deduce what group has been lost, e.g. a loss of 15 means CH₃ was lost.
- Fragmentation patterns can distinguish isomers such as propan-1-ol and propan-2-ol.
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A mass spectrometer turns molecules into positive ions and sorts them by mass-to-charge ratio. For an organic compound the heaviest ion gives the relative molecular mass, while the pattern of smaller fragment ions acts like a fingerprint…
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