Unlike IR or NMR, MS does not measure absorption — it weighs individual ions to give the molecular weight and a map of where the molecule breaks apart.
The three questions MS answers — molecular weight, fragmentation, and isotope pattern — in one glance. Structures drawn live.
1. MS Weighs the Molecule and Its Fragments
In electron ionization, ~70 eV electrons knock one electron out to form the radical cation M+•, which is sorted by mass-to-charge ratio (m/z); since almost every ion is +1, m/z is effectively mass, and only cations are ever detected.
2. The Molecular Ion (M+•) Gives the Molecular Weight
M+• is the rightmost significant peak and equals the molecular weight, but easily-fragmented molecules (branched alkanes, alcohols) can make it tiny or absent.
3. The Base Peak Is the Most Stable Fragment
The base peak is the tallest bar (set to 100%) and marks the most stable cation — for acetone, the resonance-stabilized acylium ion CH3–C≡O+ at m/z 43.
4. Weak Bonds Break to Give the Most Stable Cation
Fragmentation follows carbocation stability (3° > 2° > 1°, plus allylic/benzylic/acylium resonance), so M+• ejects a small neutral to give a peak at (M − loss) — a handful of losses cover most spectra:
| Loss | Neutral lost | Points to |
|---|---|---|
| M − 15 | •CH3 (methyl radical) | a methyl branch |
| M − 17 | •OH (hydroxyl radical) | alcohol / carboxylic acid |
| M − 18 | H2O (water) | alcohol |
| M − 28 | CO (carbon monoxide) | carbonyl |
| M − 29 | •CHO or •C2H5 | aldehyde or ethyl group |
| M − 45 | •COOH or •OC2H5 | carboxylic acid or ester |
5. Isotope Peaks Reveal Chlorine and Bromine
Heavier minor isotopes make small M+1/M+2 peaks whose relative heights fingerprint the halogens:
| M+2 : M ratio | Isotope signature | Conclusion |
|---|---|---|
| ≈ 1 : 3 (33%) | 35Cl / 37Cl | one chlorine present |
| ≈ 1 : 1 (98%) | 79Br / 81Br | one bromine present |
| tiny M+1 only | 13C (1.1% × n) | counts carbon atoms |
6. The Nitrogen Rule Counts Nitrogens from the Parity of M
An odd-mass M+• means an odd number of nitrogens; an even mass means zero or an even number — because nitrogen alone has even mass but odd valence.
7. Summary
M+• = molecular weight (rightmost peak) · base peak (100%) = most stable fragment · neutral losses −15/−18/−28/−29/−45 · M+2 ≈ ⅓ → Cl, M+2 ≈ M → Br · odd M → nitrogen · combine with degrees of unsaturation and IR.
Worked example
- M+ = 46 fits the formula C2H6O.
- Losing 15 (46 → 31) means loss of a •CH3 radical.
- m/z 31 is the resonance-stabilised CH2=OH+ oxocarbenium from α-cleavage of ethanol.
Answer. Ethanol (CH3CH2OH): M+ 46, fragment 31 from loss of •CH3.
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Bromine. The 79Br/81Br isotopes are nearly 1:1, so one bromine gives an M+2 peak about equal in height to M (e.g. bromoethane at 108/110).
It is the acylium ion CH3–C≡O+ (loss of a •CH3 radical, M−15). It is resonance-stabilized across the C≡O+, making it the most stable — and therefore most abundant — cation.
By the nitrogen rule, an odd-mass molecular ion means an odd number of nitrogen atoms — most likely one nitrogen in the molecule.
Water (H2O). An M−18 loss is the signature dehydration of alcohols; you may also see M−17 from loss of •OH.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.