Key structures for this topic — drawn live.
How the spectrum is made
In electron ionization (EI) mass spectrometry, a molecule is bombarded with high-energy electrons. One electron gets knocked out, producing a radical cation — a species with both a positive charge and an unpaired electron, written M+•. This is the molecular ion, and its mass-to-charge ratio (m/z) equals the molecular weight of the compound (since the charge is +1). Locating M+• on the far right of the spectrum is usually the first step, because it gives you the molecular weight directly.
The excess energy also shatters some molecular ions into smaller fragments. Each peak in the spectrum is a cation of a particular mass. The tallest peak is the base peak; by convention it is assigned 100% relative abundance, and every other peak is scaled against it. The base peak represents the most stable (most readily formed) cation.
Isotope peaks: spotting halogens
Most elements have minor heavier isotopes, which produce small peaks one or two mass units above the main peak (M+1, M+2). The pattern is a powerful clue for halogens:
- Bromine — 79Br and 81Br occur in nearly a 1:1 ratio, so a compound with one bromine shows an M and M+2 of about equal height.
- Chlorine — 35Cl and 37Cl occur in roughly a 3:1 ratio, so one chlorine gives an M+2 about one-third the height of M.
- Carbon — the small M+1 peak comes from 13C; its size (about 1.1% per carbon) can hint at how many carbons are present.
A prominent M+2 peak is therefore an immediate flag: roughly equal means bromine, roughly one-third means chlorine.
The nitrogen rule
The nitrogen rule is a quick parity check. A molecule with an odd number of nitrogen atoms has an odd-mass molecular ion; a molecule with zero or an even number of nitrogens has an even-mass molecular ion. So if you find M+• at an odd m/z, the compound contains an odd number of nitrogens (one, three, ...). This falls out of the fact that nitrogen is the only common element with an even atomic mass but an odd valence.
Common neutral losses
When the molecular ion fragments, it usually ejects a small neutral molecule or radical, and you see a fragment peak at (M − loss). Memorizing a few characteristic losses lets you read structure straight off the spectrum:
- M − 15 — loss of a methyl radical (CH3).
- M − 18 — loss of water (H2O), typical of alcohols.
- M − 28 — loss of carbon monoxide (CO), common for carbonyls.
- M − 29 — loss of CHO or an ethyl group (C2H5); a peak at 29 is often the CHO+ of an aldehyde.
- M − 45 — loss of COOH (or OC2H5), pointing to a carboxylic acid or ester.
Why certain fragments dominate
Fragmentation is not random — it favors pathways that give stable cations. The same stabilization ideas from the rest of organic chemistry apply: a fragment is abundant when its positive charge is well stabilized. Common examples include the acylium ion (R–C≡O+, from cleaving next to a carbonyl), and resonance-stabilized allylic and benzylic cations. For instance, a strong peak at m/z 91 is the classic tropylium/benzylic cation from alkylbenzenes. Reading these tells you which bonds broke and reconstructs the structure.
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.