Mass spectrometry (MS) answers two questions: how much does the molecule weigh, and how does it fall apart. The sample is vaporized and bombarded with electrons, knocking one electron off to create a radical cation called the molecular ion (M⁺·). Because this ion has essentially the mass of the intact molecule, the peak it produces tells you the molecular weight directly — the single most useful piece of MS data.

The instrument separates ions by their mass-to-charge ratio (m/z) and plots abundance against m/z. Two features carry most of the information:

The molecular ion peak (M⁺) gives molecular weight. A few diagnostic isotope patterns are worth memorizing: an M+2 peak about one-third the height of M signals chlorine (³⁵Cl/³⁷Cl in ~3:1), and an M+2 peak nearly equal to M signals bromine (⁷⁹Br/⁸¹Br in ~1:1). The nitrogen rule is another quick tool: an odd molecular weight implies an odd number of nitrogen atoms.
Fragmentation peaks. The molecular ion is unstable and breaks apart at its weakest bonds, producing smaller fragment ions. The key principle mirrors carbocation logic from Track 14: fragmentation favors the most stable cation, so bonds that break to give tertiary, allylic, benzylic, or resonance-stabilized cations dominate the spectrum. The base peak (tallest, set to 100% abundance) is usually the most stable fragment. Common losses — 15 (CH₃), 29 (CHO or C₂H₅), 18 (H₂O from alcohols) — help you piece the skeleton back together.

The mental model: molecular weight from M⁺, elemental clues from isotope patterns, and structural connectivity inferred by reasoning backward from which fragments are most stable.

"Weigh the Pieces"