22.2.1—Analyse mass spectra

A mass spectrum plots relative abundance against m/e. Peaks may represent molecular ions, fragments or isotopic variants; the tallest peak is the base peak, not necessarily the molecular ion.

Use the isotope pattern and peak spacing to support identity. The working of the instrument is not required, but the distinction between abundance and mass-to-charge value is essential.

A peak at m/e 43 can be a common fragment, while a higher-mass molecular-ion peak gives the parent mass if it is present. Chlorine and bromine give distinctive M:M+2 patterns.

The base peak is not automatically M⁺, and a fragment’s m/e is not the relative molecular mass of the whole compound.