3.2.8 (HL)—Mass spectrometry

The molecular-ion peak provides a mass constraint, while fragmentation peaks reveal structural features of the organic molecule. Use the supplied fragment data rather than assuming an operational mechanism not given.

For the commonly supplied singly charged positive ions, z = +1 so the numerical m/z value can be read as the ion mass; state or check that assumption if charge is not specified. Build a candidate grid: first match the molecular ion to Mr, then ask whether each proposed structure can conserve atoms while producing every supplied diagnostic fragment. A fragment supports a substructure, but cannot by itself prove the full connectivity.

Separate two jobs: the molecular-ion peak constrains relative molecular mass, while a fragment peak constrains a possible substructure. The tallest base peak is the most abundant detected ion and need not be M⁺; accept a candidate only if its formula can account for the supplied fragment masses.