Negative ion mass spectra of dicarboxylic acids

The negative ion mass spectra of dicarboxylic acids show M]^?˙ and prominent M – H]^?ions. These ions can therefore be used to determine the molecular weight of dicarboxylic acids which do not give positive molecular ions. The C₂H₃]^? ion is a base peak in the spectra of maleic and fumaric acids. Isomeric phthalic acids are readily differentiated.