Ion chemistry of phthalamic acids. III. The origin of [M ? 1]+ ions under electron impact

Electron impact mass spectra and collisional activation/mass-analysed ion kinetic energy spectra of some phthalamic acids and their deuterium labelled analogues suggested that the genesis of M ? 1]⁺ ions is due to the loss of an aromatic hydrogen ortho to the amidic group, as for aromatic amides and thioamides.