Letter: Hydroxide and oxygen atom attachment to dichlorophthalic anhydride in negative-ion chemical ionization with collisionally-activated dissociation in an external-source ion trap mass spectrometer

This study reports the investigation of some unusual adduct ions of hydroxide and oxygen atoms in both negative-ion chemical ionization (NCI) and collisionally-activated dissociation (CAD) processes in an external-source ion trap mass spectrometer. The [M + OH](-) and [M + OH - Cl](-) adduct ions in the NCI spectrum of 4,5-dichlorophthalic anhydride are attributed to an ion/molecule reaction due to the presence of trace amounts of water. The formation of the unusual [M + 0 - Cl]- ions during CAD of the M- ions from 3,6- and 4,5-dichlorophthalic anhydride isomers is attributed to reactive collisions of the fragment ions with residual oxygen.