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 + O – 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.