Formation of gas-phase clusters monitored during electrospray mass spectrometry: a study of quaternary ammonium pesticides

Gas-phase cluster formation between the quaternary ammonium pesticides para quat, diquat, difenzoquat, chlormequat and mepiquat, and chloride and aceta te anions present in a liquid chromatography (LC) mobile phase, has been st udied using electrospray mass spectrometry. The clusters of paraquat, mepiq uat and chlormequat were revealed over the entire m/z range of the mass spe ctrometer, and their formation is dependent on the concentrations of both t he cationic and the anionic species. Mepiquat and chlormequat form clusters of the type [2M(q)(+) + A(-)](+) where M-q(+) is the quaternary ammonium c ation and A- is the anion. Paraquat forms a cluster species with ammonia an d also an ion-pair complex with chloride anions. Diquat and difenzoquat did not form observable ion-pair complexes or clusters with any of the anions studied. Competitive binding of acetate and chloride anions reflects the hi gher charge density of chloride, which forms the dominant clusters with mep iquat and chlormequat. The formation of cluster species has implications fo r the quantification of quaternary ammonium pesticides and may have an infl uence on the linearity of calibrations. Copyright (C) 2001 John Wiley & Son s, Ltd.