Determination of Quaternary Ammonium Herbicides by Capillary Electrophoresis/Mass Spectrometry

Abstract

Capillary electrophoresis (CE) has been used for the characterization of the herbicides mepiquat, chlormequat, diquat (DQ), paraquat (PQ) and difenzoquat. A volatile running electrolyte solution containing acetic acid–ammonium acetate 50 mM (pH 4) with 10% of methanol as modifier was used to reduce adsorption of solutes to the capillary wall and be compatible with the mass spectrometer. The influence of pH, methanol content and ammonium acetate concentration was investigated. Capillary electrophoresis was coupled to a mass spectrometric system using a coaxial coupling device and a pneumatically-assisted electrospray as interface. CE/MS spectra of these quaternary ammonium herbicides were acquired and these mainly showed the singly charged cation for all compounds except DQ, which gave a [Cat−H]+. In addition, doubly charged cations were observed for PQ and DQ. Tandem mass spectrometry was used to obtain structural information for the confirmation of identity. Detection limits (signal-to-noise ratio=3:1) ranging from 0.7 to 20 μg/L were obtained when standard solutions were used, with 5 to 20 times higher detection limits when drinking water was analysed.