Capillary electrophoresis–mass spectrometry for the analysis of quaternary ammonium herbicides

Conditions for the simultaneous determination of the three herbicides paraquat, diquat and difenzoquat and the two plant growth regulators chlormequat and mepiquat by pressure-assisted capillary electrophoresis coupled to mass spectrometry (ion-trap) using electrospray as ionisation source have been established. A 200 mM formic acid–ammonium formate buffer solution at pH 3.0 with 50% of methanol was used as carrier electrolyte. Some capillary electrophoresis–mass spectrometry parameters such as sheath liquid and sheath gas flow-rates, sheath liquid composition, electrospray voltage and the CE capillary position were optimised. The MS and MS–MS spectra of positive ions were studied in order to obtain structural information for the confirmation of the identity. The use of labelled standards allowed to confirm fragment ions assignation. The detection limits, based on a signal-to-noise ratio of 3:1, were between 0.5 and 2.5 mg l−1 with hydrodynamic injection (10 s) and between 1 and 10 μg l−1 with electrokinetic injection (20 s, 10 kV) using standards in ultrapure water. Quality parameters such as linearity and run-to-run precision (n=6) were established. Quantitation was carried out using labelled standards. The method has been applied to the analysis of contaminated irrigation water and spiked mineral water samples.