Determination of diquat and paraquat in water by liquid chromatography-(electrospray ionization) mass spectrometry

A method for the determination of the herbicides diquat and paraquat in water was developed using liquid chromatography-(electrospray ionization) mass spectrometry LC-(ESI)MS]. The analytes were isolated on an ENVI-8 DSK solid phase extraction (SPE) disk and eluted with 5-M trifluoroacetic acid (TFA). The eluate was evaporated to dryness and the analytes were redissolved in the mobile phase (7% methanol/93% water/25-mM TFA). The extract was analyzed by liquid chromatography (C1 column) with postcolumn addition of propionic acid/methanol followed by (ESI)MS. Diquat was detected using the M²⁺ ? H⁺] ion (M²⁺ = dication) at m/z 183, whereas paraquat was detected using the mono-trifluoroacetate ion pair M²⁺/^?OOCCF₃] at m/z 299. Quantitation was done by isotope dilution mass spectrometry using d₄-diquat and d₈-paraquat and the corresponding ions M²⁺ ? D⁺] and M²⁺/^?OOCCF₃] at m/z 186 and m/z 307, respectively. Detection limits of 0.1 and 0.2 μg/L, respectively (based on the dications), were adequate to meet the Ontario Drinking Water Objectives of 70 and 10 μg/L, respectively, and the Ontario Provincial Water Quality Objective for diquat of 0.5 μg/L. Precision and accuracy were 14% and 6% for diquat and 12% and 3% for paraquat.