Automated sample preparation with extraction columns followed by liquid chromatography ionspray mass spectrometry - Interferences, determination and degradation of polar organophosphorus pesticides in water samples

The determination of polar and/or thermally labile organophosphorus pesticides trichlorfon, dichlorvos, dimethoate, oxydemeton-methyl, mevinphos (cis and trans), demeton-S-methyl, fenamiphos, fenitrothion, fenthion and diazinon in water samples was investigated using solid-phase extraction followed by liquid chromatography-ionspray mass spectrometry (LC-ISP-MS). Pesticides were spiked at 0.2 mu g/l in ground water samples and 200 ml were preconcentrated by using an ASPEC XL system. To improve the recovery of the most volatile pesticides such as dichlorvos, a further drying step with a Baker Spe 12 G apparatus with the vacuum set at 15 p.s.i. was included. A further evaporation step was applied with a gentle stream of nitrogen. A variety of materials (Amberchrom, LiChrolut EN, cyclohexyl, SDB, C-18 and Isolute ENV) were used for the isolation of the target pesticides. The elution step took place at 1 ml/min, with a 5-min contact time between the eluent and the sorbent. LC-ISP-MS permitted the identification of diethyl and di-n-butyl phthalates present as impurities in the cartridge materials. The presence of transformation products of the pesticides, e.g., fenthion sulfoxide, was also observed. The influence of pH and the water matrix (distilled and ground water) on the extraction efficiency was examined. The use of LC-ISP-MS permitted cis- and trans-mevinphos to be distinguished in the water sample owing to the presence of a specific ion at m/z 99. The limits of detection of the overall method were in the range 0.01-0.20 mu g/l using LC-ISP-MS with selected-ion monitoring.