DETERMINATION OF PARTS-PER-TRILLION LEVELS OF ORGANOPHOSPHORUS PESTICIDES IN GROUNDWATER BY AUTOMATED ONLINE LIQUID-SOLID EXTRACTION FOLLOWED BY LIQUID-CHROMATOGRAPHY ATMOSPHERIC-PRESSURE CHEMICAL-IONIZATION MASS-SPECTROMETRY USING POSITIVE AND NEGATIVE-ION MODES OF OPERATION

Liquid chromatography/atmospheric pressure chemical ionization mass sp ectrometry with positive and negative ion modes of operations was used for the trace determination of several organophosphorus pesticides, ( E)- and (Z)-mevinphos, dichlorvos, azinphos-methyl, azinphos-ethyl, pa rathion-methyl, parathion-ethyl, malathion, fenitrothion, fenthion, ch lorfenvinphos, and diazinon, in groundwater, This method required only 100 mL of water, and it was combined with a prior automated online li quid-solid extraction step using an OSP-2 autosampler containing C18 c artridges, The limit of quantitation (LOQ) varied between 5 and 37 ng/ L in positive ion (PI) mode, Under negative ion (NI) mode of operation , only the parathion group (both parathions and fenitrothion) had a be tter sensitivity as compared to that in PI mode, with a LOQ of 5-15 ng /L, whereas the rest of pesticides had 2-4 times higher LOQs as compar ed to those in PI mode, Selected ion monitoring of the group-specific fragment of the organophosphorus pesticides, e.g., [(CH3O)(2)PO2](-) o r the [M + H](+) ions, under NI or PI mode, respectively, was used, Sa mple cone voltage varied from 10 to 130 V. This parameter influenced t he transmission and fragmentation of quasi-molecular ions, and it was optimized to achieve identification capabilities with the highest sens itivity, At 20 V, good fragmentation was obtained for most of the stud ied analytes, The system was used for the certification of a groundwat er sample spiked at the nanograms per liter level with organophosphoru s pesticides provided by Aquachek.