Determination of organophosphorus pesticides and their transformation products in river waters by automated on-line solid-phase extraction followed by thermospray liquid chromatography-mass spectrometry.

The trace-level determination of ten priority organophosphorus (OP) pesticides (e.g., chlorpyrifos-methyl, diazinon, disulfoton, fenthion, fenamiphos) and various transformation products (TPs; e.g., disulfoton sulfoxide, fenthion sulfoxide etc.) using automated on-line solid-phase extraction (SPE) with C18 precolumns followed by LC-MS and thermospray interface with time-scheduled selected-ion monitoring (SIM) was developed. Two main ions (usually [M+H]+ and [M+NH4]+ or [M+CH3CN]+) were used for each pesticide in the positive ion (PI) detection mode, while [M-H]- and [M+HCOO]- ions were used in the negative ion (NI) mode. The proposed method requires 100 ml of sample for a limit of detection (LOD) of 0.01-0.1 microgram/l. Calibration graphs were constructed by preconcentrating 100 ml of water spiked with the pesticide mixture at various concentrations (from 0.025 to 2 microgram/l). Good linearity was observed for most of the analytes studied. The experimental setup described in this paper was applied to study the kinetics of degradation of ten organophosphorus pesticides in spiked river water samples. The different samples were first analyzed by an automated on-line precolumn exchange system (OSP-2) followed by LC with diode array detection. To confirm the identity of the organophosphorus pesticides detected, the samples were then analyzed by automated on-line SPE-LC-MS. The method permitted unequivocal identification of many of the TPs formed during the experiments, e.g., the oxo-derivatives of chlorpyrifos-methyl, temephos and pyridafenthion, fenamiphos sulfoxide. Many of these TPs are here reported for the first time since previously used MS-based techniques were not sensitive enough.