Mass selective detection of 11 pesticides in vegetable extracts with and without clean up

Clean up procedures in a multiresidue method for organochlorine and organophosphate pesticides were modified to give higher recoveries of the more polar pesticides (methamidophus, chlorothalonil, diazinon, metalaxyl and metribuzin). Modified clean up involved partitioning an acetonitrile extract of vegetables with 6% diethyl ether (EE) in petroleum ether (PE) instead of PE and elution of pesticides from a 1-g Florisil column by sequential 10-mL aliquots of 6, 15 and 35% EE in PE. This solvent scheme did not elute green bean pigments (chlorophylls) which were eluted by 50% EE in PE in the original procedure.

The influence of the modified clean up versus no clean up on pesticide recovery was investigated for acetonitrile extracts of 4 vegetables (potatoes, cabbage, green beans and turnip greens) spiked with 11 pesticides (methamidophus, EPTC, trifluralin, diazinon, dinoseb, chlorothalonil, metalaxyl, metribuzin, metolachlor, parathion and dacthal) at 2 different levels. The concentration of each pesticide was determined by selective ion monitoring (SIM) using a gas chromatographic (GC) method with mass selective detection and electron ionization (EIMS) and an internal standard (anthracene-dlO) method. The relative response factor of each pesticide changed significantly from day to day of analysis and was different between the two levels of spiking. Best recoveries (closest to 100%) were obtained for trifluralin, metribuzin, metalaxyl, metolachlor and dacthal from all vegetables in extracts receiving no clean up and at lower spiking concentrations (0.05-0.5 ppm). Modified clean up procedures improved recoveries of parathion from vegetables except cabbage spiked at the lower level (0.2 ppm), of EPTC from vegetables (except potatoes) spiked at the lower level (0.2 ppm), of dinoseb from green beans and turnip greens spiked at the lower level (0.4 ppm), and of chlorothalonil from spiked (0.05 ppm) green beans. Diazinon was recovered well (85 - 134%) from all vegetable extracts, with and without clean up, at the lower spiking level (0.5 ppm.). Recovery of methamidophus was much greater than 100% from most extracts mainly due to interference with its detection at M/z 94.

Instrument failure of the mass spectrometer during the experiment caused many samples spiked at higher levels (0.25 - 2.5 ppm) to be stored too long (up to 1 month at -18°C) resulting in low recoveries. However, the results at the lower spiking levels (0.05 - 0.5 ppm) showed that SIM can be used successfully for the analysis of most of the 11 pesticides in vegetables even if recoveries were dependent upon the pesticide and the vegetable and if the vegetable extract received clean up before GC analysis.