DETERMINATION OF MALATHION URINARY METABOLITES BY ISOTOPE-DILUTION ION TRAP GC/MS

Human exposures to malathion insecticide are monitored by determination of malathion diacid (MDA) in urine using an isotope dilution GC/MS method. The procedure relies on solid-phase extraction with strong anion exchange cartridges to isolate urinary acids. Samples are adjusted to pH 8 and spiked with deuterium-labeled MDA prior to extraction, and acids eluted from the resin are methylated with diazomethane. Use of isotope dilution quantitation improves method precision (relative to conventional external standard quantitation), resulting in a 3-fold reduction in the method detection limit (MDL). Method accuracy also is improved as biases associated with extraction and derivatization are minimized. The quadrupole ion trap GC/MS detects as little as 5 pg of each metabolite in the mass scanning mode and affords an MDL of 6.5 ng of MDA/mL in human urine. The new isotope dilution method was used to monitor malathion exposures in date plantation workers.