Revised method for routine determination of urinary dialkyl phosphates using gas chromatography-mass spectrometry

Among urinary organophosphorus pesticide (OP) metabolites, dialkyl phosphates (DAPs) have been most often measured as a sensitive biomarker in non-occupational and occupational OP exposure risk assessment. In our conventional method, we have employed a procedure including simple liquid-liquid extraction (diethyl ether/acetonitrile), derivatization (pentafluorobenzylbromide, PFBBr) and clean-up (multi-layer column) for gas chromatography-mass spectrometry (GC-MS) analysis starting from 5-mL urine samples. In this study, we introduce a revised analytical method for urinary DAPs; its main modification was aimed at improving the pre-derivatization dehydration procedure. The limits of detection were approximately 0.15 microg/L for dimethylphosphate (DMP), 0.07 microg/L for diethylphosphate (DEP), and 0.05 microg/L for both dimethylthiophosphate (DMTP) and diethylthiophosphate (DETP) in 2.5-mL human urine samples. Within-run precision (percent of relative standard deviation, %RSD) at the DAP levels varying in the range of 0.5-50 microg/L was 6.0-19.1% for DMP, 3.6-18.3% for DEP, 8.0-25.6% for DMTP and 9.6-27.8% for DETP. Between-run precision at 5 microg/L was below 15.7% for all DAPs. The revised method proved to be feasible to routine biological monitoring not only for occupational OP exposure but also for environmental background levels in the general population. Compared to our previous method, the revised method underscores the importance of adding pre-derivatization anhydration for higher sensitivity and precision.