Simultaneous determination of neutral and acidic pesticides in natural waters at the low nanogram per liter level

A new method for the simultaneous identification and quantification of neutral and acidic pesticides at the low nanogram per liter concentration level in natural waters is presented. It has been validated for, and applied to, three important pesticide classes, namely, the triazines (e.g., atrazine and its major metabolites desethylatrazine and deisopropylatrazine), the acetamides (e.g., alachlor, metolachlor, and dimethenamid), and the phenoxy acids (e.g., 2,4-D, dichlorprop, and mecoprop). The method includes enrichment of the compounds by solid phase extraction on graphitized carbon black, followed by sequential elution of the neutral and acidic pesticides and derivatization of the latter fraction with diazomethane. Identification and quantification of the compounds is performed with GC/MS using atrazine-d(5), [C-13(6)]-metolachlor, and [C-13(6)]dichlorprop as internal standards. Absolute recoveries from Nanopure water spiked with 4-50 ng/L were 85 +/- 10, 84 +/- 15, and 100 +/- 7% for the triazines, the acetamides, and the phenoxy acids, respectively. Recoveries from rainwater and lake water spiked with 2-100 ng/L were 95 +/- 19, 95 +/- 10, and 92 +/- 14% for the triazines, the acetamides, and the phenoxy acids, respectively, and a groundwater contaminated with landfill leaches and pesticides from agricultural applications revealed similar numbers for most analytes. Average method precision determined with fortified rainwater (2-50 ng/L) was 6.0 +/- 7.5% for the triazines, 8.6 +/- 7.5% for the acetamides, and 7.3 +/- 3.2% for the phenoxy acids, and method detection limits ranged from 0.1 to 4.4 ng/L. The method has been applied to analysis of various natural waters, including rainwater, roof runoff, surface water, and groundwater. It proved to be an excellent tool for routine multiresidue pesticide analysis even at low nanogram per liter concentrations of these analytes.