Method development for the analysis of organophosphate and pyrethroid insecticides at low parts per trillion levels in water

In the current study, organophosphate and pyrethroid insecticides including diazinon, chlorpyrifos, bifenthrin, fenpropathrin, permethrin, λ-cyhalothrin, cyfluthrin, cypermethrin, esfenvalerate and deltamethrin were analyzed in laboratory and field-collected water samples. Water samples were extracted and analyzed by gas chromatography/electron capture detector (GC/ECD) and gas chromatography/nitrogen–phosphorous detector (GC/NPD). Comparison of results from liquid–liquid extraction and subsequent normal phase solid-phase extraction cleanup (LLE–NPSPE), and reversed phase solid-phase extraction (RPSPE) showed that LLE–NPSPE was the better choice to extract trace amounts of pesticides from water. Pesticide recoveries from four spiked water samples using LLE–NPSPE ranged from 63.2 to 148.8% at four spiking concentrations. Method detection limits were 0.72–1.69 ng/L using four different water sources. The stability of the target pesticides in lake water was investigated at 4 °C for 1 h, 1 d, 4 d, and 7 d under three conditions: (1) water samples only; (2) with 20 mL hexane used as a keeper solvent; and (3) with acidification to pH 2 with HCl. Results showed that water storage without treatment resulted in slow degradation of some pesticides with storage time, storage using water acidification led to significant degradation and loss of diazinon and chlorpyrifos, while water storage with hexane as a keeper solvent showed good stability for all of the target pesticides over the 7 d storage period.