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The authors have developed an analytical method for determining trace amounts of 39 kinds of pesticides in water with high precision and accuracy by gas chromatography/ mass spectrometry with a selected ion monitor (GC/ MS-SIM). One liter of water was extracted with dichloromethane; the extract was concentrated to 1 ml with a Kuderna-Danish concentrator. After the internal standards, perdeuterated n-alkanes (C16, C20 and C24), were added to the concentrate, quantitative analyses by GC/ MS-SIM were performed to measure the recovery efficiencies of the 39 compounds spiked into water samples at concentrations of below µg/ 1. The recovery efficiencies of the tested compounds, except monocrotophos and bentazone, yielded more than 95% and their relative standard deviations were less than 6%. The developed method was applied to the determination of pesticides in river and tap water. Sixteen pesticides ranging from ng/ 1 to µg/ l were detected. This method is useful to elucidate the fates and the concentration levels of pesticides in the aquatic environment.

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