J. Sci. Res. Chula. Univ., Vol. 26 No. 1(2001) 35 Determination of Organophosphate Pesticides in Vegetable Farm Drained Water Using Solid Phase Extraction Followed by High Performance Liquid Chromatography Pranee Tethgatuk, 1,+ Wanida Jinsart 1, 2 and Peter Arnold 3 Organophosphorus pesticides in water from four vegetable farms in Patumtanee Province were analyzed by solid-phase extraction (SPE) and reverse- phase high performance liquid chromatography (HPLC). SPE experiments were performed under conditions of an ODS phase, a sorbent mass of 100 mg, solution volume of 100 ml, elution solvent of 60/40 acetonitrile/water, elution volume of 5 ml and pressure of 17 in. Hg. Extraction method recovery was confirmed by spiking standard pesticides at 1 ppm. Quantification by an external standard method implied a 90% average recovery of standard profenofos. Profenofos was detected in 27 water samples from three vegetable farms. The detected concentration range was 0.11 to 1.11 ppm with a mean of 0.44 ± 0.07 ppm. The chemical structures of pesticides were identified by mass spectra. Key words: Pesticide, organophosphate, solid phase extraction. 1 Inter-department of Environmental Science, Graduate School, Chulalongkorn University. 2 Corresponding author: Department of General Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. Tel: 662-218-5188 Fax: 662-218-5180 e-mail jwanida@chula.ac.th. 3 Department of Chemistry, Faculty of Natural Sciences, University of Hertfordshire, UK. + Current address: Scientific and Technological Research Equipment Center, Chulalongkorn University.