Thesis (M.Sc.)--Chulalongkorn University, 2012

Dispersive liquid-liquid microextraction (DLLME) using a low-density organic solvent as an extraction solvent was developed for simultaneous determination of organophosphorus and organochlorine pesticides. A high-density organic solvent as an auxiliary solvent was used for adjusting the density of a low-density organic solvent as an extraction solvent. As a result, the density of mixed solvent was higher than water and a phase separation by centrifugation and recovery of the extract were simplified. In this work, enrichment factor and extraction recoveries were optimized when using 40 µL of a mixture of n-heptane as extraction solvent and tetrachloroethylene as auxiliary solvent (ratio 1:3) and 1.4 mL of acetonitrile as a disperser solvent. The mixed solution was injected into 5 mL of aqueous sample by syringe, rapidly forming a cloudy solution. After centrifugation, the sediment phase at the bottom was removed by microsyringe and directly analyzed by gas chromatography-mass spectrometry. For method validation, the extraction recoveries for 5.0 µg/L of pesticides were in the range of 40.7 – 90.5 % and relative standard deviations were between 6.6 and 10.6% (n=6). The limits of detections for the method were range from 0.3 to 1.2 µg/L. The enrichment factors were in the range of 89 – 198. The developed method was then applied for determination of pesticide residues in water and tomato sample. The proposed method is simple, cheap, rapid, high enrichment factor and reduced the use of toxic solvent in conventional DLLME, which is environmental friendly.