HPLC ANALYSIS OF PESTICIDES IN BROWN RICE USING UV ABSORPTION AND POSTCOLUMN PHOTOLYSIS-FLUORESCENCE DETECTION

A simple and selective HPLC method was developed for the systematic de termination of bentazone (BEN), imazaril (IMZ), inabenfide (INA), pyra zoxyfen (PYR) and chinomethionate (CIN) in imported brown rice. A rice sample was extracted with 70% MeCN and divided into three fractions ( acidic, BEN; basic, IMZ/INA; neutral, PYR/CIN). The acidic and basic f ractions were purified by partition between n-hexane-dichloromethane ( 1:1) and either 0.01 M berate buffer (pH 10.4) for BEN or 0.1 M HCl fo r IMZ/INA, immediately before injection into the HPLC system. The PYR/ CIN fraction was redissolved in isopropanol-MeCN (1:1) to remove lipid components. A sample was analyzed on an ODS column with a mobile phas e of a mixture of MeCN-5 mM K2HPO4 [BEN: (20:80), pH 2.5; IMZ/INA: (35 : 65), pH 4.5; PYR/CIN: (55 :45)] at 45 degrees C. IMZ and INA were d etermined from the UV absorption at 220 nm. Other pesticides were dete rmined by using a postcolumn photolysis-fluorescence detection system at an excitation wavelength (Ex) of 329 nm and emission wavelength (Em ) of 415 nm for BEN, Ex 333 nm/Em 405 nm for PYR, and Ex 377 nm/Em 450 nm for CIN. Average recoveries ranged from 80.9 to 89.1% at a fortifi cation level of 0.025 mu g/g. The limit of determination was 0.005 mu g/g for INA and 0.01 mu g/g for other pesticides. The method was appli ed to 24 brown rice samples, and all appeared to be free from the five pesticides.