Determination of pyrethroid residues on paddy rice by reversed-phase high-performance liquid chromatography

Several synthetic pyrethroids and the synergist piperonyl butoxide have been determined as aged residues on paddy rice by reversed-phase high-performance liquid chromatography with detection at 225 nm. These compounds are commonly used as protectants for stored grains. Studies on the comparative rates of extraction of both the pesticides and interfering material from the grain were conducted with acetone, methanol, and hexane as extracting solvents. Acetone was the best of these solvents because it provided quantitative extraction of the pesticides over a 48-h period, and did not give high levels of ballast material. Pyrethroids present in the extract at levels in excess of 0.5 microgram/ml could be determined by direct injection, but at lower concentrations, clean-up and preconcentration were required. Clean-up of acetone extracts was accomplished with either Florisil or alumina pre-columns, and up to a tenfold preconcentration was achieved by adsorption of the pesticide on a C18 pre-column, or by concentrating the extract through evaporation of the solvent. These approaches gave good recoveries and linear calibration plots. Detection limits were of the order of 0.05 microgram/ml.