Determination of methamidophos residues in food remnants

Over the past 7 years, acute food poisoning arising from the consumption of methamidophos-tainted vegetables has occurred sporadically in Hong Kong. To enable prompt remedial and regulatory actions to be taken, a simple and rapid gas chromatographic method was developed to determine methamidophos residues in food remnants. Samples were homogenized and extracted with acetone. Acetone in the resultant extract was removed by rotary evaporation, leaving behind an aqueous solution. After washing with petroleum ether and addition of sodium sulphate, the aqueous solution was re-extracted with chloroform. The chloroform extract was concentrated to dryness and the residue was reconstituted in absolute ethanol for determination by gas chromatography with flame photometric detection. The method was found to be applicable also to the determination of acephate, dimethoate, omethoate and trichlorfon. Mean recoveries of the five organo-phosphorus pesticides at two fortification levels, viz. 1 mg/kg and 10 mg/kg, ranged from 74 to 113%. Relative standard deviations lay between 1.3 and 6.1%. Method detection limits ranged from 0.02 to 0.04 mg/kg. In 1994, 75 food poisoning cases in Hong Kong were suspected to be related to the consumption of methamidophos-tainted vegetables; 13 food remnants were received for analysis, four of them were found to contain high concentrations of methamidophos.