Flow injection determination of methamidophos using online photo-oxidation and fluorimetric detection

A photochemical method for the determination of methamidophos using a flow injection system is proposed. The method is based on the rapid decomposition of methamidophos in the presence of peroxydisulphate upon irradiation with UV light. The phosphate generated in the photochemical process is made to react with molybdate in dilute nitric acid to form phosphomolybdic acid, which oxidises thiamine to thiochrome. The thiochrome can then be fluorimetrically monitored at 440 nm with excitation at 375 nm. The method shows a linear range between 14 and 1400 ng ml−1 with a limit of detection of 1.7 ng ml−1. The repeatability was 0.3% expressed as relative standard deviation (n=10) and the reproducibility, studied on five different days, was ±3%. The sample throughput was 70 injections per h. The reliability of the method for routine analysis of water and vegetable samples is demonstrated.