Field screening method for above-tolerance residues of dithiocarbamate fungicides

A field screening method has been developed for detecting above-tolerance residues of dithiocarbamate (DTC) fungicides on fruits and vegetables. A harvested crop might be condemned as unfit for market if above-tolerance DTC residues are present; however, by using this screening method, a grower might be able to postpone harvesting a crop until the screening test indicates that residues have dissipated below the tolerance level. The method depends on carbon disulfide generated from DTC fungicides at an elevated temperature into the headspace gas above the contents of a septum-sealed reaction flask, with hydrochloric acid and stannous chloride present. The syringe-withdrawn, headspace carbon disulfide then reacts with the appropriate chromophore reagent already in the syringe. The lower limit of detectability is about 2 ppm DTC fungicide from 30 g chopped crop. A single test can be completed in about 90 min; the average time per test when several are conducted sequentially is considerably shorter.