In this study the sample pretreatment of the carbon disulfide evolution method for the determination of dithiocarbamate residues in lettuce, described by Keppel, was examined and improved. The following observations were made. The distribution of dithiocarbamates in lettuce heads is uneven. By deheading the lettuce and mixing the leaves, the sample taken for analysis is insufficiently homogeneous. The relative standard deviation varies from 10 to 34%. Thiram (a widely used dithiocarbamate) decomposes rapidly in chopped lettuce (50% within half an hour). When hydrochloric acid (2.4 M) containing stannous chloride is added immediately after chopping, decomposition does not occur. The existing method (deheading and mixing of lettuce leaves) is compared with the modified method (chopping and acid addition) for samples from the field treated with thiram as well as zineb. The mean results of both methods correspond well, with a much higher reproducibility for the modified method. The relative standard deviation of the modified method varies from 1 to 4%.
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Vuik et al. (1992) studied this question.