Summary A newly developed chemical assay was used to determine the degradation in soil of two sulfonylurea herbicides, primisulfuron‐methyl and metsulfuron‐methyl, under both controlled and field conditions. The results from the chemical assay were compared with those from traditional bio‐assays for determination of persistence in the field. Phytotoxic effects of these herbicides were not observed after 6 weeks following application to an acidic (pH 5.7) soil with high organic matter content (7.3% o.c). Half‐lives of 13 to 29 days were measured for primisulfuron‐methyl at different soil‐water contents and temperatures while those for metsulfuron‐methyl ranged from 8 to 36 days. The rate of degradation of metsulfuron‐methyl was more sensitive to temperature than that of primisulfuron‐methyl. Persistence in the field was shorter than expected considering the results from the controlled environment studies. However, determination of the persistence by both chemical assay and bioassay methods produced very similar results.

Sulfonylureas are extracted from water samples using solid-phase extraction (SPE) with Cl8-silica/ Teflon disks. Soil samples are extracted with 0.1 M sodium hydrogen carbonate, and the acidified extracts are processed as for water samples. The concentrated eluents are treated with diazomethane in ethyl acetate, which forms the thermally stable N,N'-dimethyl derivatives of the herbicides. Residues are screened by capillary GC using effluent splitting to electron capture and nitrogen-phosphorus detectors. Residues are confirmed by GC-MS using selected ion monitoring. Detection limits were below 0.1 #ig/L for water and below 1 /¿g/kg for soil samples. Accuracy and precision at 0.5 and 0.1 Mg/L each were, respectively, for chlorsulfuron 95 ± 2% and 110 ±16% and for metsulfuron-methyl 90 ± 6% and 98 ± 11%. At 5 and 1 gg/kg each in soil the recoveries were, respectively, for chlorsulfuron 78 ± 20% and 69 ± 6% and for metsulfuron-methyl 92 ± 18% and 105 ±17%. The methods are superior to previous methods for these herbicides based on GC because the intact herbicides are determined rather than thermal degradation products.

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A method is described for the determination of sulfonylurea herbicides by gas chromatography (GC). Chlorimuronethyl, cinosulfuron, primisulfuron-methyl, thifensulfuron-methyl, triasulfuron and tribenuron-methyl were converted into thermostable derivatives by reaction with diazomethane in ethyl acetate. The resulting N,N-dimethyl derivatives were suitable for analysis by GC. Two of the herbicides formed multiple derivatives. The identity of the derivatives was confirmed using chemical ionization and electron impact mass spectrometry and nuclear magnetic resonance. The proposed structures of the ions in the electron impact mass spectra were codrmed using highresolution, metastable ion decomposition and deuterium-labelling techniques. The derivatization was incorporated into multimethods for the residue analysis of the six sulfonylurea herbicides in water, soil and wheat samples. Recoveries ranged from 88% to 122% in water (0.1 pg I-'), 53% to 99% in soil (1 pg kg-') and 41% to 120% in wheat (0.2 mg kg-').

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