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Carrots grown from seed in soils spiked with [14C = O]linuron or [14C-ring]3,4-dichloroaniline (DCA), were found to contain radioactivity equivalent to 0.73 ppm linuron or 0.60 ppm DCA. After exhaustive extraction with methanol these tubers still retained 35% and 67% of the original 14C-bioincurred residues, for linuron and DCA respectively. To examine the bioavailability of these residues, rats were dosed by gavage (water vehicle) with unextracted tubers and extracted tubers. For comparison purposes 14C-spiked carrot tubers and [14C] standards were also administered. Animals were maintained for 3 days in metabolism cages; urine, faeces, expired gases and several tissues were collected for radioassay. The following relative amounts of radioactivity (expressed as a percentage of the applied dose) were excreted (faeces/urine) for linuron or DCA dosing respectively: unextracted carrots, 28/31, 51/10; extracted carrots, 51/0, 73/3; spiked carrots, 11/43, 63/20; and linuron and DCA standards, 11/46, 26/81. The data demonstrated that approximately 31% and 10% of bioincurred 14C-residues (from linuron and DCA respectively) in unextracted carrot tubers were bioavailable to rats. 14C-Bound residues (extracted tubers) were much less bioavailable (0% and 3% respectively). The disposition of radioactivity in tissues, blood and expired gases was very low (less than or equal to 1.3% per sample) for any dose studied.

A novel method was developed for the simultaneous determination of diquat and paraquat residues in potatoes. Potato tissues were spiked at several levels and extracted with acid using a micro‐reflux procedure with 5 g of sample; this was followed by adjusting the hydrolyzate pH to 9 to 10 and using a silica Sep‐Pak for rapid clean‐up and preconcentration. Aliquots of the final eluate were taken to dryness, dissolved in the h.p.l.c. mobile phase and analyzed as their heptanesulfonate ion‐pairs by u.v.‐h.p.l.c. (reverse phase column chromatography) at 254 and 313 nm for paraquat and diquat, respectively. A detection limit of approximately 0.05 mg kg−1 dication in a 5‐g sample of spiked potato (i.e. 0.25 μg ml−1 final extract) was achieved. Recoveries of 79.5 to 97.6% were obtained at spiking levels of 0.05 to 5.0 mg kg−1 for diquat and paraquat with coefficients of variation not greater than 8.27%. The method was developed and validated using 14C‐radiolabelled diquat and paraquat; u.v.‐h.p.l.c. recoveries were comparable with recoveries determined by radioassay. Several parameters affecting the extraction, adsorption and chromatography of diquat and paraquat were evaluated. The formula weights of diquat and paraquat were determined and their importance described; they were determined as mono‐ and tetra‐hydrates, respectively.