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J. Agric. Food Chem. 1999, 47, 3872−3878
Flupyrsulfuron Soil Dissipation and Mobility in Winter Wheat Crops Jean Rouchaud,*,† Olivier Neus,† Karolien Cools,‡ and Robert Bulcke‡ Laboratory of Phytopharmacy, Universite´ Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium, and Research Center for Weed Science, Universiteit Gent, 9000 Gent, Belgium
Residues of the sulfonylurea herbicide flupyrsulfuron were extracted from cropping soils with 0.1 M NaHCO3. The soil extracts were cleaned up by partitioning and repeated thin-layer chromatography. Flupyrsulfuron was transformed by diazomethane into N-(4,6-dimethoxypyrimidine-2-yl)N-(3-methoxycarbonyl-6-trifluoromethylpyridine-2-yl)methylamine (2), which was analyzed by gasliquid chromatography with electron capture detection, and confirmation for several samples was made by gas chromatography combined with mass spectrometry. The sensitivity limit was 0.5 µg of flupyrsulfuron kg-1 of dry soil. Bioassays using sugar beet as test plant qualitatively confirmed the results of the chemical analyses. Flupyrsulfuron [10 g of active ingredient ha-1] was applied in autumn on plots in two winter wheat crops on a sandy loam soil, the first crop being made in 19961997 and the second one in 1997-1998. In the 0-8 cm surface soil layer of both crops, the flupyrsulfuron soil half-lives were 123 and 92 days, respectively. Flupyrsulfuron was also applied post-emergence in March to other plots in the same crops; the half-lives in the 0-8 cm surface soil layer were similar in both seasons, that is, ∼58 days. During all crop trials, flupyrsulfuron remained in the 0-8 cm surface soil layer and was not detected in the 8-10, 10-15, and 15-20 cm surface soil layers. The surface-2 cm soil layer contained the greatest flupyrsulfuron soil concentration, but the residues progressively moved down into the 2-4 and 4-6 cm soil layers. At the winter wheat harvest date for each trial, flupyrsulfuron was not detected in any of the soil layers (90%. Spectra of Compounds 2-4. Compound 2: IR 3113, 2954, 2925, 1734 (CO), 1640, 1601, 1564, 1449, 1409, 1386, 1341, 1276, 1233, 1184, 1113, 904, 791; 1H NMR 3.58 (d, 6H, pyrimidine-OCH3), 3.72 (s, 3H, NCH3), 3.93 (s, 3H, CO2CH3), 5.12 (s, 1H, H-5 pyrimidine), 7.28 (d, 1H, H-4 pyridine), 8.19 (d, 1H, H-5 pyridine); MS 372 (M+, 36), 341 (M - OCH3, 8), 313 (M - CO2CH3, 100), 298 (313 - CH3, 7), 283 (313 - OCH3 + H, 11), 168 [C4HN2(NCH3)(OCH3)2 , 8], 139 [C4HN2(OCH3)2, 21]. Compound 3: IR 3109, 3071, 2956, 2926, 1730 (CO), 1693 (CO), 1612, 1545, 1449, 1368, 1346, 1282, 1196, 1152, 1095, 1056, 799; 1H NMR 3.56 (s, 3H, NCH3), 3.94 (s, 6H, OCH3), 6.20 (s, 1H, H-5 pyrimidine), 7.59 [d, 1H, H-5 pyrido(2,3-d)pyrimidine], 8.69 [d, 1H, H-6 pyrido(2,3-d)pyrimidine]; MS 383 (M+, 96), 382 (M - 1, 100), 368 (M - CH3, 13), 353 (M - OCH3 + H, 15), 325 (M - NCH3CO - H, 14), 311 (325 - N, 13), 297 (311 - CH3 + H, 12). Compound 4: IR 2957, 2926, 2855, 1744 (CO), 1701 (CO), 1595, 1575, 1367, 1300, 1189, 1155, 1130, 1079, 804; 1H NMR 3.33 (s, 3H, CONCH3), 3.37 (s, 3H, CON′CH3), 3.97 (s, 6H, pyrimidine-OCH3), 4.02 (s, 3H, CO2CH3), 5.78 (s, 1H, H-5 pyrimidine), 7.92 (d, 1H, H-4 pyridine), 8.28 (d, 1 H, H-5 pyridine); MS 493 (M+, 32), 372 (M - SO2NCH3CO, 29), 313 (372 - CO2CH3, 12), 268 [C5H2N(CF3)(CO2CH3)(SO2), 15], 225 [C4HN2(OCH3)2(NCH3CONCH3), 78]; 204 [C5H2N(CF3)(CO2CH3), 76], 196 [C4HN2(OCH3)2(NCH3CO), 100].
Flupyrsulfuron Soil Dissipation and Mobility in Wheat Residue Analysis of Flupyrsulfuron in Soil. Soil (100 g) was stirred with 0.1 M NaHCO3 in water (200 mL, 20 min, 20 °C), and the mixture was centrifuged (3000 rpm, 15 min) and the supernatant removed. The extraction was repeated, and the supernatants were combined and washed with dichloromethane (2 × 150 mL). The dichloromethane layer was discarded, and the aqueous phase was brought to pH 2.2 with 1 N hydrochloric acid and extracted two times with ethyl acetate (2 × 200 mL); the ethyl acetate solution was dried (Na2SO4), concentrated successively to 40 and 15 mL in a vacuum rotary evaporator (at 30 and 20 °C, and in 1 L and 50 mL flasks, respectively), and then concentrated further to 0.5 mL under a slow stream of nitrogen (20 °C). The extract was applied to a TLC plate along with flupyrsulfuron 1 free acid (∼5 µg, i.e., the lowest amount to make the spot visible at the fluorescence) in a separate lane. Elution with 1:1.5 acetone/ hexane (v/v) gave a band corresponding to flupyrsulfuron at Rf ) 0.49, which was scraped off; the silica gel was extracted with acetone (40 mL) in a small column, and the extract was concentrated to 15 mL in a vacuum rotary evaporator at 20 °C and then concentrated further to 1 mL under a slow stream of nitrogen (20 °C). Ethyl acetate (5 mL) was added, and then a solution of diazomethane in ether (7 mL) was added until the yellow color persisted. After 2 h at 20 °C, the solution was concentrated to 0.5 mL under a slow stream of nitrogen (20 °C) and applied to a second TLC plate, together with compound 2 (∼5 µg) in a separate lane. Elution with 2:1 ethyl acetate/ hexane (v/v) gave a band corresponding to compound 2 at Rf ) 0.55, which was scraped off and extracted with acetone. The concentrated extract was applied to a third TLC plate. Elution with ether gave a band corresponding to compound 2 at Rf ) 0.51 which was separated and extracted with acetone. The concentrated extract (between 0.08 and 0.3 mL) was analyzed by GC with confirmation by GC/MS. When the final extract was not sufficiently clean (interferences at the GC or GC/MS analyses), the third TLC step was repeated. Quantification Standards and Recovery Experiments. For calibration of the GC and GC/MS chromatograms, 10 mg of flupyrsulfuron free acid 1 was dissolved in ethyl acetate (10 mL), and the diazomethane solution in ether (∼5 mL) was added until persistence of the yellow color. After 2 h at room temperature, the volume of the mixture was reduced to 5 mL by a current of nitrogen (20 °C), and the volume was adjusted to 10 mL with ethyl acetate. Several dilutions of this solution with ethyl acetate gave calibration solutions containing 0.410 × 10-9 g of flupyrsulfuron µL-1. Injection of these solutions (external standard) gave curves correlating the GC or GC/MS signals to the amounts of flupyrsulfuron. Calibration was repeated several times during a daily series of analyses. Compound 2 (8 mg) dissolved in ethyl acetate (10 mL) gave a solution, which by dilutions gave several calibration solutions corresponding to 0.4-10 × 10-9 g of flupyrsulfuron µL-1. These solutions gave GC and GC/MS calibration curves similar to the ones obtained by methylation of flupyrsulfuron 1, taking into account that at this level methylation transformed flupyrsulfuron quantitatively into compound 2. The calibration solutions were kept at -20 °C and were stable for >1 month. For recovery experiments, soil was taken at Melle in field plots not treated with flupyrsulfuron. The soil was air-dried for 24 h at 20 °C in a ventilated hood. Ten milligrams of flupyrsulfuron free acid 1 was dissolved in 10 mL of acetone. By dilution, a solution in acetone containing 10-5 g mL-1 was made; 1 mL of this solution was dissolved in 100 mL of water, giving a solution in water containing 10-7 g mL-1. Of this solution 0.5, 2, 5, or 10 mL was dissolved in water to make a final volume of 15 mL, and this was mixed with 100 g of soil, resulting in fortification levels of 0.5, 2.0, 5.0, and 10 µg kg-1. The spiked soil samples were kept overnight at 12 °C and extracted. To observe the possible effect of residue aging, some soil samples were kept at 12 °C for 48 h before extraction, the soil humidity being maintained at 15%. Incubation time between 20 and 48 h had no significant influence on flupyrsulfuron recoveries. At the levels of 5, 2, and 0.5 (sensitivity limit) µg of flupyrsulfuron kg-1 dry soil, recoveries were 8397, 81-92, and 74-89%, respectively. Other soil samples
J. Agric. Food Chem., Vol. 47, No. 9, 1999 3875 Table 5. Effect of Known Concentrations of Flupyrsulfuron Free Acid 1 on Sugar Beet Grown in the Greenhouse 15 Days after Planting µg of flupyrsulfuron kg-1 of dry soil
fresh shoot weight, % of untreated ( SD
injury ratinga
1 2 3 5 7 8 10
96 ( 8 85 ( 9 71 ( 8 47 ( 6 29 ( 5 23 ( 5 19 ( 4
0 2 3 4 7 7 8
a Injury ratings 2 weeks after planting: 0 ) no visible effect; 1-3 ) slight stunting and chlorosis; 4-6 ) moderate stunting, yellowing of the leaf edges, and chlorosis; 7-9 ) tissue necrotic, stem green, reduced emergence, and shoot growth; 10 ) all plants killed.
containing 5 or 10 µg of flupyrsulfuron kg-1 were extracted after 6 days of incubation at 12 °C, with the soil humidity maintained at 15%. The decrease of their recoveries was 90%. The quantitative transformation of flupyrsulfuron at the residue level into a derivative measured by GC and GC/ MS has been optimized here. Before this analysis method is extended to another sulfonylurea, its efficiency for this sulfonylurea should be verified as the reactivity of each sulfonylurea depends on its chemical structure. For the soil analysis of flupyrsulfuron, in the present work the aqueous sodium bicarbonate extracts of soil were cleaned by partitioning and three successive TLC steps. For the Rf evaluations, standards of the analytes were applied on separate lanes of the TLC plates. In the soil extract, residues of parent flupyrsulfuron were first isolated from its potential soil metabolites. Methylation of flupyrsulfuron at the 1 mg or lower level quantitatively generated compound 2 (Figure 1), which was formed by elimination of SO2NCH3CO from the flupyrsulfuron dimethyl derivative 4. Compound 2 was measured by GC-ECD in the cleaned up soil extracts with confirmation by GC/MS. The GC and GC/MS
chromatograms were free of interfering peaks close to those of compound 2. For calibration of the GC and GC/MS signals, the solutions obtained by dilutions of the methylation products of 10 mg of flupyrsulfuron and the solutions made starting with compound 2 gave the same signal/ quantity relationships. This confirmed that methylation of low amounts of flupyrsulfuron generated compound 2. Recovery experiments were made during the 20-48 h period following fortification of soil. Within this period of time, the recoveries did not change significantly, indicating the absence of bonding of the residue to the soil, which could lower the extraction efficiency. Extraction of the spiked soils after 6 days of incubation at 12 °C gave recoveries lower by
