Herbicides, sulfonylurea

Du Pont has recently developed a new group of selective herbicides, the sulfonylureas, from which three candidates have shown extraordinary activities DPX 4189, DPX-T 6376 and DPX 5648. 

DPX 4189, 1 -(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl)urea (chlorsulfuron, 1), (Levitt, 1978, 1979, 1980) is synthesised by adding an equivalent of 2-chlorobenzenesulfonyl isocyanate to a suspension of 2-amino-4-methoxy-6-methyl-l,3,5-triazine in acetonitrile (Levitt et al., 1981). The compound is moderately soluble in acetone and acetonitrile. The sodium salt of chlorsulfuron has a solubility of 5-10% in water. 

Chlorsulfuron is the active ingredient in Du Pont Glean Weed Killer. A very important feature of this new selective herbicide compound is its very high herbicidal activity at very low application rates. The compound controls most broad-leaved weeds in small grains at 10-40 g/ha. It is nonvolatile and is active due to uptake through both foliage and root system. 

The first weed-control reports in cereals and fallow w6re published in 1979 (Hageman and Behrens, 1979 Miller and Nalewaja 1979). After the first summarised development report of 1979 (Levitt et ai, 1980) Palm et al. (1980) gave a world-wide review of the chlorsulfuron. 

In the winter cereal belt in North America and Australia, similar rates also controlled Helianthus, Erodium, Lithospermum, Amsinckia, Lamium, Oxalis and Emex. At rates of 35-70 g active ingredient/ha early postemergence treatments controlled significantly Lolium, Seteria and Poa. 

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Sulfonylurea herbicides ate weak acids and, in general, ate not strongly sorbed to soils. Sorption of chlotsulfuton and metsulfuron—methyl is inversely related to soil pH (407) and is positively correlated to soil organic matter (408). 

The EPA has not issued HAs for any of the sulfonylurea herbicides (269) and data on the occurrence of the sulfonylurea herbicides in SW or GW are not available. Additional sulfonylurea herbicides include bensulfuron [99283-01-9] and metsulfuron, methyl [74223-64-6]. 

Extensive use of two more recendy developed classes of herbicides will further dramatically reduce the amount of appHed to control weeds. The sulfonylurea herbicides are extremely active compounds first discovered in the mid-1970s at DuPont they have been discussed extensively (78). Sulfonjiurea herbicides have experienced a rapid and widespread success since thek commercial introduction in 1982 with chlorsulfuron (Table 5). The sulfonylureas are appHed at rates of 2—75 g/ha. The chemistry of the sulfonylurea molecule permits the synthesis of a very large number of useful analogues, consequentiy many new herbicides are anticipated for crop production. As of this writing (1996), over 350 patents have been issued to about 27 agricultural companies covering tens of millions of stmctures known or expected to be herbicidaHy active. 

Agrochemical Products. The principal thiophene derivative in herbicidal protection, one of a range of sulfonylurea herbicides, is Harmony [79277-27-3] (Du Pont) (60), based on the intermediate methyl 3-aminothiophene-2-carboxylate (9). The product is characterized by a rapid biodegradabHity in the soil. Many other thiophene derivatives have been shown to have agrochemical activity, but few of these have been developed to the commercial level. 

A multiresidue analytical method based on sohd-phase extraction enrichment combined with ce has been reported to isolate, recover, and quantitate three sulfonylurea herbicides (chlorsulfuron, chlorimuron, and metasulfuron) from soil samples (105). Optimi2ation for ce separation was achieved using an overlapping resolution map scheme. The recovery of each herbicide was >80% and the limit of detection was 10 ppb (see Soil chemistry of pesticides). 

The rationale of validation experiments with fatty matrices is the high amount of fat extracted with many organic solvents. If analytes are not fat soluble and extraction is performed with water or aqueous buffer solutions, the troublesome fat is not extracted together with the analyte. Such extractions are typical for, e.g., the class of sulfonylurea herbicides. Examples exist where in such cases the applicability of an analytical method to fatty matrices was accepted by the authority without particular validation. 

LC/MS/MS is the preferred means of detection, quantitation, and confirmation of sulfonylurea herbicides in biological and environmental matrices. Therefore, recommendations for establishing and optimizing LC/MS/MS analyses common to all matrices are given first, followed by specific rationales for methods and sample preparation techniques for plant, soil, and water matrices. 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

Table 1 Example HPLC conditions for the determination of sulfonylurea herbicides by LC/MS/MS...

If sulfonylurea herbicides can reasonably be expected to be present in an analytical sample (based on prior knowledge), one parent-to-daughter ion transition is usually considered sufficient to confirm its presence. In other cases where little is known about the sample history, two parent-to-daughter ion transitions are generally considered to be necessary for a definitive confirmation. Suggested ion transitions for most of the registered sulfonylurea herbicides are listed in Table 2. Furthermore, the ratio of the signals for the two transitions obtained for the sample should match that of an authentic standard within 30%, at most. ... 

Sulfonylurea herbicides are generally applied to crops as an early post-emergent herbicide. Crops that are tolerant to these herbicides quickly metabolize them to innocuous compounds. At maturity, residues of the parent compound in food and feed commodities are nondetectable. Metabolites are not considered to be of concern, and their levels are usually nondetectable also. For this reason, the residue definition only includes the parent compound. Tolerances [or maximum residue limits (MRLs)] are based on the LOQ of the method submitted for enforcement purposes and usually range from 0.01 to 0.05 mg kg (ppm) for food items and up to O.lmgkg" for feed items. There is no practical need for residue methods for animal tissues or animal-derived products such as milk, meat, and eggs. Sulfonylurea herbicides are not found in animal feed items, as mentioned above. Furthermore, sulfonylurea herbicides intentionally dosed to rats and goats are mostly excreted in the urine and feces, and the traces that are absorbed are rapidly metabolized to nontoxic compounds. For this reason, no descriptions of methods for animal-derived matrices are given here. 

Hydrolysis rates of sulfonylurea herbicides in water are heavily dependent upon pH. In general, acidic conditions promote faster hydrolysis, usually by cleavage of the sulfonylurea bridge. Neutral to alkaline conditions favor the compounds existing in... 

At the present time, LC/MS/MS with triple-quadrupole instruments is the analytical method of choice for the determination of residues of sulfonylurea herbicides. We can expect to see improved triple-quadrupole instrumentation become more available and affordable as time passes, so that more analytical laboratories will have this capability. Time-of-flight (TOP) instrumentation may also play an increasingly important role in sulfonylurea analysis. Even though the metabolites are innocuous, stricter regulatory requirements may mandate that they be monitored, and LC/MS/MS is the method of choice for these compounds also. 

J. Strahan, Development and application of an enzyme-linked immunosorbent assay method for the determination of multiple sulfonylurea herbicides on the same microwell plate, in Environmental Immunochemical Methods, ed. J.M. Van Emon, C.L. Gerlach, and J.C. Johnson, American Chemical Society, Washington, DC, pp. 65-73 (1996). 

Sample preparation consists of homogenization, extraction, and cleanup steps. In the case of multiresidue pesticide analysis, different approaches can have substantially different sample preparation procedures but may employ the same determinative steps. For example, in the case of soil analysis, the imidazolinone herbicides require extraction of the soil in 0.5 M NaQH solution, whereas for the sulfonylurea herbicides, 0.5M NaOH solution would completely decompose the compounds. However, these two classes of compounds have the same determinative procedure. Some detection methods may permit fewer sample preparation steps, but in some cases the quality of the results or ruggedness of the method suffers when short cuts are attempted. For example, when MS is used, one pitfall is that one may automatically assume that all matrix effects are eliminated because of the specificity and selectivity of MS. 

ESI performs well for the more polar compounds such as imidazolinone herbicides, sulfonylurea herbicides, triazine herbicides, phenoxy acid herbicides, and carbamate pesticides (to name a few). ESI also performs well with proteins and peptides. 

C.R. Powley, Sulfonylurea herbicides , in Handbook of Residue Analytical Methods for Agrochemicals , ed. P. W. Lee, John Wiley Sons Ltd, Chichester, Vol.l, (2003). 

Only a few models applicable to paddy field conditions have been developed. RICEWQ by Williams, PADDY by Inao and Kitamura," and PCPF-1 by Watanabe and Takagi are useful for paddy fields. EXAMS2 by the United States Environmental Protection Agency (USEPA), a surface water model, can also be used to simulate paddy fields with an appropriate model scenario and has been used for the prediction of sulfonylurea herbicide behavior in paddy fields. The prediction accuracy of PADDY and PCPF-1 is high, although these models require less parameter... 

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