Alachlor, acetochlor, and metolachlor

Chloroacetanilides are soil-applied herbicides used for pre- and early post-emergence control of annual grasses and broadleaf weeds in crops. Representative chloroacetanilide compounds, alachlor, acetochlor, and metolachlor, are extensively used worldwide. Other chloroacetanilides with limited usages include propachlor, bu-tachlor, metazachlor, pretilachlor, and thenylchlor. Public environmental concerns and government regulatory requirements continue to prompt the need for reliable methods to determine residues of these herbicides. There now exist a variety of analytical methods to determine residues of these compounds in crops, animal products, soil, and water. The chemical structures and major crops in which these compounds are used are summarized in Table 1. 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

The method for chloroacetanilide soil metabolites in water determines concentrations of ethanesulfonic acid (ESA) and oxanilic acid (OXA) metabolites of alachlor, acetochlor, and metolachlor in surface water and groundwater samples by direct aqueous injection LC/MS/MS. After injection, compounds are separated by reversed-phase HPLC and introduced into the mass spectrometer with a TurboIonSpray atmospheric pressure ionization (API) interface. Using direct aqueous injection without prior SPE and/or concentration minimizes losses and greatly simplifies the analytical procedure. Standard addition experiments can be used to check for matrix effects. With multiple-reaction monitoring in the negative electrospray ionization mode, LC/MS/MS provides superior specificity and sensitivity compared with conventional liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/ultraviolet detection (LC/UV), and the need for a confirmatory method is eliminated. In summary,... 

Weber, J.B., Peter, C.J. (1982) Adsorption, bioactivity, and evaluation of soil tests for alachlor, acetochlor, and metolachlor. Weed Sci. 30, 14-20. 

A variety of neutral degradation products of the chloracetanilide herbicide alachlor was identified [63, 115]. However, the ionic metabohtes such as the oxoethanesulfonic acid derivative appear to be of more significance, as they are readily leached to groundwater. WMe alachlor itself is amenable to GC-MS, its ionic metabolites are not. Initially, GC-MS, LC-UV-DAD, and fast-atom bombardment MS-MS were applied in the analysis and identification of such metabolites [119]. Subsequently, the potential of LC-ESI-MS in this area was recognized [120]. Both oxanilic acid and oxoethanesulfonic acid metabohtes of alachlor, acetochlor and metolachlor were identified in snrface water and gronnd water, and subsequently determined with detection limits at the 0.01-pg/l level using off-hne SPE in combination with LC-MS [120]. 

Limits of detection for each of the three parent herbicides in surface and groundwater were determined using results obtained from control samples analyzed along with hundreds of surface and ground water sets during the years 1995-2001. In each of these years, the calculated LODs (minimum detectable true concentrations/detection) were below 0.03 pg for acetochlor and metolachlor and 0.05 pg for alachlor. A detection criterion is a measured concentration threshold that defines a likely upper bound for samples not containing the analyte. If the actual concentration of an analyte is at this detection limit or greater, there is at least a 95% chance of detection. 

LOQs for each of the three parent herbicides in surface water were determined using all the analytical results (not corrected for background) of samples fortified at the lowest fortification level, 0.05 pgL , during the analysis in years 1995-2001. The calculated LOQs were below 0.05 pgL for acetochlor and metolachlor and approximately 0.05 pgL for alachlor. If the true concentration of an analyte is at the LOQ or greater, the standard error of individual measured concentration values relative to the true concentration is at most 10%. 

The focus of this article is to describe the residue methodologies for alachlor, acetochlor, metolachlor, and propachlor. Four residue analytical methods are discussed ... 

The complexity of the metabolism of alachlor, acetochlor, butachlor, and propachlor has led to the development of degradation methods capable of hydrolyzing the crop and animal product residues to readily quantitated degradation products. Alachlor and acetochlor metabolites can be hydrolyzed to two major classes of hydrolysis products, one which contains aniline with unsubstituted alkyl groups at the 2- and 6-positions, and the other which contains aniline with hydroxylation in the ring-attached ethyl group. For alachlor and acetochlor, the nonhydroxylated metabolites are hydrolyzed in base to 2,6-diethylaniline (DBA) and 2-ethyl-6-methylaniline (EMA), respectively, and hy-droxylated metabolites are hydrolyzed in base to 2-ethyl-6-(l-hydroxyethyl)aniline (HEEA) and 2-(l-hydroxyethyl)-6-methylaniline (HEMA), respectively. Butachlor is metabolized primarily to nonhydroxylated metabolites, which are hydrolyzed to DEA. Propachlor metabolites are hydrolyzed mainly to A-isopropylaniline (NIPA). The base hydrolysis products for each parent herbicide are shown in Eigure 1. Limited interference studies have been conducted with other herbicides such as metolachlor to confirm that its residues are not hydrolyzed to the EMA under the conditions used to determine acetochlor residues. Nonhydroxylated metabolites of alachlor and butachlor are both hydrolyzed to the same aniline, DEA, but these herbicides are not used on the same crops. 

Multi-residue analytical method for the determination of acetochlor, alachlor, and metolachlor in aqueous samples... 

Acetochlor, alachlor, and metolachlor are determined in ground and surface water samples. Deuterated internal standards are added to each water sample, and analytes are extracted using an SPE column. After elution and concentration to an appropriate volume, the analytes are quantitated by GC/MS. 

This analytical method determines levels of major oxanilate and sulfonate soil metabolites of acetochlor, alachlor, and metolachlor in groundwater and surface water. The method consists of analysis of environmental samples by direct aqueous injection reversed-phase LC/MS/MS. 

R.A. Yokley, L.C. Mayer, S.-B. Huang, J.D. Vargo, Analytical method for the determination of metolachlor, acetochlor, alachlor, dimethenamid, and their corresponding ethanesulfonic and oxanillic acid degradates in water using SPE and LC-ESI-MS-MS, Anal. Chem., 74 (2002) 3754. 

Chloroacetanilide herbicides (e.g., alachlor, metolachlor, and acetochlor) are of an important class of herbicides used to control grass weeds in various crops. Alachlor and metolachlor have been widely used (both in the U.S.A and in Europe) for more than 20 years. Acetochlor, a herbicide used for maize, has been on the United States market since 1994, following approval by the U.S. Environmental Protection Agency. This approval will be renewed, however, only if the total quantity of other herbicides used on this crop, including atrazine, decreases. Acetochlor was approved in France in 2000 and is now used in substitution programs. 

Fig. 2 Structures of parent herbicides and transformation products under investigation. Structures I-X represent alachlor and its transformation products structures XI-XIX represent metolachlor and its transformation products structures XX-XXIV represent acetochlor and its transformation products structures XXV-XXVIII are those transformation products that can result from either metolachlor or acetochlor structures XXIX-XXX represent dimethenamid and its transformation product and structures XXXI-XXXV are triazine herbicides and their transformation products. Reprinted with permission from [50]...

The reaction was also tested in soils and sand for chloroacetanilide herbicides. Enhanced herbicide dissipation occurred in sand or sandy soils, but became insignificant in clayey soils (29). The reduced enhancement in clayey soils may be attributed to the strong adsorption of herbicides to soil, which may have limited the interaction between the herbicide (adsorbed) and thiosulfate anion (in solution). In sand or sandy soils, the transformation of propachlor was the most rapid, which was followed by acetochlor and alachlor. Metolachlor transformation was only marginally enhanced. 

Figure 1. Chemical structures of (a) alachlor, (b) acetochlor, and (c) metolachlor.

Acid amide herbicides are nonionic and moderately retained by soils. The sorption of several acid amide herbicides has been investigated (369). Acetochlor [34256-82-1] is sorbed more than either alachlor or metolachlor, which are similarly sorbed by a variety of soils. Sorption of all the herbicides is well correlated to soil organic matter content. In a field lysimeter study, metolachlor has been found to be more mobile and persistent than alachlor (370) diphenamid [957-51-7] and napropamide [15299-99-2] have been found to be more readily leached (356). 

Enzyme-Linked Immunoassays (ELISA) were used to determine chloroacetanilide levels in water, with detection limits of 0.06, 0.3, and 0.4 /rg/l for metolachlor, alachlor, and acetochlor, respectively. " ... 

To explore the risk implications of mixtures of pesticides and their transformation products occurring in aquatic systems, we have taken data from a recent monitoring study of pesticides and transformation products in US streams [45]. This study looked at the occurrence of the parent compounds alachlor, metolachlor, acetochlor, dimethamid and atrazine as well as transformation products associated with each of these parent compounds. The monitoring was done in the spring and autumn at a number of sites. 

In aquifers of the midwestem U.S., some of these herbicide metabolites have been detected more frequently and at higher concentrations than the parent molecule (3). Researchers in Wisconsin reported that alachlor, metolachlor, and acetochlor were detected in 20 and 40% of monitoring and private wells tested, respectively, while only 2% of the municipal sanq les contained these herbicides... 

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