Evaluation of an ELISA Method for Acetochlor Analysis in the Le Sueur River Watershed

In the spring of 2009, the Minnesota Department of Agriculture (MDA) conducted an evaluation of the use of enzyme-linked immunosorbent assays (ELISA) for the analysis of acetochlor in surface water samples collected from within the Le Sueur River Watershed in south central Minnesota. Both composite and grab samples were collected from a network of existing stream gage locations and submitted to the University of Wisconsin-Stevens Point Trace Organics Laboratory for ELISA analysis. Of the 90 samples submitted for ELISA analysis 39 were also submitted for base neutral pesticide analysis including acetochlor, using gas chromatography/mass spectrometry (GC/MS) at the MDA Laboratory. ELISA results indicated that 11 percent of the samples exceeded an acetochlor concentration of 1 ?g/L. No samples collected had concentrations greater than the current Minnesota aquatic life standard for acetochlor of 3.6 ?g/L. An acetochlor cross-reactivity analysis was completed using samples that had both ELISA and GC/MS analysis. The results of the acetochlor analysis for the two methods indicated a strong relationship existed (R2=0.80) between the ELISA and GC/MS methods. The relationship improved (R2 =0.92) when the acetochlor ELISA results were compared to the cumulative GC/MS results of acetochlor and metolachlor. Approximately 10 percent of the ELISA samples were submitted for additional liquid chromatography/mass spectrometry (LC/MS) analysis for eight chloroacetanilide degradates. There did not appear to be cross-reactivity between the acetochlor ELISA method and the chloroacetanilide degradates although the data collected in this study was limited. Metolachlor appeared to be the primary analyte causing cross-reactivity concerns in the ELISA method. Results of this study strongly suggest that the acetochlor ELISA analysis is a reasonable screening tool for evaluating acetochlor concentration in surface water especially when combined with GC/MS analysis of an appropriate number of split samples above a pre-determined critical concentration to verify the presence of acetochlor and metolachlor. However because of the potential for cross-reactivity with metolachlor, which is a common contaminant in Minnesota waters, and a slight bias towards high (more protective) values in sampling results, the acetochlor ELISA is not suitable as a method for the precise quantification of the concentration of acetochlor