2 QUALITY ASSURANCE MEASURES FOR PESTICIDE ANALYSIS OF WELLWATER - DEGRADATION IN STORAGE AND GC ELISA COMPARISON/

At the request of Environmental Protection Agency (EPA) project coordi nators, two special quality assurance components were included in a st udy of herbicides in rural wells in Iowa. Since the study involved dai ly sampling of 88 rural wells for a period of four to five weeks, it w as anticipated that samples would be in refrigerated storage for up To eight weeks during which microbial and chemical activity could lead t o analyte loss. The sample degradation study reported here was conduct ed to insure that water samples containing three herbicides (atrazine, alachlor and metolachlor) did not undergo excessive losses during sto rage. Results indicate no reduction in pesticide concentrations in six refrigerated water samples analyzed weekly during an eight-week stora ge period.Due to budget and time limitations, the rural well-water stu dy employed enzyme-linked immunosorbent assay (ELISA) techniques To de termine atrazine, alachlor, and metolachlor concentrations in well-wat er samples. Since ELISA techniques generally are considered to be less accurate than the more costly gas chromatography (GC) technique, a GC /ELISA comparison study was designed to evaluate the accuracy of the w ater quality data from this study. Results of the GC/ELISA comparison show that the ELISA method may over-estimate pesticide concentrations or register a positive detection for a chemical that is not present. B ased on results of this study and information from the ELISA test kit manufacturers, it is believed that ELISA results for this study were a ffected by cross-reactive parent compounds and/or metabolites. Deethyl atrazine and prometon were identified as compounds that may have cross -reacted with the antibody of the atrazine test kit.