Determination of Alachlor and Its Sulfonic Acid Metabolite in Water by Solid-Phase Extraction and Enzyme-Linked Immunosorbent Assay

Solid-phase extraction (SPE) and enzyme-linked immunosorbent assay (ELISA) were combined for the trace analysis of the herbicide alachior and its major soil metabolite, ethanesulfonic acid (ESA). The anti-alachlor antibody cross-reacted with ESA, which produced false-positive detections of alachior in water samples by immunoassay screens. Alachior and ESA were isolated from water by SPE on a C₁₈ resin and eluted sequentially with ethyl acetate and methanol. Alachior is soluble in ethyl acetate while the anionic ESA is not. Thus ESA remained adsorbed on the C₁₈ resin and was eluted later with methanol. The combination of SPE with ELISA effectively separated and quantified both aiachior and ESA using the same antibody for two ELISA methods. The general method may have applicability for the separation of other herbicides and their ionic, metabolites. The SPE-ELISA method has a detection limit of 0.01 µ/L for aiachior and 0.05 µg/L for ESA, with a precision of ±10%. Analyses of surface and ground water samples were confirmed by gas chromatography/mass spectrometry and high-performance liquid chromatography with photodiode-array detection. Results showed widespread occurrence of ESA in surface and ground water of the midwestern United States, with concentrations ranging from <0.10 to >10 µ/L.