Coupling Solid-Phase Extraction and Enzyme-Linked Immunosorbent Assay for Ultratrace Determination of Herbicides in Pristine Water

Solid-phase extraction (SPE) and enzyme-linked immunosorbent assay (ELISA) were coupledfor automated trace analysis of pristine water samples containing 2-chloro-4-ethy lamino-6-isopropylamines-triazine (atrazine) and 2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide (alachlor). The isolation of the two herbicides on a C₁₈-resin involved the selection of an elution solvent that both removes interfering substances and is compatible with ELISA. Ethyl acetate was selected as the elution solvent followed by a solvent exchange with methanol/water (20/80, % v/v). The SPEELISA method has a detection limit of 5.0 ng/L (5 ppt), >90% recovery, and a relative standard deviation of ±10%. The performance of a microtiter plate-based ELISA and a magnetic particlebased ELISA coupled to SPE was also evaluated. Although the sensitivity of the two ELISA methods was comparable, the precision using magnetic particles was improved considerably (±10% versus ±20%) because of the faster reaction kinetics provided by the magnetic particles. Finally, SPEELISA and isotope dilution gas chromatography/mass spectrometry correlated well (correlation coefficient of 0.96) for lake-water samples. The SPE-ELISA method is simple and may have broader applications forthe inexpensive automated analysis of other contaminants in water at trace levels.