Development of Enzyme-Linked Immunosorbent Assays for the Insecticide Chlorpyrifos. 2. Assay Optimization and Application to Environmental Waters

Monoclonal antibody-based enzyme-linked immunosorbent assays (ELISA) to chlorpyrifos were optimized for the analysis of this insecticide in environmental water samples. By optimizing the immunoreagent conditions and the composition of the competition buffer, assays with up to 1 order of magnitude better sensitivities were obtained. Reduction of the detergent (Tween 20) concentration provided most of the sensitivity improvement. Other factors studied, such as pH and ionic strength, behaved consistently with the nonpolar nature of the analyte and contributed in lesser extent to improve assay characteristics. Under optimized conditions, the chlorpyrifos concentration giving 50% reduction of maximum ELISA signal ( I 50 ) was in the 0.8−1.0 nM range, and the assays showed acceptable specificity for both chlorpyrifos and chlorpyrifos-methyl. Preliminary evaluation of assay performance in several water samples showed the absence of significant matrix effects for both formats evaluated. ELISAs provided a limit of detection of 0.10−0.14 μg/L and a working range in the 0.21−2.50 μg/L range and allowed the direct determination of submicrograms per liter chlorpyrifos in environmental water samples with acceptable analytical parameters.