Evaluation of an enzyme‐linked immunosorbent assay (ELISA) for the direct analysis of molinate (Ordram®) in rice field water

Abstract

A direct ELISA for the thiocarbamate herbicide molinate was used to study distribution and dissipation of the compound in a treated rice field. No sample preparation other than buffering and dilution was required for the analysis of field water samples. Analyses were performed in 96‐well microplates and required less than 0–5 man‐hour per sample (three dilutions per sample, four replicate wells per dilution). Spiked samples and selected field samples were split for analysis by ELISA and gas chromatography. Two control samples of 92 and 93 ppb (after dilution) had between run coefficients of variation of 13.8 and 13.9% for 37 ELISA runs. A nested ANOVA analysis revealed that the largest source of error for the ELISA was due to within replicate variability, partly attributable to interwell variability of the 96‐well plates. Practical aspects of reducing assay error and handling ELISA data are discussed. Quality control data showed that reliability of the direct ELISA is comparable to the gas chromatography method for molinate. ELISA data from field samples showed concentration differences among sites in the same check which coincided with differences in water flow. The half‐life of molinate in the field, as determined by ELISA, was comparable to the value determined by chromatography.