Evaluation of ELISA microtitter plate-based assays for the direct determination of isoproturon in water samples and soil extracts

Trials were carried out on the commercially available Millipore isoproturon ELISA microtiter plate tests and on laboratory assays developed by Hirst as part of a Joint European Union research project (BIOPTICAS). The lowest detectable dose (LDD) was determined as three times the standard deviation of the blanks. Depending on the calibration curves obtained on different days with different plates, the LDD varied from 0.020 to 0.064 μg/L for the Millipore test and from 0.080 to 0.329 μg/L for the Hirst test. The mean coeficients of variation within a single plate for triplicate determinations of standard solutions in the 0.05 to 0.5 μg/L range were 5.5 and 3.6% for Millipore and Hirst respectively. Cross-reactivity was studied for mono- and di-demethylated isoproturon, chlortoluron, diuron and linuron. The highest cross-reactivity with both tests was that of mono-demethylated isoproturon (22% for Millipore, 4% for Hirst). This molecule was the only one to show significant cross-reactivity in the Hirst test, whereas in the Millipore test, the di-demethylated isoproturon also cross-reacted (4%). Natural water samples, 19 ground-, 53 lysimetric plate and 47 suction cup water samples, and 32 soil samples were also analysed with the ELISA tests. HPLC with a diode array detector was used as a validated control technique for the natural samples. For each water type, ELISA concentrations of both tests were significantly correlated with the HPLC values (r⪖ 0.937; p < 0.001). For the soil extracts, the correlations were also significant (p < 0.001), but the scatter in the data was greater. Overall, the Millipore correlation coefficients were higher than those of Hirst.