Inhibition-based determination of metrifonate in liquid and solid samples using the triple integration chemical hydrolysis–pervaporation–enzymic derivatisation

An innovative continuous flow approach consisting of the triple integration of chemical hydrolysis, analytical pervaporation and enzyme inhibition-based reaction for the determination of metrifonate is presented. The method is based on chemical degradation of the pesticide, a separation step consisting of analytical pervaporation of its volatile metabolite and inhibition action of this on the acetylcholinesterase catalysis. The subsequent derivatisation reaction is a two-step reaction involving choline oxidase (ChOD) and horseradish peroxidase (POD) with fluorimetric detection (λex=310 nm and λem=415 nm) of the dimer formed by the action of hydrogen peroxide. The efficiency of the inhibitory effect was increased using an open–closed flow system. Applied to liquid samples, the method has a linear determination range of 0.0025–0.15 g l−1 (n=8, r=0.9993) with a precision, expressed as RSD, of 3.2–6.7% and a sampling frequency of 3 h−1. When applied to solid samples the method shows a linear determination range of 0.0026–0.13 g kg−1 (n=5, r2=0.9981, RSD 2.7–7.7%) and a sampling frequency of 2 h−1. The approach has been applied to the determination of metrifonate in natural water and spiked soil samples with recoveries ranging between 94.3 and 107.8% for liquid samples and between 86.5 and 99.6% for solid samples.