Validation of an enzymatic biosensor with various liquid chromatographic techniques for determining organophosphorus pesticides and carbaryl in freeze-dried waters

An enzymatic biosensor based on the reaction of acetylthiocholine chloride, which is enzymatically hydrolysed to thiocholine giving a disulphide compound by dimerization, was validated with various liquid chromatographic techniques for determining the organophosphorus pesticides fenitrothion, fenamiphos, parathion-ethyl and the carbamate pesticide carbaryl in two different batches of freeze-dried waters at levels varying from 0.1 to 5.7 μg 1−1 for each individual compound. Calibration graphs for the biosensor were performed for a concentration range of 1 × 10−5–1 × 10−10 M for paraoxon, and the limit of detection was taken as the insecticide concentration that resulted in 10% inhibition of the enzyme activity. Depending on the liquid Chromatographic systems used -either liquid-liquid extraction of 11 water with dichloromethane or an on-line automated precolumn exchange system (Prospekt) of 100 ml water - followed by liquid chromatography with diode array detection, good reproducibility was obtained between 0.05 and 10 μg 1−1, the limit of detection lying between 0.01 and 0.05 μg l−1, depending on the compound. The determination of the total amount of pesticides by the biosensor was carried out either after liquid-liquid extraction or using a flow-injection system. In both cases the batch of water containing the higher amount of pesticides gave also a higher signal when using the biosensor. The total amount of pesticide determined with the biosensor could be used as a positive indication of the presence of the target pesticides at a level of 3.9–10.3 μg 1−1 total target pesticides present in the sample.