Improvements in the determination of organophosphorus pesticides in ground- and wastewater samples from interlaboratory studies by automated on-line liquid-solid extraction followed by liquid chromatography diode array detection

On-line liquid-solid extraction coupled to liquid chromatography and diode array detection (LSE-LC-DAD) was validated by participating in interlaboratory calibration studies organized by Aquacheck (Medmenham, UK). The quantitation of 11 organophosphorus pesticides ground- and wastewater samples at levels varying from 30 to 600 ng/l respectively was carried out. The solution containing the organophosphorus pesticides and the water sample was provided by Aquacheck. Between 90 and 150 ml of wastewater and ground respectively were percolated through the Prospekt C-18 precolumns (Spark Holland). The average error in comparison to the certified values from Aquacheck varied from 4 to 65%. The main problems encountered in the determination of these pesticides were the low UV absorbance of dichlorvos, mevinphos and malathion, the early breakthrough of dichlorvos and mevinphos and matrix interferences, specially abundant in the wastewater samples. In this type of water samples, the spiked level was one order of magnitude higher than in groundwater, but the error percentage was higher due to the matrix. One of the main advantages of this technique is that it permits the identification of two isomers of mevinphos, contrary to the rest of the participants who only reported trans-mevinphos. Moreover, by percolating 150 ml of water, limits of detection (LODs) below 0.1 mu g/l are attained. The system behaved linearly over a concentration range of 0.4 to 5 ng/ml with 150 ml of water preconcentrated.