Validation of an automated precolumn exchange system (PROSPEKT) coupled to liquid chromatography with diode array detection. Application to the determination of pesticides in natural waters

Abstract

An automated precolumn exchange system (PROSPKET) coupled to liquid chromatography with diode array detection (LC-DAD) has been used for the determination of trace pesticides in natural (drinking and ground) waters. Relevant parameters such as pH and type of precolumn (C 18 and PLRP-s) were optimized for a variety of organophosphorus pesticides, herbicides and pesticide transformation products. Calibration graphs at low levels of determination (0.1–1.5 μg/l) were constructed. The validation of the present automated method has been guaranteed by participating in Aquacheck interlaboratory exercises where more conventional gas chromatographic determinations are being used. The overall relative standard deviation between values obtained in our laboratory and the average value obtained by 14–15 other laboratories varied between 1.6 and 36% for most of the studied compounds. Problems encountered in the determination of certain organophosphorus pesticides led to high variations in the mean values and errors of many laboratories. In this sense, problematic compounds like mevinphos, which gave two peaks corresponding to the cis- and trans-isomers, parathion-methyl and diazinon, which showed coelution problems, or malathion, which presented poor quantitation due to low absorption in the UV range. In addition, fenthion, which was not determined gave 3–4 compounds, corresponding to various degradation products. Confirmation of the different pesticides exhibiting an UV maximum > 220 nm was feasible due to their characteristic UV spectra and satisfactory levels of 0.02 μg/l were found when preconcentrating 150 ml of ground water sample.