Multiresidue analysis of pesticides using new laminar extraction disks and liquid chromatography and application to the French priority list.

The national French lists of priority pesticides to be monitored in drinking and/or surface water contain various pesticides with a wide range of physico-chemical properties and can be modified on a regional scale with the addition of other pesticides, depending on local agricultural usage. A multiresidue extraction method is presented using new disk formats engineered for high throughput that are particularly well adapted to the extraction of compounds from high volumes of dirty samples. However, because of the occurrence of very polar and very apolar compounds in the lists, two procedures are required for the extraction step. Divinylbenzene disks were used to extract the more polar compounds, as well as the moderately polar or acidic ones. For apolar pesticides, a C18 silica disk was used, and 10% methanol was added to the water sample before percolation in order to avoid adsorption on the flasks and connecting tubes. Since 1 l of surface water is extracted in less than 5 min without previous filtration using these new laminar disks, the percolation time is no longer a limiting factor in the analysis scheme of surface water samples containing suspended matter. The sample volume can be easily increased in order to reach lower detection limits, provided that the extraction conditions have been optimized in order to minimize the amount of co-extracted and interfering substances. A considerable decrease in the effect of the humic and fulvic substances was achieved using divinylbenzene disks, which allows the samples to be handled at pH 6, for the polar pesticides. Moreover, the necessary addition of 10% methanol in the samples for the extraction of the apolar ones also considerably decreases the amount of co-extracted analytes. The time of the whole sample preparation sequence, i.e., conditioning of the disks, sample extraction, sample clean-up and desorption, is achieved within 10 min for six simultaneous samples. Detection limits in the range 0.01-0.05 microgram l-1 are easily obtained for most pesticides contained in the national lists from 1 l of real surface water samples. The two procedures described in this work allow the handling of any compounds having a water-octanol constant, log Kow, in the range 1-6.