Extraction and stability of pesticide multiresidues from natural water on a mixed-mode admicellar sorbent.

The suitability of a mixed-mode sorbent made up of admicelles of sodium dodecyl sulphate (SDS) and tetrabutylammonium (TBA) to extract and preserve pesticides from river and underground water was assessed. Pesticides belonging to different structural groups (i.e. triazines, carbamates, phenylureas, anilides, chloroacetanilides, organophosphorus and phenoxyacids), most of them well known by their instability, were selected for this purpose. Extraction of pesticides with the admicellar sorbent was optimized. Percolation of 250 mL of sample through the SPE cartridge, elution with 1 mL of methanol (neutral and basic pesticides) or 2 mL of 0.3M NaOH:methanol (90:10, v/v) (acidic pesticides) and direct analysis of the extract by liquid chromatography/UV detection, permitted to obtain method detection limits in the range 2-60 ng L(-1). Their degradation on the SPE cartridges after sample percolation was investigated under a variety of experimental conditions, namely darkness, natural illumination and different temperatures (room temperature, 4 °C and -20 °C) for 3 months. Under darkness, most of pesticides were stable for 1 month at room temperature and 3 months at 4 °C. Only atriazine and clorfenvinfos did not follow this general behaviour. No influence of matrix components on the stabilization of pesticides in the admicellar sorbent was observed. The stabilizing capability of surfactant aggregates surpassed that of C18 and black carbon and was similar to that obtained in some polymeric materials, which have the disadvantage of requiring large volumes of solvents for analyte elution. The stabilization of pesticides on the admicellar sorbent was long enough to permit easy shipping and storage of the cartridges in monitorization campaigns.