Effects of sample pretreatment and storage conditions in the determination of pyrethroids in water samples by solid-phase microextraction and gas chromatography-mass spectrometry.

The aqueous instability of pyrethroids and other compounds usually found in commercial pesticide formulations has been demonstrated in this work. Several types of sample treatment have been studied to avoid analyte losses during sample manipulation and storage. Analysis was performed by SPME-GC-MS. Addition of sodium thiosulfate to tap water prevented pyrethroid degradation as a result of oxidation by free chlorine. The amount added was optimized to minimize the effect of the salt on the analytical results. Analysis of samples that had been stored at 4 degrees C for several days revealed loss of some of the pyrethroids in the first period of storage. The effect of freezing the samples was studied and it was confirmed that samples could be stabilized for at least one week by freezing. Finally, addition of a miscible organic solvent, for example acetone, led to improvement of the analytical precision. The quality of the SPME-GC-MS method was studied. Linearity (R > 0.993), repeatability (RSD < 15%), and sensitivity (detection limits between 0.9 and 35 pg mL(-1)) were good. When the procedure was applied to real samples including run off and waste water some of the target compounds were identified and quantified.