Stir bar sorptive extraction coupled with liquid chromatography-mass spectrometry for the quantification of pesticides in water samples

Stir Bar Sorptive Extraction (SBSE) is a solvent free sample preparation technique described in the literature [1, 2, 3] to extract organic compounds from aqueous samples by sorption into a thick film of polydimethysiloxane (PDMS) on a glass-coated magnet. Stir bar sorptive extraction followed by liquid desorption and high performance liquid chromatography with mass spectrometric detection (SBSE-LD-LC-MS/MS) was applied for the determination of ultra-traces of thirteen pesticides or metabolites (azoxystrobin, carbendazim, chlorpyrifos, diuron, dichloroaniline, 3-(3,4-dichlorophenyl)-1-methylurea (DCMU), linuron, dimethomorph, flufenoxuron, fenitrothion, isoproturon, spiroxamine and tebuconazole) in water samples. Systematic studies were carried out in water samples spiked at the 0.1 or 0.5 mg/L level, with the purpose of optimizing several important parameters which could influence the SBSE-LD efficiency. Optimized conditions were established as follows concerning extraction time and stirring speed, aqueous medium characteristics (ionic strength and polarity) and back desorption solvent and time, respectively: 3 h (800 rpm), addition of 10% of sodium chloride, no addition of methanol as organic modifier, and 15 min ultrasonic desorption in equivolume mixtures of ACN-MeOH. Recoveries of the pesticides, from 20 to 84%, were lower than the theoretical values, which were calculated from their octanol-water equilibrium coefficients (KO/W). Nevertheless, repeatability and reproducibility for these pesticides were remarkable (<10%), except for flufenoxuron for which both repeatability and reproducibility were poor. Excellent linear dynamic ranges up to 10.0 mg/L and low quantification limits (0.002-0.042 µg/L) were achieved for twelve compounds (except for DCMU). This method was then applied to river water samples and showed a good conformity to the results obtained from parallel analyses using solid phase extraction-LC-MS/MS procedure.