Centrifuge-less dispersive liquid-liquid microextraction base on the solidification of switchable solvent for rapid on-site extraction of four pyrethroid insecticides in water samples

An on-site dispersive liquid liquid microextraction base on the solidification of switchable solvent has been developed as a simple, rapid and eco-friendly sample extraction method for the fast determination of pyrethroid insecticides in aqueous samples using high-performance liquid chromatography with ultraviolet detection. In this extraction method, medium-chain saturated fatty acids (n ≥ 10), which can rapidly solidify at low temperatures (<20 °C), were investigated as switchable hydrophilic solvents. The fatty acids were converted into the hydrophilic form by adding sodium hydroxide. Microdroplets of the fatty acids were generated when injected into an acidic sample that had been pretreated by the addition of sulfuric acid. The formed cloudy solution was cooled to a low temperature to turn the fatty acids into a solid, which was then separated by filtration, thus avoiding the time-consuming step of centrifugation. The microextraction process was performed in a 10 mL syringe and the pretreatment process could thus be finished in 5 min. No external energy resources were required in this method, which makes it a potential method for on-site extraction. The optimal experimental parameters were as follows: 350 μL of decanoic acid (1 mol/L) was used as the extraction solvent, 150 μL of sulfuric acid (2 mol/L) was used to decrease the pH of the samples, no salt was added, and the temperature of the samples was in the range of 20–40 °C. Finally, the sample was cooled in an ice bath for three minutes. Under these optimal conditions, good responses for four pyrethroid insecticides were obtained in the concentration ranges of 1–500 μg/L, with coefficients of determination greater than 0.9993. The recoveries of the four pyrethroid insecticides ranged from 84.7 to 95.3%, with relative standard deviations ranging from 1.6 to 4.6%. The limits of detection based on a signal-to-noise ratio of 3 were in the range of 0.24–0.68 μg/L, and the enrichment factors were in the range of 121–136. The results demonstrate that this method was successfully applied to determine pyrethroid insecticides in real water samples. No centrifugation or any special apparatus are required, make this a promising method for rapid field-sampling procedures.