Dispersive Solid-Phase Extraction Clean-up Combined with Dispersive Liquid–Liquid Microextraction for the Determination of Neonicotinoid Insecticides in Vegetable Samples by High-Performance Liquid Chromatography
Dispersive solid-phase extraction (DSPE) clean-up combined with dispersive liquid–liquid microextraction (DLLME) has been developed as a new approach for the extraction of neonicotinoid insecticides in vegetable samples prior to high-performance liquid chromatography with diode array detection. In the DSPE–DLLME method, neonicotinoid insecticides were first extracted with acetonitrile from vegetable samples, followed by clean-up by a DSPE with primary secondary amine and multi-walled carbonnanotubes as sorbents. A 2.5-mL aliquot of the resulting extract was then added into a centrifuge tube containing 10 mL of water, 0.8 g NaCl, and 200-μL chloroform (as the extraction solvent) for DLLME procedure. Under the optimum conditions, the enrichment factors for the compounds were in the range between 110 and 243. The linearity of the method was in the range from 5.0 to 300 ng g−1 with the correlation coefficients (r) ranging from 0.9989 to 0.9998. The detection limits of the method were 0.5–1.0 ng g−1. The repeatability of the method expressed as the relative standard deviations by five parallel experiments at the concentration levels of 10 and 50 ng g−1 each of the neonicotinoid insecticides in tomato or cucumber samples varied from 3.6% to 5.8%. The developed method has been successfully applied for the analysis of target neonicotinoid insecticides (acetamiprid, imidacloprid, thiacloprid, and thiamethoxam) in tomato and cucumber samples. The recoveries of the method for the target neonicotinoid insecticides from the vegetable samples at spiking levels of 10.0 and 50.0 ng g−1 were in the range between 84.6% and 97.5% (n = 5).