Deep eutectic solvent-based clean-up/vortex-assisted emulsification liquid-liquid microextraction: Application for multi-residue analysis of 16 pesticides in olive oils

A new hydrophobic deep eutectic solvent (DES) composed of thymol and vanillin (1:1) was prepared and used as the extraction solvent in vortex-assisted emulsification liquid-liquid microextraction of 16 pesticides in olive oil before GC-μECD analysis. Due to the complexity of oil samples, a DES-based liquid-liquid solvent system (n-hexane/acetonitrile/DES) was developed to achieve an effective clean-up. Among the four examined hydrophilic DESs, [choline chloride]:[urea] led to improved sensitivity, and clean chromatograms with low noise and drift from the sample matrix. Besides, the effect of the significant factors such as extraction solvent volume, pH, ionic strength, and vortex time was investigated. At the optimized conditions, the validation parameters such as linear range 0.04–250 μg kg−1, the limits of detection of 0.01–0.08 μg kg−1, intra-day, and inter-day relative standard deviations (RSDs) < 6.8 and < 9.7%, were obtained. This method was followed by GC-μECD and applied to determine pesticides in five olive oil samples. The relative recoveries were in the range of 63.1–119.4%.