Liquid Chromatographic Determination of Trace Bioavailable Neonicotinoids in Soil with Dispersive Liquid–Liquid Microextraction and Its Application for Experimental Monitoring

Dispersive liquid–liquid microextraction (DLLME) was applied to extract bioavailable neonicotinoids involved in the uptake from soil through roots to plants. To quantitatively extract bioavailable neonicotinoids with the proposed DLLME, 3.5 mL of dichloromethane (extractant)/acetonitrile (dispersive solvent) (6:1, v/v) was injected into 5 mL of aqueous soil extracts in which 1 g of sodium chloride was previously dissolved. The separated dichloromethane phase after sonication and centrifugation was evaporated, reconstituted with a mobile phase, and determined with high-performance liquid chromatography. The established method showed sufficient analytical performance to quantify the amount remaining in soil in trace amounts. In a pilot trial conducted in the field, the changes in the concentrations of bioavailable neonicotinoids were confirmed using the method. After showing rapid degradation in soil, degradation of clothianidin and imidacloprid slowed after about 100 days of treatment, but it continued to be detected at around 0.02–0.05 μg/g-dried weight until 1097 days. This result suggests that once these neonicotinoids are treated in soil, they might remain for long periods, which supports the possibilities of crop contamination and exposure to pollinators.