Determination of some sulfonylurea herbicides in soil by a novel liquid-phase microextraction combined with sweeping micellar electrokinetic chromatography

A novel method for the determination of five sulfonylurea herbicides in soil was developed by a dispersive solid-phase extraction (DSPE) clean-up followed by dispersive liquid–liquid microextraction (DLLME), prior to sweeping micellar electrokinetic chromatography (MEKC). In the DSPE-DLLME, 10 g of soil sample was first extracted with 10 mL of acetonitrile containing 5% formic acid (pH 3.0). The extract was then cleaned-up by a DSPE with C₁₈ as sorbent. A 1 mL aliquot of the resulting extract was then added into a centrifuge tube containing 5 mL of water adjusted to pH 2.0 and 60.0 μL chlorobenzene (as extraction solvent) for DLLME procedure. Then, the organic sample extraction solution was evaporated to dryness, and reconstituted with 20.0 μL of 1.0 mmol L⁻¹ Na₂HPO₄ (pH 10.0) for sweeping-MEKC analysis after DLLME. Under optimized conditions, the method provided as high as 3,000- to 5,000-fold enrichments factors. The linearity of the method was in the range of 3.3–200 ng g⁻¹ for chlorimuron ethyl and bensulfuron methyl, and in the range of 1.7–200 ng g⁻¹ for tribenuron methyl, chlorsulfuron and metsulfuron methyl, with the correlation coefficients (r) ranging from 0.9965 to 0.9983, respectively. The limits of detection (LODs) ranged from 0.5 to 1.0 ng g⁻¹. The intraday relative standard deviations (RSDs, n = 5) were below 5.3% and interday RSDs (n = 15) within 6.8%. The recoveries of the method for the five sulfonylureas from soil samples at spiking levels of 5.0, 20.0, and 100.0 ng g⁻¹ were 76.0–93.5%, respectively. The developed method has been successfully applied to the analysis of the target sulfonylurea herbicide residues in soil samples with a satisfactory result.