A novel hydrophobic deep eutectic solvent-based paired-drop microextraction method for the trace determination of metribuzin in urine samples

Abstract A hydrophobic deep eutectic solvent (DES) was molecularly tailored from tributylphosphine oxide and octanoic acid via hydrogen-bonding and employed as an efficient extractant in a novel ultrasound-assisted paired-drop microextraction (UAE-PDME) method for metribuzin determination in urine. The DES structure was confirmed by FTIR analysis. The innovative paired-drop design addresses a key practical challenge: while ultrasound efficiently disperses the extractant, quantitatively retrieving a minute volume (e.g., 2 µL) of dispersed solvent from an aqueous sample is nearly impossible. This method solves this by using a second, hanging DES droplet that coalesces with the dispersed microdroplets during gentle stirring, enabling efficient, complete, and centrifugation-free collection of the enriched phase. After optimization of key parameters, the method was rigorously validated using spiked urine samples. It demonstrated excellent performance: a wide linear range of 5–500 µg/L, low limits of detection and quantification of 0.20 and 0.67 µg/L, respectively, high precision (RSD < 3.6%), quantitative recoveries (99.5–103.1%) with negligible matrix effect (− 4.2%) and high solvent phase recovery (98.5%). The high extraction efficiency is attributed to the DES’s rational molecular design. The DES-UAE-PDME method represents a green, rational, and effective sample preparation strategy for trace pesticide monitoring using conventional HPLC–UV instrumentation.