Natural Thymol–tert-Butylhydroquinone–Based Deep Eutectic Solvent/Vortex-Assisted Dispersive Liquid–Liquid Microextraction for Organophosphorus Pesticide Extraction inWater Samples

Abstract

Monitoring trace levels of pesticide residues in water remains a major global analytical challenge. We synthesized a bio-derived hydrophobic deep eutectic solvent (DES), [thymol:tert-butylhydroquinone] (Thy:TBHQ), and coupled it with vortex-assisted dispersive liquid–liquid microextraction (DLLME) prior to gas chromatography (GC) with micro electron capture detection (μECD) for pre-concentration of organophosphorus pesticides (OPPs) (phosphamidon, diazinon, chlorpyrifos).DES formation and stability were confirmed by Fourier transform infrared spectroscopy (O–H red shift) and 1H-NMR (deshielded phenolic signals). Univariate optimization identified acetonitrile (ACN) as a disperser and ACN/DES = 1:1 (v/v), 100 mL sample, 3 min extraction, and pH 2–6 as optimal. The method showed excellent linearity (coefficients of determination = 0.991–0.997), limits of detection (LODs) of 3.44–15.43 ng L−1 and limits of quantification (LOQs) of 10.32–51.38 ng L−1, all well below the EU 100 ng L−1 perpesticide limit, with enrichment factors of 13–27. Precision supported routine application (intra-day relative standard deviation [RSD] 1.67%–3.79%; inter-day 5.25%–9.66%). Spiked real samples across environmental waters: tap (≈90%–100% relative recovery), river (≈60%–82%), and seawater (≈54%–66%), each with RSD < 10% and no background residues detected. The DES retained >85% of its extraction performance over ≥9 adsorption–desorption cycles. Satisfactory extraction efficiency is probably due to the synergistic π–π, hydrogen bonding, van der Waals, and hydrophobic interactions between aromatic OPPs and the DES. Overall, the [Thy:TBHQ]-based vortex-assisted DLLME/GC with μECD platform delivers sensitive, precise, and reusable trace analysis with reduced solvent use, offering a green alternative for monitoring pesticides in diverse water matrices.