Development of a natural dispersive liquid-liquid micro-back-extraction technology based on hydrophobic deep eutectic solvent for the enrichment and determination of heavy metals in soy sauce.

To solve the issues of low extraction efficiency and poor stability arising from the direct injection of organic solvents in traditional dispersive liquid-liquid microextraction, this study established a novel dispersive liquid-liquid micro-back-extraction method for the determination of Pb(II) and Cd(II) in high-salt food matrices. This method effectively mitigated the interference caused by the sodium chloride matrix in high-salt food samples, enabling efficient enrichment and trace determination of Pb(II) and Cd(II). A natural deep eutectic solvent synthesized with L-menthol and dodecanoic acid at a molar ratio of 2 : 1 was adopted as a green extraction solvent to simultaneously realize the enrichment of Pb(II) and Cd(II) and to mitigat sodium chloride induced matrix interference. Acidic hydrogen peroxide was innovatively used to break the coordination bonds of metal complexes, facilitating the dissociation of target ions from the organic phase and their back-extraction into the aqueous phase for accurate and stable determination of Pb(II) and Cd(II). The sensitivity and accuracy of the proposed method were greatly enhanced by establishing a synergistic system integrating two-step dispersive liquid-liquid microextraction and back-extraction. Key parameters, including the dosages of extraction solvent, complexing agent, disperser solvent, and hydrogen peroxide, as well as ultrasonication time, were systematically investigated. Under optimal conditions, the proposed method achieved satisfactory linearity from 0.15 to 50 ng mL-1, with detection limits, relative standard deviations, and enrichment factors ranging within 0.012-0.021 ng mL-1, 3.06-4.57%, and 46.7-48.6, respectively. With its excellent sensitivity and selectivity, the optimized method is applicable for the determination of heavy metals in high-salt food matrices, and it has been successfully utilized for the detection of real soy sauce samples.