A guanidinium ionic liquid-based surfactant as an adequate solvent to separate and preconcentrate cadmium and copper in water using in situ dispersive liquid–liquid microextraction

Abstract

An in situ dispersive liquid–liquid microextraction (DLLME) method together with flame atomic absorption spectroscopy (FAAS) is developed for the extraction and determination of copper and cadmium from aqueous samples using as an extraction solvent a low cytotoxic ionic liquid (IL)-based surfactant, specifically decylguanidinium chloride (C₁₀Gu-Cl). The IL-based surfactant 1-hexadecyl-3-butylimidazolium bromide (C₁₆C₄Im-Br) is also used for comparative purposes. In the method, N,N′-bis(salicylidene)thiocarbohydrazide is the complexing agent, and the metathesis reaction in the in situ DLLME is accomplished using lithium bis[(trifluoromethyl)sulfonyl]imide. The main parameters affecting the extraction efficiency of the method, i.e. pH of the sample, NaCl content, and vortex time, are properly optimized with a Doehlert experimental design. The method using C₁₀Gu-Cl exhibited a limit of detection (LOD) four times lower and with slightly better reproducibility than that of the imidazolium IL-based surfactant. The validation of the in situ DLLME-FAAS method with C₁₀Gu-Cl is accomplished for the simultaneous extraction and quantification of copper and cadmium from water. The optimum method requires 10 mL of aqueous sample and ∼25 μL of the IL-based surfactant to obtain a microdroplet of ∼21 μL, which is diluted up to 120 μL with acetonitrile to perform the analytical determination of both metals. LODs down to 0.3 and 0.5 μg L⁻¹ were reached with the entire in situ IL-DLLME-FAAS for copper and cadmium, respectively. The inter-day relative standard deviation values were lower than 17% for both metal ions; with relative recoveries of 97.8% and 102%, and enrichment factors of 85 and 94, for copper and cadmium, respectively, at a low spiked level (4 and 6 μg L⁻¹, respectively). Furthermore, the developed method shows adequate precision and accuracy evaluated with a certified reference material, the absence of matrix effects, and high tolerance limits towards common cations (between 80 and 600) for the analysis of tap water, while accomplishing the requirements of green analytical chemistry.