Simultaneous Determination of Trace Amounts of Cobalt and Nickel in Water and Food Samples Using a Combination of Partial Least Squares Method and Dispersive Liquid–Liquid Microextraction Based on Ionic Liquid

Abstract

A new, sensitive, and simple combined method including ionic liquid (IL)-based dispersive liquid–liquid microextraction and partial least squares method (PLS) was developed for simultaneous preconcentration and determination of cobalt and nickel in water and food samples. In this work, a small amount of an IL 1-hexyl-3-methylimmidazolium bis (trifluormethylsulfonyl)imid ([Hmim][Tf₂N]) as an extraction solvent was dissolved in ethanol as a disperser solvent and then the binary solution was rapidly injected by a syringe into the water sample containing Co²⁺ and Ni²⁺, which were complexed by 1-(2-pyridylazo)-2-naphthol. After preconcentration, the absorbance of the extracted ions was measured in the wavelength range of 200–700 nm. The partial least squares method was then applied for simultaneous determination of each individual ion. The parameters controlling the behavior of the system were investigated and optimum conditions were selected. Eleven binary mixtures of cobalt and nickel were selected as the calibration set. The calibration models were validated with four synthetic mixtures containing the metal ions in different proportions, which were randomly designed. The best calibration model was obtained by using PLS-1 regression. Calibration graphs were linear in the range of 2.0–20.0 and 2.0–15.0 ng mL⁻¹ with a limit of detection of 0.65 and 0.32 ng mL⁻¹ for cobalt and nickel, respectively. The root mean square errors of prediction for cobalt and nickel were 0.4032 and 0.2980, respectively. Satisfactory results were reported for simultaneous determination of trace levels of cobalt and nickel in water, food, and geological certified reference material samples.