A validated analytical method to measure metals dissolved in deep eutectic solvents

This work presents the first validated method to analyze metals dissolved in deep eutectic solvents (DES) on a microwave plasma atomic emission spectrometer (MP-AES), which is key to the success of the upcoming field of solvometallurgical processing. The method was developed and validated for eleven metals: alkali metals: lithium (Li); alkaline earth metals: magnesium (Mg); transition metals: iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), palladium (Pd); and post-transition metals: aluminum (Al), tin (Sn), and lead (Pb) in choline chloride based DES. The proposed method was validated with respect to linearity, limit of detection (LOD), limit of quantification (LOQ), accuracy, precision, and selectivity. Our method's selectivity was evaluated for three DES matrices: (1) choline chloride: ethylene glycol, (2) choline chloride: levulinic acid, and (3) choline chloride: ethylene glycol in the presence of iodine, which is an oxidant often used in solvometallurgy. In all three matrices, the linearity range was plotted with at least 5 levels of standard solutions. All the parameters satisfied the acceptability criteria suggested by international organizations, such as the International Council for Harmonization, AOAC International, and the International Union of Pure and Applied Chemistry. Specifically, the calculated LOD and LOQ are comparable with aqueous matrices on MP-AES and with other analytical methods. The metal with the lowest reported LOD (0.003 ppm) and LOQ (0.008 ppm) was Cu, while the highest LOD and LOQ were obtained for Mg at 0.07 and 0.22 ppm, respectively. The recovery and precision for the three DES matrices were acceptable, i.e., between 95.67–108.40% and less than 10%, respectively. Finally, to compare the proposed method with the standard analytical method used to measure metals dissolved in aqueous solutions, we used 2 ppm standard solutions in DES and found that the accuracy was unacceptable without using the proposed method. Therefore, it is evident that our method will be pivotal in the field of solvometallurgy, as it will allow accurate and precise detection and quantification of metals dissolved in DES and eliminate quantification errors, which were estimated in excess of 140% without using the method developed and proper DES matrix-matched calibrations.