Green Approaches for Preparation of Natural Deep Eutectic Solvents for Determination of As, Cd, and Pb in Plant Samples by ICP-MS

Natural deep eutectic solvents (NADESs) and amino acid-based deep eutectic solvents (AADESs) are considered green solvents due to their biodegradable characteristics, low toxicity, low cost, simple preparation and handling, negligible volatility, and, especially, modulable physicochemical properties (density, viscosity, and polarity). In this work, two green methods for preparation of NADES and AADES were developed and evaluated with characterization of the solvents produced. Eutectic solvents based on citric acid/β-alanine/water (Ala-CA), citric acid/xylitol/water (Xyl-CA), and citric acid/malic acid/water (MA-CA) were prepared either by stirring without heating or by using a rotary evaporator under reduced pressure. Comparison was made to the solvent obtained by the reference preparation method involving stirring with heating. Fourier transform infrared spectroscopy (FTIR), in attenuated total reflectance (ATR) mode, showed wavenumber shifts, such as that of the CO band from 1696 to 1710 cm^-1, indicating the occurrence of interactions for formation of the solvents. Determination of the physicochemical properties of the solvents revealed significant differences between the methods, with viscosity values from 5.99 ± 0.02 to 10.9 ± 0.01 mPa·s for the stirring method and from 18.9 ± 0.03 to 26.02 ± 0.02 mPa·s for the rotary evaporator method. The melting points obtained by differential scanning calorimetry (DSC) were 240-251 K (stirring method) and 223 K (rotary evaporator method). The deep eutectic natures of the solvents were confirmed by using phase diagrams and thermodynamic calculations to estimate the ideal eutectic points of the ternary mixtures. These solvents provided satisfactory results in microwave-assisted extraction of plant material, with recoveries of 87-106% for determination of Cd and Pb, using the NADES prepared by the stirring method, and for As and Pb, using the AADES prepared by the stirring and rotary evaporator methods. The results demonstrated the effectiveness of the proposed methods for the preparation of NADES/AADES, with the potential to modulate the physical properties, such as viscosity and melting point, of these water-based solvents. Evaluation using EcoScale resulted in 99 points for the stirring method and 98 points for the rotary evaporator method, representing high green scores for these methods.