# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## Recent Applications of DESs in Environmental Analysis
### Analysis of Inorganic Compounds in Environmental Samples
Additionally, considering that the freezing point of the HDES was much lower than that of water, the UA-solidified aqueous phase (SAP)-LLME of 15 mL of water samples with 30 \(\upmu\)L of extraction solvent was carried out and the DES extract was 3-fold diluted prior to its injection into the HPLC-UV system for the determination of DES-based metal complexes. However, despite the good results obtained, freezing 15 mL of water is not a quick task to carry out. In addition, since it is an HDES, although it is lighter than water, its collection can be easier than the proposed SAP procedure. Although most applications of DESs for the analysis of environmental samples have been based on LLME procedures, a few works on extractions with sorbents based on DESs can be found in the literature. This is the case of the work of [PERSON] et al. [40] who synthesised a DES-based magnetic sorbent for the preconcentration of Pb(II) and Cd(II) from soil, river, tap, well and sea water and their determination by flame atomic absorption spectrometry (FAAS). Bare nanoparticles of Fe\({}_{3}\)O\({}_{4}\), Al\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\), and DESs of ChCl with oxal acid, urea and ethylene glycol were evaluated. The best results were obtained with the DES of ChCl:urea 1:2.5 and Fe\({}_{3}\)O\({}_{4}\) magnetic nanoparticles (m-NPs), as the most suitable sorbent due to the ease of separating the phases thanks to its magnetic properties. Interestingly, according to the authors, 200 \(\mu\)L of DES and 20 mg of m-NPs were added to 60 mL of sample solutions at pH 6.5 and stirred for 10 min. At this step, both processes took place: the interaction of DES with the metal ions and the adsorption of DESs in the m-NPs surface. After the separation of the sorbent with an external magnet, the analytes were re-extracted with 600 \(\mu\)L of 1M HNO\({}_{3}\). The recovery of 95-105% and LODs between 0.1 and 0.4 \(\mu\)g/L were obtained. Despite the good results obtained in this work and other similar applications, the combination of hydrophilic DESs with m-NPs for its use in aqueous samples is a controversial topic. According to [PERSON] et al. [40], the DES acts as a hydroxy functionalised IL and it is adsorbed on the surface of m-NPs, probably through strong hydrogen bonds and electrostatic interactions, losing its liquid state, and it allowed its separation from the aqueous solution. However, two major issues must be considered at this point. Firstly, the supposition that a hydrophilic DES-based sorbent can be formed in situ in an aqueous solution, more so when the instability DES based on ChCl has been amply demonstrated due to its decomposition in the sample. Secondly, and related to the first point, the authors stand up for the possibility that the binding forces of the sorbent are hydrogen bonds and electrostatic interactions that can easily be broken in the sample solution. In a similar work, [PERSON] et al. [42] used m-NPs combined with ChCl:acetic acid 1:3 for the extraction of plant growth regulators from edible oils. In this case, in which a fatty sample was evaluated, and hydrogen bonds can be formed between DESs and the m-NPs, the authors indicated that when an external magnet was used, the m-NPs moved with the magnet while DES dropped to the bottom of the tube. This suggests that interactions of this kind of DESs with the m-NPs are very weak and contradict the idea proposed by [PERSON] et al. [40]. Apart from that, in none of these works were two important aspects evaluated. Firstly, the extraction capacity of the unmodified sorbent versus the one modified with DESs, which would respond to the need of combining both materials. Secondly, a thorough characterisation of the sorbent before and after the extraction procedure. Although [PERSON] et al. [40] performed Fourier transform infrared spectroscopy analysis of bare and modified m-NPs after the extraction procedure, these tests are not conclusive to determine if both materials remain in the final phase.