# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## 3 Conclusions  
The introduction of DESs in analytical chemistry to replace traditional toxic and volatile organic solvents is, without any doubt, a practice that is growing and consolidating due to their remarkable benefits, such as their easy and inexpensive preparation, low volatility and toxicity, biodegradable and biocompatibility. These solvents exhibit a high tunability of their physicochemical properties (melting point, viscosity, conductivity, vapour pressure, density, miscibility and polarity) mainly by changing the starting components and their respective molar ratios, as well as a high solvation capacity, which together make them suitable as extraction solvents of both organic compounds (pharmaceuticals, pesticides, dyes, UV-filters, etc.) and inorganic species (metal ions, metal oxides, metal salts, ammonia, etc.) from different environmental matrices. In this sense, DESs and NaDESs have been applied in several liquid-phase extraction methods, especially in miniaturised approaches, which require low volumes of solvents to obtain satisfactory extraction efficiencies. Due to the viscosity of DESs, it has sometimes been necessary to apply different assistance methods (ultrasound, vortex, air cycles, etc.) in order to disperse them through the sample solution, and consequently, enhance the mass transfer of the target analytes into the extraction phase. Apart from that, sorbent-based extraction techniques using DESs combined with other nanomaterials as extractant agents can also be found in the field  
Figure 2: Maximum solubility as the mass fraction \(\omega_{\text{max}}\) (wt%) of metal compounds in seven different DESs: (**a**) oxides; (**b**) chlorides; (**c**) acetates; and (**d**) phosphates. The mass fractions represent an average of three independent measurements. Reprinted from [PERSON]; [PERSON]; [PERSON] Deep Eutectic Solvents as Extraction Media for Metal Salts and Oxides Exemplarily Shown for Phosphates from Licinated Sewage Sludge Ash. Green Chem., 2019, 21 (2), 321–328 with permission from Royal Society of Chemistry [32]. of environmental analysis. Likewise, other applications of DESs such as gas absorption, extraction of bioactive compounds, revalorisation of industrial wastes, as well as the recovery of value-added substances, have also been reported in the literature. In order to overcome the limitations of extracting a group of compounds of interest from aqueous solutions, HDESs have been widely employed for the analysis of environmental water samples, although quasi-HDESs have also been developed for this purpose, fundamentally for the extraction of organic compounds. Likewise, hydrophilic DESs have also been used for the extraction of organic compounds and metal species from non-aqueous media or aqueous samples with the use of additional steps. Regarding the combination of DESs with analytical instruments, chromatographic techniques and spectrophotometric methods have been the most frequently used. However, to the best of our knowledge, these solvents have not been applied to modify chromatographic techniques in the analysis of environmental matrices. Although the use of DESs has not only given rise to highly selective and sensitive sample preparation steps but also easier analytical performance, further studies should be addressed in terms of the characterisation, greenness and reuse of these solvents. Conceptualisation, B.S.-R. and R.R.-R.; resources, R.R.-R. and A.S.-M.; writing--original draft preparation, B.S.-R., R.R.-R., A.S.-M.; writing--review and editing, B.S.-R., R.R.-R., A.S.-M.; supervision, B.S.-R. and M.A.R.-D.; project administration, M.A.R.-D.; funding acquisition, M.A.R.-D. All authors have read and agreed to the published version of the manuscript. This work has been funded by the Spanish Ministry of Economy, Industry and Competitiveness (project AGL2017-89257-P). Not applicable.