# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## Recent Applications of DESs in Environmental Analysis
### Analysis of Organic Compounds in Environmental Samples
Although the addition of a third component significantly improved the extraction performance, as well as allowed to control density, the melting point and viscosity of the solvent met the requirements of the SFOD technique. Among the evaluated TDESs, the combination of nonanoic, decanoic and dodecanoic acids, at a molar ratio of 1:1:1, was selected as the most appropriate extraction solvent since it provided a better chromatographic separation and peak shapes of target analytes. The phase separation was assisted by air cycles of pulling in/pushing out, avoiding the use of additional laboratory devices. The floating organic droplet obtained was frozen, then retrieved with a spatula, and 2-fold diluted with acetonitrile (ACN) prior to HPLC-photodiode array detector (PDA) analysis. The optimised methodology achieved low LODs between 0.96 and 2.30 \(\upmu\)g/L and recovery values in the range of 90-104% for river and tap water samples. Finally, the green profile of the method was assessed by analytical eco-scale, obtaining an 89-point score that proved its eco-friendly behaviour. As can be seen in Figure 1, Raynie and GAPI pictograms also confirmed these results. However, other studies in which the phase separation has only been favoured by mechanical assistance (i.e., VA, SA, UA and air-assisted (AA) methods) can also be found in the literature [11; 15; 17; 18; 19; 21]. In these cases, the analytical procedures are simpler, faster and cheaper since the use of additional solvents or solutions is avoided. In addition to HDESs, quasi-HDESs have also been used as extraction solvents of organic contaminants from environmental samples. As an example, [PERSON] et al. [15] prepared a DESs from tetrabutylammonium bromide (TBAB) and various fatty acids  
Figure 1: Greenness assessment of the developed and other reported DLLME methods for the determination of EDCs by Raynie (**A**) and GAPI (**B**) pictograms. Reprinted from [PERSON]; [PERSON] A Green Air Assisted-Dispersive Liquid-Liquid Microextraction Based on Solidification of a Novel Low Viscous Ternary Deep Eutectic Solvent for the Enrichment of Endocrine Disrupting Compounds from Water. J. Chromatogr. A, 2020, 1629, 461498 with permission from Elsevier [16]. (i.e., heptanoic, decanoic and 4-phenylbutyric acids) for the removal of five organophosphorus pesticides (OPPs) and two dyes from wastewater and water and soil destined to agricultural activities by the VA-LLME technique. In this case, the centrifugation step for phase separation was avoided by embedding the synthesised DESs into a sponge. After the removal process, the residual OPPs and dyes were determined by HPLC and UV-visible spectrophotometry analysis, respectively. In this way, the authors found that TBAB:decanoic acid at an optimised molar ratio of 1:3 gave rise to the highest removal efficiency (70-95%) of the pollutants. However, the main drawback of this work was the lack of characterisation of the DES phase after the removal step, since as previously demonstrated by [PERSON] et al. [26], the hydrogen bonds responsible for the formation of tetrabutylammonium chloride (TBAC):decanoic acid are partially broken in aqueous solution and the hydrophilic compound is solubilised into the water phase according to its water solubility. Therefore, considering that the counter anion of the quaternary ammonium salt has no influence on the DESs, the removal process that occurs in this study could be mainly attributed to the hydrophobic interactions between pollutants and one of the components of quasi-HDESs, decanoic acid, since TBAB could be suspected to leak into the aqueous phase. However, it could facilitate the dispersion of the fatty acid in the aqueous phase, as well as the mass transfer of the target analytes to the extraction phase by the salting-out effect. The sponge for its part also promoted the removal by the adsorption of pollutants into its pores. The proposed method achieved a point-score of 80 and 92 by the analytical eco-scale for OPPs and dyes, respectively, which indicated its green nature.