# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## Recent Applications of DESs in Environmental Analysis
### Analysis of Organic Compounds in Environmental Samples
The best extraction efficiency for the target analytes was obtained with ChClethylene glycol and thymoloctanoic acid as disperser and extraction solvents, respectively. In addition, the authors found that doping the extraction medium with beta-cyclodextrin(\(\beta\)-CD) gave rise to a higher and more selective extraction performance. These results were explained based on the structure of the \(\beta\)-CD, since its hydrophilic surface allowed its solvation in the hydrophilic DES, whereas its hydrophobic cavity broke the HDESs into finer droplets, facilitating their dispersion in the aqueous sample, as well as the mass transfer of target analytes to the extraction medium. Likewise, additional sorption sites for tetracyclines were provided. Under the optimised extraction conditions, the limits of detection (LODs) between 1.37 and 4.38 \(\mathrm{\SIUnitSymbolMicro g}\)/L and good recovery values in the range of 74-113% were obtained for well, rainforest, coastal sea, gardening and mineral water samples. Finally, an evaluation of the greenness of the methodology was carried out. Both the Green Analytical Procedure Index (GAPI) pictograms and the 76-point score obtained by the analytical eco-scale showed the good greenness of the analysis method, mainly because the proposed method completely eliminated the use of volatile and toxic solvents. Following this trend of avoiding the use of conventional organic solvents to favour phase separation after the extraction process, [PERSON] and [PERSON] [13] developed an LLME procedure influenced by pH changes for the analysis of two antibiotics, levofloxacin and ciprofloxacin, in feed, tap and wastewater samples. The extraction protocol consisted of a homogenous solution from which the target analytes migrated to a water-immiscible phase thanks to the phase separation induced by adjusting the solution pH. For this purpose, the phase transition behaviour was studied on DESs prepared from thymol and medium-chain fatty acids including hexanoic, octanoic, decanoic and dodecanoic acids, changing the pH medium with the addition of base and acid solutions. The results showed that thymol:hexanoic acid at a molar of 2:1 (100 \(\mathrm{\SIUnitSymbolMicro\mu L}\)) was the most suitable extraction solvent for the target analytes, due to the stronger hydrogen bond between the hydroxyl group (OH) of the fatty acid and thymol. Among the different affecting parameters investigated, the concentration levels of base and acid solutions played an important role in promoting the formation of a homogeneous mixture and breaking it down to separate the DES phase containing the target analytes from the aqueous sample, respectively. In this way, 100 \(\mathrm{\SIUnitSymbolMicro L}\) of a 6 M potassium hydroxide (KOH) solution and 150 \(\mathrm{\SIUnitSymbolMicro L}\) of a 6 M hydrochloric acid (HCI) solution were necessary to achieve a good extraction performance with recovery values in the range of 95-111%. After the extraction procedure, the DES droplet was collected and 10-fold diluted with methanol prior to the high performance liquid chromatography (HPLC)-UV determination. Another strategy to favour phase separation has been the solidification of the floating organic droplet containing the analytes. This is the case of the work carried out by [PERSON] and [PERSON] [16] for the extraction of five endocrine disrupting compounds (EDCs) (i.e., plastic migrants, hormones and UV-filters) from environmental waters. For selecting the best extraction solvent, binary and ternary combinations of dodecanoic acid with other fatty acids (octanoic, nonanoic and decanoic acids) were studied at different molar ratios. The authors found that the length of the fatty acid chain significantly affected the extraction efficiency of the compounds of interest, since better results were obtained using binary DESs based on nonanoic acid compared with those based on octanoic acid due to their higher affinity to the target analytes through hydrophobic interactions.