# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## Recent Applications of DESs in Environmental Analysis
### Analysis of Organic Compounds in Environmental Samples
Apart from liquid-phase extraction methods, DESs have also been explored in sorbent-based extraction techniques, although to a significantly lesser extent. In this sense, [PERSON] et al. [24] prepared a ferrofluid based on a nanocomposite of montmorillonite clay@Fe3O4 modified with a menthol-based DES for the m-u-dSPE of eleven explosives (nitrogen compounds) from water and soil samples. In order to successfully act as a carrier, the DES should meet some requirements, such as establishing interactions with the nano-material to form a stable ferrofluid, establishing interactions with the analytes to improve the extraction performance, showing good stability in water, among others. In this regard, among the seventeen carboxylic acids evaluated as HBDs, only those formed by octanoic, decanoic, dodecanoic and oleic acids were chemically stable in water during the extraction process, with the combination of menthol:decanoic acid (1:2) being the one that provided the highest extraction efficiency for all target analytes. In addition, the authors investigated the effect of the nanocomposite, DES and ferrofluid on the extraction efficiency, separately, finding that the nanocomposite could be used for the extraction of the target analytes, but irreversible adsorptions could occur for some of them. While using the ferrofluid, this effect did not take place, and additionally, a higher extraction efficiency was obtained. Under the optimised extraction conditions, low LODs between 0.22 and 0.91 ug/L, as well as recovery values in the range of 88-104%, were achieved using only 50 uL of the ferrofluid. In this way, the limitation of DESs to be applied only in solvent-based extraction techniques is overcome, and simultaneously, green sorbent-based extraction methods are developed. In summary, DESs have been widely exploited for the extraction of organic compounds from environmental matrices, especially, different types of waters. In this sense, DESs composed of natural components such as terpen and fatty acids stand out, although DESs prepared from synthetic quaternary ammonium or phosphonium salts have also been explored, and both have led to the development of green analytical methods [12; 15]. These solvents have mainly been applied in solvent-based extraction methods, in which the phase separation after the extraction process was promoted by different types of forces, both chemical and physical, either by the use of organic solvents or DESs as dispersing agents, as well as by pH changes in the extraction medium, and different agitation methods, among others. Likewise, quasi-HDESs have also been used in this field, the hydrophobic part being mainly responsible for the extraction of the target analytes, while the water-soluble component acts simultaneously as salting-out and disperser agent favouring the extraction process. In addition to liquid-phase extraction methods, the use of DESs in sorbent-basedextraction techniques has also been investigated thanks to their combinations with other nanomaterials to form ferrofluids. With respect to the compatibility of DESs with the separation and determination techniques, their combinations with liquid chromatography coupled to conventional detectors and to a lesser extent to mass spectrometry (MS) detectors stand out above all others. In some cases, the final extracts are diluted with organic solvents or a mobile phase prior to injection into the chromatographic systems in order to avoid possible DES residues that could cause damage to the needle, column or detector, as well as to reduce the irreproducibility problems due to the high viscosity of DESs and improve the shapes of the peaks obtained.