# Recent Applications of Deep Eutectic Solvents in Environmental Analysis
## 1 Introduction  
The extensive use of chemicals in the industry and daily life has led to the concerning occurrence of hazardous compounds in the environment which brings about considerable contamination in ecosystems and has a negative effect on the population's health. In this sense, it is of utmost importance that the development of methodologies enables the reliable remediation and determination of these kinds of substances, especially at the low levels at which they can be harmful for humans [1]. The combinations of efficient sample preparation methods together with the most sensitive and robust analytical techniques are the gold tools to reach suitable determinations in this field, particularly due to the complexity of environmental samples, which usually present a great number of interferences and other components that can hinder the analysis. However, nowadays, sensitivity, selectivity, and robustness are not enough criteria to define the suitability of an analytical methodology. As part of the current trend to develop more sustainable systems that comply with the goals of green analytical chemistry, the search for environmentally friendly strategies is fundamental to demonstrate the viability of the method and its adequate application. In this sense, it is essential that the development of procedures does not endanger the environment by the generation ofadditional waste, and at the same time, allows to decrease the operator risk during the method performance--thus saving time, cost, and energy [2; 3]. Diverse strategies have been tested and successfully applied in the different areas of analytical chemistry to reach these objectives. Among them, the application of direct analyses, the development of miniaturised and automated techniques and the use of novel green materials have been the main lines of research [4]. Great effort has been made by the scientific community to seek new materials such as nanomaterials or alternatives to the use of toxic conventional organic solvents. In this context, in recent years, the application of deep eutectic solvents (DESs) has gained relevance with respect to other previously used solvents such as the generally recognised as safe (GRAS) solvents, supramolecular solvents (SUPRAS), switchable solvents or even ionic liquids (ILs) [5], due to the unique and advantageous properties of these new materials [6]. DESs have been described as mixtures of at least two substances, a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD) interacting by hydrogen bonds, with a freezing point much lower than that of either of each component. However, it is known that other types of interactions such as van der Waals forces or electrostatic interactions could also be established and not all eutectic mixtures present a considerable decrease in the freezing point, which has produced considerable controversy in a large number of works published in recent years, in which diverse mixtures have been used without the clear characterisation and evaluation of the materials used [4; 7; 8]. For this reason, a stricter definition has been proposed to describe them, namely eutectic mixtures that are liquid at room temperature and whose eutectic point is below that of an ideal liquid mixture [8]. Within the scope of this definition, diverse classifications of DESs have been proposed, as can be seen in different recently published review articles [7; 8; 9]. Among them, DESs can be differenced into hydrophilic DESs, when they are soluble in water, hydrophobic DESs (HDESs), exclusively constituted by hydrophobic components, and quasi-HDESs, when at least one component is soluble in water. These two last groups have gained great attention in the field of analytical chemistry since their poor solubility in water simplifies their separation from aqueous samples in analytical applications [7]. Such an aspect is especially important in aqueous environmental matrices, for which those have been used, particularly for the evaluation of organic compounds.