## 7 Sorbents Used in the TFME Technique
### Deep Eutectic Solvents  
Deep eutectic solvents (DES) are systems formed from a mixture of various types of cationic and anionic Lewis or Brensted acids and bases [118]. The ions in the mixture, as a result of hydrogen bonding, form a eutectic mixture. A eutectic mixture is characterized by a much lower melting point than the melting point of each of its components [111]. They are mainly obtained by mixing the quaternary ammonium salt with metal salts or a hydrogen bond proton donor (HBD). A hydrogen bond proton donor (HBD) has the ability to form a complex with the halide anion as a hydrogen bond acceptor (HBA) from the quaternary ammonium salt. Figure 21 shows quaternary ammonium salts and hydrogen bond proton donors (HBD) widely used in DES formation [111].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sorption Phase** & **Preparation** & **Analyte** & **Matrix** & **Detection** & **Reference** \\ \hline HVImBr/MMA & application to the Petri dish & sulfathiazole & milk, honey & SF & [114] \\ \hline p-PIL-AcO & dip coating & nonsteroidal & urine & LC-MS/MS. & [115] \\ \hline p-Poly-(MMA-IL) & dip coating & sulfonamides & water & HPLC-DAD & [116] \\ \hline poly-(MMA-BVImBr) & application to the Petri dish & aniline & water & LC-MS/MS & [117] \\ \hline \hline \end{tabular}
\end{table}
Table 11: Examples of the use of polymeric ionic liquids in TF-SPME.  
DES have similar properties to ionic liquids, but unlike them, they are less toxic, non-flammable, and biodegradable [119]. The physicochemical properties of deeply eutectic solvents depend both on the properties of each component of the eutectic mixture, and on the interaction between them. The values of the physical quantities, such as freezing point, viscosity, electrical conductivity, and pH value, can be modified by an appropriate combination of different quaternary ammonium salts (e.g., ChCl) and hydrogen bond donors (HBD) [111]. The density of deeply eutectic solvents ranges from 1.041 to 1.63 g:cm\({}^{-3}\). Its value is influenced by temperature (with an increase in temperature, the density of DES decreases linearly) and the molar ratio of the components of the eutectic mixture [120]. Most deep eutectic solvents have higher viscosity and surface tension at room temperature than conventional solvents. The extensive interconnection of DES-forming particles via hydrogen bonds, van der Waals forces, and electrostatic interactions are responsible for the high viscosity and surface tension values [120].  
DES are increasingly used in analytical chemistry, mainly as mobile phase modifiers in chromatography and extractants in various microextraction techniques to the liquid phase, and in recent years, also in microextractions to the solid phase [121]. A significant number of DES between room temperature and 70 \({}^{\circ}\)C occur in the liquid state [111]. However, there are solid eutectic systems at room temperature that can be used as sorption phases in microextraction to the solid phase.  
Table 12 summarizes examples of the use of DES in the TF-SPME technique, which has been described in scientific publications, to perform surface water analysis.