## 7 Sorbents Used in the TFME Technique
### Ionic Liquids and Poly(Ionic Liquids)  
Ionic liquids (ILs) are salts containing organic cations and organic, or inorganic anions with a melting point equal to or below 100 \({}^{\circ}\)C. The cation of an ionic liquid is usually a branched organic structure with low symmetry. The cations forming ionic liquids include imidazolium (IM), pyrrolidine (Pyrr), pyridine, tetraalkylammonium, or tetalkylphosphonium. The anions included in the ionic liquids are the halide anions Cl\({}^{-}\), Br\({}^{-}\) I\({}^{-}\), but also triflate ([TfO\({}^{-}\)]), bis(trifluoromethylsulfonyl) amide ([TFSI\({}^{-}\)]), tetrafluoroborate ([BF\({}_{4}\)\({}^{-}\)]), or hexafluorophosphate ([PF\({}_{6}\)\({}^{-}\)]) [109]. Figure 21 show the molecular structures and nomenclature of various cations and anions commonly found in synthesizing second- and third-generation ionic liquids [110]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Sorption Phase** & **Preparation** & **Analytic** & **Matrix** & **Detection** & **Reference** \\ \hline aptamer/cellulose & dip coating & codeine acetaminophen & urine, vegetable juice water & ESI-IMS & [106] \\ \hline aptamer/CDs/cellulose & dip coating & methamphetamine & urine, plasma saliva & ESI-IMS & [107] \\ \hline aptamer/cellulose & dip coating & codeine & urine & ESI-IMS & [108] \\ \hline \end{tabular}
\end{table} 
Table 10: Examples of the use of aptamers in TF-SPME. Figure 21: Molecular structures of substrates found in synthesizing ILs and DES [110; 111]. Ionic liquids are among chemical compounds with unique physicochemical properties, such as high thermal stability, near-zero vapor pressure, high viscosity, non-flammability, and the ability to dissolve a wide range of organic and inorganic chemicals. Physical properties such as miscibility and thermal stability depend primarily on the anion. In contrast, other properties, such as viscosity and surface tension, are controlled by the length of the alkyl chain on the cation and the shape and symmetry of the cation [110]. The negligible vapor pressure of ionic liquids, their good thermal stability, and controllable viscosity, as well as their miscibility with water and organic solvents, are the main properties that make ionic liquids applicable in microextraction to the solid phase. The high viscosity of ILs facilitates their application to the substrate and positively influences the homogeneity of the sorption film. Simultaneously, their liquid state makes it easier to conduct the extraction process due to the higher diffusion rates of analytes compared to solid phases. The ability to design ILs by matching the type of cation and anion to the target analyte is a factor that determines high selectivity and sensitivity to determinations carried out using solid-phase microextraction [112]. Ionic liquids (ILs) are used in SPME microextraction, while poly(ionic liquids) PILs--in TF-SPME techniques. Poly(ionic liquids) are a subgroup of polyelectrolytes that can be obtained by polymerizing ionic liquid monomers or post-polymerization processes [109]. The polymer chains of PILs repeat ionic units: cationic, anionic, or zwitterionic. In the cationic structure, groupings endowed with a positive electrical charge are covalently attached to the polymer backbone. Typical cationic PILs are based on imidazole, pyridine, pyrrolidine, and quaternary ammonium cations. Groupings with a negative electrical charge, such as sulfonates, carboxylates, and phosphates, covalently link to the polymer chain in anionic PILs. In the case of zwitterionic PILs, both cations and anions are covalently attached to the polymer chains [113]. The physicochemical characteristics of polymeric ionic liquids, such as high thermal, chemical, and mechanical stability and excellent plasticity, make them applicable in various fields of science and technology, including microextraction techniques.