## 7 Sorbents Used in the TFME Technique
### Polymeric Adsorbents  
In TF-SPME, polymers are used as sorptive phases in the form of self-supporting TFME membranes and as sorptive phase dispersion media or as a glue that attaches the solid sorptive phase to the support. A developed internal surface and the presence of various types of functional groups characterize polymeric adsorbents, which are often cross-linked copolymers. Non-specific dispersive interactions mainly occur between the macromolecules of the polymer and analyte molecules. Various types of specific interactions can also occur, which are related to the presence of functional groups in the polymer macromolecule. Specific interactions allow the formation of hydrogen or ionic bonds between analyte particles and polymer macromolecules. Polymers frequently used in the TF-SPME technique include PDMS, polyacrylamide(PAN), polystyrene (PS), and the natural polymer cellulose (Figure 16). Figure 16: Structural formulas: (**a**) polydimethylsiloxane, (**b**) polyacrylamide, (**c**) polystyrene, (**d**) cellulose. Figure 15: Types of materials used as extraction phases in the TFME technique. PDMS belongs to the group of first polymers used in the TFME technique. Polydimethylsiloxane is a polymer classified as siloxane. Its macromolecule contains Si-O-Si silicon bridges in its structure, in which each silicon atom is connected to methyl groups (Figure 16a). PDMS is commonly used as a sorbent in solid-phase microextraction techniques because of its properties, which include the following:  
1. High thermal resistance;
2. The ease with which it undergoes desorption;
3. Chemical and physiological inertness;
4. Flexibility and mechanical strength;
5. Low chemical reactivity. In addition, PDMS is resistant to matrix effects. Its sorption properties do not change even in highly complex matrices such as plasma, animal adipose tissues, or digestive juices [30]. Despite its many positive features, PDMS, as an extraction phase, also has its limitations. It shows low selectivity toward apolar compounds; thus, analyzing samples with complex matrices becomes difficult when used as an extraction phase. As an apolar material, it shows a low affinity to polar analytes. In order to increase the microextraction efficiency of polar analytes, combinations of polydimethylsiloxane with other sorbents, including divinylbenzene (PDMS/DVB) and carboxene (PDMS/CAR), are currently applied in TF-SPME techniques. TFME membranes, made of PDMS-coated carbon mesh and DVB/PDMS or CAR/PDMS, are also found in commercially produced TFME microextraction kits by Markes International, among others. Polyacrylonitrile (PAN) is a synthetic resin formed by the polymerization of acrylonitrile (Figure 16b). It belongs to the hard and rigid thermoplastic materials. Due to the presence of highly polar nitrile (-CN) groups in the macromolecule, it does not dissolve in typical organic solvents. In the TF-SPME technique, it mainly performs the function either of a dispersion medium, or of a linker of the sorption phase to the substrate. Polystyrene (PS) is a polymer of the thermoplastic group (with limited elasticity) obtained by polymerization of styrene (Figure 16c). It is soluble in aromatic and chlorinated hydrocarbons and in ketones. It does not dissolve in aliphatic hydrocarbons, alcohols, and water. In microextraction to the solid phase, it is used as both a sorption phase and a dispersant. Cellulose is a natural polymer commonly found in nature. It is a component of plant cell walls. It belongs to natural raw materials extracted mainly for industrial purposes from wood and cotton. Cellulose products are environmentally safe as they decompose through a natural carbon cycle [32]. The polymer chain of cellulose has a simple, unbranched structure consisting of glucose rings linked by \(\beta\)-1,4-glycosidic bonds (Figure 16d). In view of the absence of hydrocarbon side chains, the cellulose chains form ordered and fibrous semi-crystalline structures. The -OH groups, bonded to carbon atoms, determine the chemical and physical properties of cellulose.