## 7 Sorbents Used in the TFME Technique
### Metal-Organic Frameworks  
The term metal-organic frameworks (MOFs) is used to describe solid polymeric materials containing metal cations or secondary building units (SBUs) in their structure.  
Figure 19: MIP-TFME preparation [58].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Template** &
\begin{tabular}{c} **Functional** \\ **Monomer** \\ \end{tabular} & **Crosslinking Reagent** & **Analytic** & **Matrix** & **Detection** & **Ref.** \\ \hline
2-thiophenacarboxylateable & 1-vinylimidazole & bisphenol dimethacrylate & PASHs & seawater & GC-MS & [59] \\ \hline
2-[diethoxy(sulfanylidene)-\(\lambda\)-phosphanyl]aminoacetic acid & \begin{tabular}{c} methacrylic acid \\ \end{tabular} & \begin{tabular}{c} ethylene glycol \\ dimethacrylate \\ \end{tabular} & OPPs & water & LC-MS/MS & [60] \\ \hline catechol & 4-vinyl benzoic acid & \begin{tabular}{c} ethylene glycol \\ dimethacrylate \\ \end{tabular} & phenols & water & UHPLC-PDA & [61] \\ \hline (3-(10,11-dihydro-5H-dibenzo[b,fazepin-5-yl]propyl)) carbamate & \begin{tabular}{c} methacrylic acid \\ \end{tabular} & \begin{tabular}{c} ethylene glycol \\ dimethacrylate \\ \end{tabular} & TCAs & plasma & LC-MS/MS & [62] \\ \hline phenol & \
\begin{tabular}{c} itaconic acid, \\ 4-vinylpyridine, \\ styrene \\ \end{tabular} &
\begin{tabular}{c} ethylene glycol, dimethacrylate, \\ triethylene glycol \\ dimethacrylate, \\ pentaerythtitol triacrylate \\ \end{tabular} & phenols & water & LC-MS & [63] \\ \hline mycophenolate mofetil & 4-vinylpyridine & ethylene glycol dimethacrylate & MPA & plasma & UPLC & [64] \\ \hline \hline \end{tabular}
\end{table}
Table 5: Examples of the use of MIPs in TF-SPME.  
SBUs are metallic clusters in which coordination bonds link the metal ion to multifunctional organic groups (linkers) such as carboxylates [65]. The most common transition metal ions used in MOF syntheses are, for example: Cr\({}^{3+}\), Fe\({}^{3+}\), Co\({}^{2+}\), Zn\({}^{2+}\), and Cu\({}^{2+}\) acting as electron pair acceptors, which are provided by ligands containing atoms with free electron pairs, oxygen, nitrogen, or sulfur atoms. The function of ligands in MOF structures can be performed, inter alia, by polycarboxylic acids, for example, terephthalalic, 1,3,5-benzetricarboxylic [66].  
MOFs are porous materials with the largest specific surface areas. MOF-5 is the first synthesized metal-organic skeleton-type structure, reported in _\"Nature\"_ in 1999. Its skeleton is formed by zinc [Zn\({}_{4}\)O]\({}^{6+}\) clusters and tetraphthalic acid anions. It has a specific surface area (Langmuir) of 2900 m\({}^{2}\)/g [67].  
Considering vast specific surface areas, MOFs are characterized by good thermal and mechanical stability, homogeneous nanoscale cavity structure, uniform pore topology, ultra-low density, and high affinity to adsorption of various analytes [68]. The characteristics of MOF-type polymeric materials make them suitable for a variety of applications in science and technology (including catalysis, gas adsorption, and storage). Extraction techniques also apply to them as drug carriers and sorbents [69].  
The adsorption of analytes on the surface of the MOF, used as a sorption phase, occurs through various types of interactions. Ligands in the crystal structure interact hydrophobically with analytes, while aromatic rings are present in linkers through \(\pi\)-\(\pi\) interactions. Ligands, containing oxygen and nitrogen polar functional groups, bind analytes through hydrogen bonds, ionic interactions, and dipole-dipole interactions. Metals at nodes of the crystal lattice, which have free coordination sites, enable acid-base Lewis interactions with molecules of the corresponding analyte [31].  
Examples of the application of MOFs in the TF-SPME technique, described in scientific publications, are summarized in Table 6.