## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Solid-Phase Microextraction (SPME)
In the SPME technique, the shape and dimensions of the fiber itself force researchers to develop a method of permanent connection of the MOF with the surface of the fiber. An example is the combination of MOF-199 with graphene oxide used in the study presented in Figure 8[56]. The authors used 3-amino-propyl-triethoxysilane (APTES) as a cross-linking agent, which increased the durability and allowed the fiber to be reused more than 140 times. Composite materials created from MOFs and GO showed an increased affinity for organochlorine pesticides, resulting from the combination of the high specific surface area of the MOFs and the unique nature of graphene oxide. The developed analytical method was characterized by LOD atthe level of 2.3 to 6.9 ng/L, and the relative standard deviation for five extractions carried out using the same fiber ranged from 5.3 to 8.8%. Recoveries for the analytes studied ranged from 72.2 to 107.7%. The combination of graphene oxide with MOF, more specifically with MIL-88(Fe), was also used in another study [55]. The authors managed to connect the MIL-88(Fe)@GO composite with the steel fiber SPME using a covalent bond. The steel SPME fiber was first coated with a layer of silver, then coated with a layer of silica using mercaptopropyltriethoxysilane (MPTES) and APTES. Then, the fiber prepared in this way was immersed in MIL-88(Fe)/GO and heated to 100 \({}^{\circ}\)C for 1 hour. The fiber obtained in this way has been successfully used to determine phthalamic acid esters (PAE) from various vegetable oil samples. The developed analytical method was characterized by LOD at the level of 0.5-2.0 ng/g, repeatability in the range of 4.0 to 11.4%, and analytes were determined with recovery at the level of 83.1 to 104.1%. Another approach to creating composite materials containing MOF for use in SPME is presented in an interesting article published by [PERSON] et al. [60]. In this study, the authors determined the content of fluoroquinolones in water and honey samples using a metal-organic composite in the form of a monolith for microextraction in a solid-phase tube (IT-SPME). For this purpose, 4-vinylbenzoic acid was copolymerized with ethyl methacrylate in a molten silica capillary to form a porous monolith. Subsequently, the in-situ ZIF-8 pore surface of the monolith was synthesized (see Figure 9). The introduction of ZIF-8 increased the composite surface area. The developed analytical method allowed for the detection of compounds with LOD at the level of 0.14 to 1.1 ng/L. Relative standard deviations for all the samples were less than 10%, with recovery ranging from 80.1 to 119%. UiO-66-OH has also found its application in SPME. The authors used this MOF to determine polybrominated diphenyl ethers (PBDEs) from milk [58]. In this case, it was not decided to produce a composite material but to combine UiO-66-OH with the surface of the steel SPME fiber using silicone glue. Under optimized conditions, the developed method was characterized by excellent linearity R\({}^{2}\) = 0.9994, LOD in the range of 0.15 to 0.35 ng/L, and recovery at the level of 74.7-118.0%. The developed method is reliable and effective for determining PBDE in milk at very low concentration levels. The UiO-66 was  
Figure 8: Schematic representation of MOF-199/GO composite synthesis for SPME extraction using APTES as a linking agent, presented by [PERSON] et al. [56]. also used in another study (Figure 10) [52]. In this case, it was used in the SPME-Arrow technique, a variant of SPME characterized by a different shape of sorption fiber. MOF was used to form a composite and combined with molybdenum disulfide MoS. This technique, combined with gas chromatography, allowed for the determination of the content of sixteen selected aromatic hydrocarbons in fish samples. The composite material was bonded to the surface of the SPME fiber using silica gels. The manufactured SPME sorption fibers showed an increased specific surface area (from 2.1 to 4.5 times) compared to commercial fibers with PDMS/CAR/DVB coatings.