## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Solid-Phase Microextraction (SPME)  
The technique of SPME was developed by the team of Prof. [PERSON] in the early 1990s [49]. The SPME technique itself is based on the phenomenon of the sorption of analytes on the surface of a thin fiber. This process can be described as the equilibrium constant or the split constant of the analyte, depending on how the technique is used, between two or three phases in a direct immersion (DI) or headspace (HS) system. The HS allows the absorption of analytes found in lubricants, thick oils, and soil but limits the use of SPME to volatile and medium-volatile compounds. The great advantage of SPME is also its ability to concentrate analytes. Commercially available fibers for SPME allow multiple concentrations of analytes present in the headspace. In addition, the use of this technique is extremely simple, fast, and does not require the use of any organic solvents. Fibers with adsorbed analytes can be directly introduced into the GC inlet for the analyte desorption. Thanks to its advantages, SPME remains one of the most popular techniques for determining volatile and medium-volatile compounds. This technique constantly evolves, especially towards developing new sorption materials [50; 51]. Most publications on applying the SPME technique are the result of analyses of volatile and medium-volatile organic compounds (e.g., BETX) in combination with gas chromatography. Most MOFs, in addition to their large specific surface area and the possibility of their modification, are also characterized by relative instability in the aquatic environment. The current applications of MOF focus mainly on the sorption and separation of gases [12]. Combining the HS-SPME technique with the use of MOF as a sorption layer seems to be exceptionally highly beneficial due to the possibility of the sorption of volatile and medium-volatile compounds from the headspace, and consequently not exposing the MOFs to direct contact with the aqueous matrix of the samples. Sorbents used in the SPME technique must have high thermal stability (in the range of 250-350 \({}^{\circ}\)C) because the desorption of analytes most often occurs thermally in a gas chromatograph inlet, and MOFs demonstrate durability at the required temperatures. When working on using new sorbents in the SPME technique, it is also important to consider how to apply the sorbent to the fiber. Often, difficulties in applying a sorbent to fiber can be a significant obstacle to the development of this technique. There are several different approaches to this problem. One of the most straightforward solutions is to cover the fiber with silicone or epoxy glue (it is essential that the adhesive used be resistant to high temperatures in the range of \(\sim\)250-350 \({}^{\circ}\)C), then immerse the fiber coated with glue, e.g., in the powder of the MOF. This process can be repeated several times to obtain the desired thickness of the sorption layer [52]. Another approach involves immersing the metal fiber in a polymer and MOF composite solution [53]. Direct solvothermal synthesis of the MOF on the fiber's surface is also used, followed by direct carbonization of the sorbent layer formed in this way on the fiber [54]. The creation of composite materials made of MOF, such as graphene oxide (GO), also gives great hope. Graphene oxide is chemically bound to silica present on the surface of the fiber and to the selected MOF [55]. Due to the specificity of the SPME technique, it is most often used in combination with gas chromatography. The use of MOFs in SPME is successfully used not only for testing water samples but also fruit [56], plasma and urine [57], milk [58], soy oil [59], honey [60], and fish [52]. The most frequently studied analytes include polycyclic aromatic hydrocarbons [52], organochlorine pesticides [56], fragrances [54; 61], polybrominated diphenyl esters [58], and fluoroquinolones (FQs) [60].