## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis  
### Solid-Phase Extraction (SPE)  
SPE is one of the most popular liquid sample preparation techniques for liquid-solid extraction. Sorption of analytes occurs due to intermolecular interactions between the analyte particles and the sorbent [3; 29; 30]. The SPE technique is extremely versatile due to the possibility of selecting the appropriate sorbent and elution solvent for the analytes to be tested. In general, usable sorbents (commercially available) can be divided into three categories: low-specificity sorbents, inorganic oxides, and specific sorbents. The simplest sorbents are inorganic oxides, such as silica gel and clay. They have a polar character and can be used during the sorption of polar analytes contained in non-polar solvents. Porous polymers and modified silica gels belong to sorbents with low specificity. Silica gel-based sorbents, functionalized by hydrocarbon chains, C\({}_{2}\), C\({}_{8}\), or C\({}_{18}\), are particularly often used. They are used to extract non-polar analytes from water matrices, e.g., common organic water pollutants. The last group includes specific sorbents, which are ion-exchange resins. For this purpose, silica modified with organic groups containing, for example, NH\({}_{2}\) or SO\({}_{3}\)H are used [31]. One of the main advantages of the SPE technique is its ability to concentrate the sample repeatedly. This technique also allows for a quick change of the sample matrix, characterized by high recoveries and repeatability. It is possible to automate the sample preparation process with SPE. For effective sample preparation using the SPE technique, a vacuum or overpressure should be applied to accelerate the flow of the sample through the sorbent bed. For this purpose, vacuum collectors are used, for example, which, combined with a vacuum pump, make it possible to collect the liquid phase from several columns simultaneously. In the SPE process, the amount of organic solvents needed is usually smaller compared to the popular LLE technique, and there is no risk of emulsion formation, which hinders the sample preparation process. Samples prepared with this technique can be analyzed with both GC and LC (depending on the extraction solvent used). The SPE technique is widely used in pharmaceutical, clinical, research, forensic, environmental, or food quality  
Figure 4: Schematic representation of MOF durability, according to the theory of HSAB, prepared based on the literature [28]. control laboratories in the preparation of samples for analysis and the determination of trace amounts of, among others: pharmaceuticals and metabolites, drugs, environmental contaminants (PAHs, pesticides, PCBs, phthalates), and vitamins [32]. Not coincidentally, SPE is the most popular. A significant factor in its selection is also that it is the easiest to apply. Placing the sorption material in an empty syringe between two sintered disks is relatively easy to implement in any analytical/research laboratory. However, this technique has one significant drawback. Its use of sorbents that exhibit a large specific surface area can lead to the \"back pressure\" phenomenon, i.e., column clogging. The vacuum or positive pressure created to force the sample through the column may not be sufficient. Using sorbents characterized by tiny pores or small grain diameters can lead to clogging when analyzing samples with a complex or heavily contaminated matrix. SPE requires the use of organic solvents in a few to several mL per sample. In most cases, it is also necessary to use a relatively large amount of sorbent; these values, depending on the type of sorbent and the size of the column, can range from several dozen milligrams to a couple of grams. The use of MOF in the SPE technique may be limited simply to placing the sorbent in the SPE column, between two porous disks; it is optional, for example, to create magnetic composite materials as in the case of the MSPE technique.