## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Magnetic Solid-Phase Extraction (MSPE)  
In the D-SPE technique, solid sorbent grains are placed directly in the tested sample, where the sorption process of the tested compounds takes place. This technique ensures high extraction efficiency due to the large contact surface of the sorbent with the sample. This technique also has some drawbacks; the sorbent is most exposed to potential impurities present in the sample. The sorbent must then be filtered, which may vary according to the matrix's composition and the test sample's characteristics. A modification of this technique aimed at eliminating some of the disadvantages of D-SPE is MSPE. This technique is analogous to the D-SPE technique, with the difference that in MSPE, the sorbent exhibits magnetic properties, which allows it to be more easily separated from the sample solution, e.g., using an external magnet. MSPE imposes certain restrictions on the sorbent itself--it must exhibit magnetic properties (Figure 13). Sorbents showing magnetic properties can be used, or existing sorbents can be modified or built on, e.g., Fe\({}_{3}\)O\({}_{4}\) nanoparticles [91]. The use of nano- or micro-particles, Fe\({}_{3}\)O\({}_{4}\) (mixed nanoparticles, diameter-5-100 nm) of iron (II) oxide and iron oxide (III), to give the sorbent magnetic properties, has also become popular in the context of the use of MOFs as a sorption material, creating the Magnetic Metal-Organic Framework (MMOF). There are several ways to modify classical sorbents or MOF with Fe\({}_{3}\)O\({}_{4}\) to exhibit magnetic properties [92; 93]. Simple mixing of the initial sorbent with nanoparticles is the simplest method, although the resulting sorbent is the least stable. Often, Fe\({}_{3}\)O\({}_{4}\) nanoparticles are coated with a silica layer, which acts as a \"linker\" that connects the magnetic Fe\({}_{3}\)O\({}_{4}\) particles and the sorbent itself [91]. Iron oxide nanoparticles are coated with silica in the Stober process [94]. Iron particles can either be directly coated with silica using tetraethoxysilane (TEOS) or APTES, resulting in NH\({}_{2}\) group left outside the nanoparticle surface or first with TEOS, then APTES, leading to the formation of more regular, larger diameter nanoparticles with free NH\({}_{2}\)[94]. Both methods of coating iron oxide nanoparticles with silica are shown schematically in Figure 14. NH\({}_{2}\) groups can potentially be chemically bound to selected functional groups on the MOF surface to increase the bond stability of magnetic particles and metal-organic sorbent [12; 56]. The process of applying another silica layer using APTES is analogous to TEOS, except that no ammonia is necessary to functionalize the nanoparticles using APTES. 1 mL ethanol and 4% APTES solution were added to 1 mL of aqueous 60 mg/L solution Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\). The temperature and reaction time were the same as before [95]. One paper briefly describes depositing a silica layer on iron nanoparticles in the Stober process: nanoparticles can be placed in a 200 mL aqueous solution of 0.1 M HCl to develop their surface; the nanoparticles are then drained and placed in water [96]. An aqueous solution of 60 mg/L of magnetic nanoparticles is placed in a round-bottomed flask, heated to 40 \({}^{\circ}\)C, diluted with 200 mL ethanol, and ultrasonicated for 15 min to disperse in a solid particle solution. In this order, 9 mL of 28% aqueous solution of NH\({}_{3}\) and 1.2 mL of TEOS are added to the flask. The suspension in the flask was stirred and maintained at 40 \({}^{\circ}\)C for 2 h. The resulting Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) precipitate is collected using an external magnet to avoid collecting silica  
Figure 13: Application of the MSPE technique—variant [own elaboration]. particles that do not contain magnetic nanoparticles inside. The nanoparticles thus collected were washed three times with ethanol and water [96]. Fe\({}_{3}\)O\({}_{4}\) can also be combined with MOF in the process of their solvothermal synthesis.