## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Magnetic Solid-Phase Extraction (MSPE)
Then, in addition to organic links and salts of the corresponding metals, magnetic nanoparticles are added to the reaction mixture, which is enclosed inside the formed crystals of metal-organic lattices during the precipitation of the MOF [93]. Increasing the durability of MMOF can be achieved by converting them into Magnetic Porous Carbon, MPC. MPC can be produced by carbonizing MOF or MMOF. As a result of carbonization, organic components (linker) are transformed into a porous carbon structure, enclosing in themselves metallic nano- or micro-particles with magnetic properties. In addition, metals present in clusters are leached. In the treasured MOF, hydrolysate coordination bonds are no longer present, and the porous structure is formed only with organic linkers. The obtained MPCs are characterized by more excellent durability, but unfortunately, they lose some porosity [93]. Each time the use of the MSPE technique is associated with a magnetic field-prone component with ferromagnetic properties combined with the sorbent used. In each of the techniques discussed in this article, the role of Fe\({}_{3}\)O\({}_{4}\) nanoparticles is discussed, and more information can be found at the beginning of this section. However, interestingly, two main ways of combining the sorbent (MOF) with Fe\({}_{3}\)O\({}_{4}\) can be observed. The first of these is the use of silylating agents, such as TEOS and APTES, for binding the sorbent to the surface of iron oxide nanoparticles using a silica layer [97; 98; 99; 100]. The second of which is the direct MOF precipitation in the presence of Fe\({}_{3}\)O\({}_{4}\), thanks to which ferromagnetic nanoparticles are \"trapped\" inside the sorbent [97; 101; 102]. The frequently chosen MOF are MIL-101(Cr) [99], MIL-125-NH\({}_{2}\)[103], and MIL-100(Fe) [102]. You can also meet with the use of ZIF-8 [97; 100] and less common MOF-5 [101] and MOF-808 [36]. It is still popular to create composite materials, using e.g. graphene oxide [99; 100]. The MSPE technique combined with MOF is used to detect organophosphorus pesticides [104], benzoylurea insecticides [36], triazole fungicides [100], and others, including NSAIDs [102] in aqueous samples [98], tea [97], rice [99], honey [100] and juice [36]. A practical example of the use of MOF is the composite material Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-GO/MIL-101(Cr), created by combining the sorption properties of the MOFs and graphene oxide with the magnetic properties of iron oxide nanoparticles for the determination of selected seven triazine herbicides in rice samples (Figure 15). In combination with the HPLC-UV-Vis technique, LOD was obtained for the tested compounds at the level of 0.010-0.080 ug/kg, with the recovery of analytes in the range of 83.9-103.5%, with relative standard deviations of less than 8.7% [99]. Figure 14: Application of a silica layer to magnetic nanoparticles [94; 95; 96]. The authors of another work [103] additionally used L-Cysteine to coat iron nanoparticles to increase the dispersion of Fe\({}_{3}\)O\({}_{4}\) and increase the number of active centers on the surface of nanoparticles. In combination with the use of MIL-125-NH\({}_{2}\) and UPLC-TUV, an analytical method was developed to detect compounds present in samples at such low concentrations as (LOD) from 0.05-0.2 ug/L, with recovery ranging from 83.8-109.4%, with an RSD lower than 8.9%. The prepared Fe\({}_{3}\)O\({}_{4}\)@Cys@MIL-125-NH\({}_{2}\) is a promising magnetic adsorbent for efficiently enriching fluoroquinolones from the tap and environmental water [103]. Residues of organophosphorus pesticides in soil, water, and other agricultural products can cause neurotoxic and human diseases. Examples of such compounds include chlorpyrifos, profenofos, and phosalon. They have shown severe toxicity, mutagenic, carcinogenic effects and pose a considerable threat to mammalian life. An easy-to-apply, fast, and sensitive analytical method is required to analyze them.