## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Solid-Phase Extraction (SPE)
The authors of the paper describe the possibility of using MIL-68(Al) in combination with chitosan, creating a composite material in the form of nanofibers suitable for use in the SPE technique for determining the content of Pb\({}^{2+}\) and Cd\({}^{2+}\) in samples of Trochus erythreus, a selected species of sea snail. The tested compounds were determined using the ICP-OES technique; the LOD was 0.16 ug/L for both compounds. The authors used the electrospinning technique to produce their composite material, thanks to which the resulting material resembles a spider's  
Figure 5: Synthetic procedure for MIL-101@GO presented by [PERSON] et al. [33]. Figure 6: A simple MIL-101(Cr) synthesis and application protocol presented by [PERSON] et al. [37]. web with a developed surface, which significantly promotes the process of sorption of compounds on the material used (Figure 7). UiO-66 has also found its applications. It is often synthesized in several versions, differing from each other by the presence of one group -NH\({}_{2}\) or -OH connected to the aromatic ring in the ligand of this MOF [38; 40; 41]. UiO-66-OH has found its application in the extraction of PFCs and PFOS from aqueous samples. UiO-66-OH was used in conjunction with the MALDI-TOF-MS technique to determine macromolecules, including the described PFCs [40]. This material has been successfully used both as an adsorbent and as a matrix for the MALDI-TOF-MS technique. The research found that MOF shows high laser desorption/ionization efficiency and functions well in both positive and negative ion modes. Compared to MS spectra with conventional matrices, MS spectra obtained on the MOF matrix are characterized only by deprotonated molecule ion peaks, without background interference. UiO-66-NH\({}_{2}\), combined with molecularly imprinted polymer (MIP), was used as a sorbent to extract four selected aflat (AFB1, AFB2, AFG1, and AFG2) from grain samples [41]. As a result of the combination of MIP and MOF, a polymer with surface printing type UiO-66-NH\({}_{2}\)@MIP was obtained. Quercetin was used as a virtual template, and the MOF acts here as a carrier into which the acrylamide monomer can be copolymerized. The sorbent was successfully used to determine the discussed analytes. The developed analytical method was characterized by good linearity in the range of 0.20 to 45 \(\mathrm{\SIUnitSymbolMicro g}\)/kg. LOD ranged from 90 to 130 \(\mathrm{\SIUnitSymbol ng}\)/kg. The obtained recoveries ranged from 74.3 to 98.6%. UiO-66 and UiO-66-NH\({}_{2}\) were also used in discs with different mixed-matrix discs (MMD) to perform automated flow-through extraction of 7 selected substituted phenols in aqueous samples. UiO-66 crystals of various sizes (90, 200, and 300 \(\mathrm{\SIUnitSymbol nm}\)) have been embedded in mechanical polyvinylidene discs (PVDF). MOF-MMDs enabled the simultaneous extraction of phenols while excluding larger molecules. The best results were obtained with 90 \(\mathrm{\SIUnitSymbol nm}\) UiO-66-NH\({}_{2}\) crystals. Using the obtained sorbent, LOD was obtained in the range from 0.1 to 0.2 \(\mathrm{\SIUnitSymbol mg}\)/L. Relative standard deviations of the method ranged from 3.9 to 5.3% intra-day, and 3.7-5.7% inter-day. The obtained recoveries for the studied analytes ranged from 90 to 98%. A summary of the MOF materials used in SPE is presented in Table 1.  
Figure 7: SEM images of synthesized MIL-68 (Al) nanoparticles (**a**) electrospun spider-web-like chitosan/MIL-68 (Al) composite nanofibers (**b**) [39].