## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Solid-Phase Extraction (SPE)
MOFs are materials characterized by the presence of a complex spatial structure of micro and macro pores, which can negatively impact the rate at which the water sample will flow through the column (permeability, or the resistance that the sorbent puts to the flowing sample). However, the use of vacuum collectors can solve this problem to some extent. One of the disadvantages of using MOF in the SPE technique is the relatively large amount of sorption material that needs to be placed inside the column. In the standard case, the amount of sorbent should be between a few tens of milligrams to several grams, which is why there has been a trend towards the miniaturization of sample preparation systems in recent years. The high specific surface area of MOFs, combined with the significant concentration capacity of SPE, is an excellent combination for the analysis of compounds present in samples at very low concentration levels. MOFs, in combination with the SPE technique, are mainly used for the determination of compounds in aqueous samples [33; 34; 35; 36; 37; 38]. Materials from Institut Lavoisier (MIL) [33; 34; 37; 39] and from the University of Oslo-UiO [38; 40; 41] are widely used. MIL-101(Cr) was used in tandem with graphene hybrid aerogel to determine selected non-steroidal anti-inflammatory drugs (NSAIDs) and selective enrichment/concentration of selected proteins (ribonuclease A enrichment while excluding cytochrome C and lysozyme) [33]. A graphene aerogel (GA)-supported MOF, demonstrating a 3D structure, was made using a simple'sol-cryo' technique, which does not require the use of \"guest\" molecules whose role is to direct the growth of MOF crystals (Figure 5). Thanks to the use of the described sorbent, and using the HPLC-UV-Vis technique, the authors of the study managed to obtain satisfactory recoveries (89.2-100.7%), high sensitivity of the method, and good repeatability (3.7-8.5%) for five selected NSAIDs. Another promising application example of MIL-101(Cr) is the analysis of sulfonamides [34]. The authors used both MIL-101(Cr) and the related MIL-101(Fe) to determine four selected sulfonamides in environmental water samples using the UPLC-MS/MS technique. Using the described MOF, the authors were able to determine sulfadiazine (SDA), sulfamethazine (SMZ), sulfachloropyridazine (SCP), and sulfamethoxazole (SMX), with a LOD of -0.03-0.08 g/L. The developed analytical method was characterized by recovery for the examined analytes at the level of 83.5 to 107.3%, with repeatability expressed in RSD at the level of 0.2 to 8.0%. An extremely interesting aspect of this study was the application of computer modeling to predict the affinity of MIL-101(Cr) for selected sulfonamides, which not only accurately predicted the studied relationships but also provided a new strategy in the analysis of compounds at very low concentration levels. MIL-101 has also found its application in the determination of eleven selected UV-filtering compounds, also from aqueous samples [37]. This MOF was placed directly in a polystyrene SPE column and put down in a vacuum collector [37]. The eluted compounds were determined using GC-MS/MS. The dependence of several factors on the extraction process was investigated, such as the desorption conditions of compounds, the pH of samples, salting, and the volume of the tested samples. The LOD determined during validation ranged from 1.0 to 11.7 ng/L, and the analytes were determined with a satisfactory recovery of 82 to 105%. The highest concentration of the tested compounds was detected in water samples from swimming pools. The method of simple MIL-101(Cr) synthesis and analytical workflow for the developed SPE-GC-MS/MS method is shown in Figure 6.  
The synthesis of spider-weblike chitosan/MIL-68(Al) composite nanofibers and their application to fast SPE was described by [PERSON] et al. [39].