## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Pipette-Tip Solid-Phase Extraction (PT-SPE)
MOFs are used in the PT-SPE technique to detect, among others, levofloxacin [116], bisphenol A [109], herbicides [110], and benzodiazep drugs [112], in water [115], wastewater [116], food [114] and biological fluids [112]. The use of PT-SPE enables the use of a small amount of sorbent (even a few mg) and the testing of small sample volumes (<1 mL). In one of the studies [110], the authors used only 5 mg of UiO-66/PAN composite produced by electrospinningning to analyze only 1 mL of liquid sample. Under optimized conditions, combined with the HPLC-FD technique, the authors were able to determine four selected phytohormones in samples of watermelon and mungbeam tasks (Figure 17). The developed analytical method allowed for the detection of the investigated compounds with LOD at the level of 0.01 to 0.02 ng/mL. The linear ranges of the calibration plots were in the range of 0.06-60 ng/mL, with a correlation coefficient above 0.992. Recoveries for the analytes studied ranged from 84.4 to 111.2%, with the RSD in the range of 1.5-5.6%. Fluoroquinolones (FQs) are commonly used to treat infections in humans and animals. These compounds are poorly absorbed by humans; they are usually excreted in urine or feces in unchanged form, where they end up in municipal wastewater. Wastewater treatment plants can be poor at removing them and they then end up back in the drinking water cycle like other pharmaceuticals and personal care products. Their detection at very low concentration levels can be carried out using the PT-SPE technique, using ZIF-8 as a sorbent [115]. The authors of this study used molecular simulations to elucidate further how ZIF-8 and fluoroquinolones interact. According to the calculations, the interactions between ZIF-8 and the tested compounds consisted mainly of hydrophobic interactions and hydrogen bonding between the oxygen/fluorine atom and the hydrogen atom of 2-methylimidazole. Under optimized conditions, the authors could detect compounds in LOD water samples ranging from 0.337 to 1.707 ng/L, with recovery from 75.9 to 96.8% from an RDR of less than 8.0%. Another example of drugs ending up in wastewater is benzodiazepieresis such as nitrarepam and oxazepam [112]. They are used to treat anxiety and sleep disorders, but their excessive consumption can lead to side effects such as memory loss and depression. The content of these compounds in wastewater samples is commonly detected using chromatographic techniques. However, the low concentration levels of these compounds in the samples and the complex matrix effectively hinder the analysis. A well-designed sample preparation step is necessary, leading to the concentration of analytes and removal of the influence of interferents. For this purpose, the authors of this study [112] used the PT-SPE technique, using a sorbent MIL-53(AI) combined with polyacrylamide in the process of electrical grading. In combination with the HPLC-DAD technique, under optimized conditions, the developed research method was successfully used to analyze these drugs in wastewater and biological fluids. LOD for the tested compounds ranged from 1.5 to 2.5 ng/mL; the linearity range of the method is 5.0-1000 ng/mL, with an RSD of \(<\)7.6% determined at three levels of analyte concentrations: 50, 100, and 250 ng/mL. Recoveries for the tested compounds were 92.4 and 100.3%. A summary of MOF materials used in PT-SPE extraction is presented in Table 5.  
Figure 17: An example of a PT-SPE procedure, presented by [PERSON] et al. [110]. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline