## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Pipette-Tip Solid-Phase Extraction (PT-SPE)  
One of the most significant advantages of the SPE technique is the possibility of multiple concentration of compounds present in the tested samples. One of the disadvantages of this technique is the need to use organic solvents in quantities ranging from a few to several mL per sample and the need to use a sorbent in an amount ranging from several dozen milligrams to several grams. Both of these disadvantages are contrary to the principles of green chemistry and affect the higher cost of analysis, especially when using such a unique sorbent like MOF. The pipette-tip solid-phase extraction (PT-SPE) technique is a miniaturized version of the SPE technique aimed at eliminating some of the disadvantages of the SPE technique [109]. It can be used in the process and with smaller amounts of organic solvents and sorbents, simply using the tip of the automatic pipette as a column for SPE and the automatic pipette itself as a source of vacuum or overpressure forcing the sample to flow through the sorbent bed. PT-SPE has become an effective tool in purifying, concentrating, and isolating specific analytes from the interferents present in the sample [110]. The PT-SPE technique is an excellent example of a method that uses fewer organic solvents, making it more economical and indirectly contributing to less environmental pollution [111]. One of the problems that can be encountered when using the PT-SPE technique is the same as in SPE: high back pressure, the resistance that the sorbent puts to the liquid sample flowing through it. This may be related to a densely packed solid sorbent in a relatively small \"column\" with a small internal diameter, which is particularly important when using an MOF, characterized by tiny pores. Another factor may be not using a vacuum collector capable of creating a significant vacuum but only the piston of the automatic pipette itself. This problem does not occur in other techniques and is also less common in the SPE technique. The solution may be to reduce the sorbent packing density, which will also help maintain the initial specific surface area and the sorption properties of the material itself. This effect can be achieved by using, as sorbents, composite materials created by combining MOF and polyacrylamide (PAN) nanofibers obtained by electrospinningning. Electrospinning is a method of producing fibers of material with a diameter of several hundred nanometers. For this purpose, PAN is often used because it is characterized by adequate thermal, chemical, and mechanical resistance. Electrospinning is a simple, efficient method of producing continuous, porous fibers with a high specific surface-to-volume ratio. MOF/PAN structures, where MOFs are deposited on the nanofiber surface, are characterized by lower packing of MOF particles, preventing the problem of high back pressure [110; 112]. An example of an electrospinningning procedure for MOF/PAN nanofibers, in short: 0.48 g of UiO-66 in 3 mL DMF was dispersed using sonification, and 0.12 g PAN was added and mixed for 6 hours. After this time, another 0.2 g PAN was added to form a homogeneous, dispersed solution for electrical grading. The solution was subjected to a spinning process at 18 kV. The flow of the solution was 0.8 mL/h, and the diameter of the spine was 0.6 mm. The distance between the spindle and the collector was 20 cm. The procedure was carried out at room temperature. The resulting nanofibers were immersed in ethanol to exchange solvents and then dried overnight at 80 \({}^{\circ}\)C in a vacuum oven [110]. Electrospinning in combination with MOF has been used in practice [111; 112; 113]. The use of the electrospinningning method prevented the back pressure problem during the liquid sample flow through the tip of the pipette with a fine-grained sorbent of the MOF type. The most frequently appearing in MOF publications are again UiO-66 [110; 113], MIL-68 [114], MIL-53(Fe) [111], and ZIF-8 [115].