## 3 Application of MOFs as Sorbents in Selected Techniques of Preparing Liquid Samples for Analysis
### Dispersive Solid-Phase Extraction (D-SPE) and Micro-Solid-Phase Extraction (\(\mu\)-Spe)  
In recent years, there has been a trend towards miniaturization and simplification of sample preparation techniques for analysis. These efforts are carried out, for example, to reduce the number of organic solvents used in sample preparation and to create the possibility of testing smaller and smaller quantities of samples with complex matrices. Not without significance, there is also a possibility of using smaller amounts of solid sorbents, which is particularly important if the cost of purchasing the sorbent is relatively high or the sorbent itself is somewhat challenging to synthesize, as in the case of MOFs. One of the results of such miniaturization is the \(\mu\)-SPE technique [66; 67]. The \(\mu\)-SPE technique was initially introduced in 2006 as an alternative to multi-stage SPE [68]. Due to the development of microextraction strategies, and the large variety of ways of using them in practice, many types or variations of these techniques can be found [66]. Dispersive-Micro-Solid-Phase Extraction (D-\(\mu\)-SPE) and porous membrane-protected Micro-Solid Phase Extraction have recently gained popularity. The second version does not have its acronym; it is often simply described as \"porous membrane-protected\". D-\(\mu\)-SPE is a miniaturized version of Dispersive-Solid-Phase Extraction (D-SPE). It uses a small amount of solid sorbent placed directly in a solution of an aqueous sample. Due to the direct placement of the sorbent in the solution, it is possible to obtain significant extraction yields to the solid phase. However, in this release, the sorbent is directly exposed to the matrix, and interferents are present in the sample. The sorbent is then filtered out, and the analytes are desorbed with an adequately selected solvent. \(\mu\)-SPE protected by a porous membrane (usually polypropylene), consists of placing a small amount of sorbent (even a few milligrams) in a self-made \"envelope\" made of porous semi-permeable material (e.g., polypropylene). This technique is a simplified and miniaturized version of the SPE technique. It is simplified because the process of concentration, extraction, and purification from interferents (clean-up) takes place here in one step. A sheet of the porous membrane is properly folded, cut, and hot-welded to form a small envelope inside which the sorbent can be placed. The use of a plastic permeable membrane provides mechanical protection for the sorbent. Thanks to its use, it is possible to insert a container with sorbent directly into even very contaminated or complex samples, e.g., environmental or biological. In both D-\(\mu\)-SPE and \(\mu\)-SPE versions with a semi-permeable membrane, the sorbent can be mixed with the sample (to increase the sorption efficiency), of course using a mechanical stirrer, sometimes described as (vortex assisted, VA), but also by ultrasound (sonification assisted, SA), and microwave-assisted (MA) [66]. When using a semi-permeable membrane, the sorbent container is removed from the sample solution, and the analytes are desorbed using an organic solvent [67]. An additional advantage of this method is the possibility of using small amounts of sorbent (a dozen or several dozen mg), which favors the use of less common sorbents, the use of which in the SPE technique could be disadvantageous due to, for example, the cost of their purchase. The \(\mu\)-SPE technique can include carbon sorbents such as graphene, graphite oxide (GO), carbon nanotubes (CNTs), including single-walled carbon nanotubes (SWCNT), and multi-walled carbon nanotubes (MWCNT), molecular mapping polymers (molecularly imprinted polymers, MIP), biopolymers and, which have become increasingly popular in recent years, MOFs [66; 67].