## 1 Introduction  
The development of analytical chemistry is constantly observed, among others, in the field of improving the techniques of sample preparation for analysis. Samples delivered to the laboratory are often characterized by a complex matrix, in which, in addition to interesting analytes, there is often a significant number of interferents hindering the analysis. In addition, the analysis is often restrained by the small amount of sample provided and the low concentration of analytes contained in it. It is often necessary to change the sample matrix so that it can be analyzed using the appropriate technique. The primary role of sample preparation for analysis is usually concentration, the isolation of analytes from the sample matrix, and the removal of some or all interferents, often changing the sample matrix or transforming analytes into appropriate derivatives that allow their detection (derivatization) [1]. A big challenge may also be the repeatability of conducted tests and testing a large number of samples, with the latter being particularly important in commercial laboratories, but not only. Water-based sample preparation techniques used in laboratories can be broadly divided into those based on liquid-phase extraction and those based on solid-phase extraction [2]. In part thanks to the fact that there are a wide range of possible sorbents,solid-phase extraction techniques have been the main direction of development in recent years (Figure 1). As such, this article focuses exclusively on using variations of the solid-phase extraction technique. Additionally, the miniaturization and adaptation of the properties of the sorption material, depending on the specific application--such as increasing the selectivity and specificity of the sorbent - have become the focus of researchers in recent years. The most common and most developed analytical techniques are variations and attempts to improve  
Figure 1: (**A**) Number of recently published scientific papers, including chosen techniques for preparing samples for analysis. (**B**) Excluding SPE and SPME techniques to better show the scale for other techniques. SCOPUS database [own elaboration]. the SPE technique; these can include, for example, SPME, MSPE, PT-SPE, MSPD, and SBSE [2]. Various sorption materials are available commercially and have been adapted to many applications. However, work is still underway on both the form of sorbent application (miniaturization) and its properties (sorption efficiency, selectivity) [3; 4; 5]. Particularly interesting here seems to be the use of metal-organic frameworks (MOFs)--crystalline porous materials with high specific surfaces [6; 7]. The popularity of the discussed sample preparation techniques and MOFs as sorption materials is very well illustrated by the graphs shown in Figures 1-3. MOFs, despite being a relatively young group of sorptive materials, have undoubtedly gained increasing interest in the scientific community (Figure 2), thanks to their remarkable properties compared to other sorptive materials and their potentially wide range of applications in many fields of science and technology [7; 8]. The analysis of scientific publications on both MOFs and various sample preparation techniques allows us to observe that MOFs are eagerly used as sorptive materials in SPE technique (Figure 3). In second place, with a very similar number of publications, is the use of MOFs in SPME and MSPE techniques. The MSPE technique may not be as popular in combination with commercial sorbents due to the smaller amount of sorption material used in it, which may translate into a lower extraction efficiency. On the other hand, the binding of an MOF with another material may positively affect the durability of MOFs in the aquatic environment, and their high surface area could still provide a satisfactory extraction efficiency, even with fewer sorbents used. Each of the sample preparation techniques has its advantages and disadvantages and is best suited for specific applications.