## 1 Introduction  
Oligonucleotides (OGNs) are short analogs of nucleic acids, so they are built from nucleobases and sugar-phosphate backbone. One strand of human DNA is made up of several billion nucleotides when OGN length does not exceed a few dozens of it. An example of an important naturally occurring OGNs are microRNA, which plays a significant role in organism functions [1]. The participation of these molecules in biological processes is important for oncogenesis, which is why they are studied as a diagnostic and therapeutic factor [1, 2, 3]. Synthetic OGN often has chemical modifications and give promising results as therapeutics in many types of diseases, including cancer, metabolic, inflammatory, infectious, and neurological ones [4, 5]. Whatever the reason for the interest in a specific type of OGN their analysis has a great role; thus, sensitive methods are needed. Nowadays, high-performance liquid chromatography (HPLC) is the most widely used technique in this field [6, 7, 8]. Analysis of OGN is preceded by the sample preparation step [9]. Among the techniques of sample cleaning, solid-phase extraction (SPE) with various adsorbents is dominant [9]. Commercially available anion-exchange resigns, such as Clarity OTX(r) and Oasis WAX(r), are one of the most popular materials used during OGN SPE. Optimized procedures that use these adsorbents give high recovery values, usually in the range of 60-90% [10; 11; 12; 13; 14; 15]. However, the protocols often required additional preliminary cleaning step with the application of lysis buffer to disrupt protein binding [12; 15]. Moreover, some SPE procedures reported in the literature assume the application of a high concentration of salts, so they are not compatible with mass spectrometry (MS) detection or required additional desalination step [9; 16]. It is worth highlighting that a classical SPE is nowadays replaced by its miniaturized forms. They, among others, minimize toxic and hazardous organic solvent consumption, reduce the sample volume, and maximize extraction efficiency at the same time [17; 18]. One of the miniaturized dispersive SPEs involves the use of magnetic particles modified or coated with different adsorbents [19]. Functionalized magnetic particles, modified in different manner are intensively used in nucleic acids separation [20; 21]. This approach allows for extraction of DNA or RNA, avoiding centrifugation steps. Automation of whole process is easy and nucleic acids can be extracted from higher volumes of samples [20]. Different types of magnetic particles are used, usually coated with silica, cellulose, or synthetic or biopolymers [20]. Nucleic acids can randomly interact with the modified bead surface, by hydrogen, ionic, and many other interactions. Adsorbed DNA is usually used for further proceedings. If desired, it may be eluted by using a higher ionic strength, pH changes, heating, etc. [20]. Strong anion-exchange magnetic particles (SAX MP) were used for the extraction of short interfering RNA (siRNA) molecules by [PERSON] and [PERSON] [16]. The authors tested two commercial SAX MP surface-modified with quaternary functional groups. The elution of analytes was possible with a high (1 M) concentration of salts. The application of volatile salts (triethylammonium bicarbonate, ammonium bicarbonate, and ammonium chloride) helped to solve the problem of incompatibility with MS detection. The recovery was dependent on OGN and SAX MP type. Moreover, there were differences in the extraction recovery of analytes with different lengths. These observations suggest that the extraction protocol must be developed every time and assessed empirically [16]. In our previous studies, we developed the method of OGNs extraction with the usage of poly(ionic liquid)s [22]. The dispersive micro-solid-phase extraction was characterized by high recoveries and good repeatability. A drawback of this procedure was the application of centrifuge as a phase separation tool [22].