## 1 Introduction  
Positron Emission Tomography (PET) is an important molecular imaging modality that has long been used for clinical diagnostics, particularly in the field of oncology. It allows quantification of physiological and pathological processes in living subjects on a molecular level through detection of radiotracer--biologically active or drug-like molecule labeled with positron-emitting radionuclides--biodistribution. Among cyclotron-produced radionuclides, [11]C, [13]N, and [15]O, the [18]F possesses the longest half-life (109.8 min) and favorable physicochemical and nuclear properties. Low positron energy of 0.635 MeV allows for highest theoretical spatial resolution due to the shortest maximum range of positrons in tissue (2.4 mm in water). Over 300 GBq per batch of the radionuclide can be easily produced in a low-to-medium energy cyclotron via the [18]O(p,n)[18]F reaction occurring during proton irradiation of [18]O-enriched water in a high pressure target [1]. With this production approach, starting [18]Fl fluoride and corresponding labeled tracers are available in high molar activity (A\({}_{\mathrm{m}}\)) allowing for microdosing approach in receptors-binding studies. Ca. 2-h half-life permits complex synthetic approaches, providing access to a large variety of [18]F-labeled radiopharmaceuticals (RPs) and also allows shipping of multiple clinical doses from a centralized production site to hospitals without access to cyclotron and radiochemistry/radiopharmacy facilities. [18]Fl fluoride is currently the most practical and generally available source of this radionuclide, and in this form, it is used in the vast majority of the [18]F-labeling procedures. Nucleophilic substitution of an appropriate leaving group in a precursor molecule with [18F]fluoride is the most prominent method for the production of a variety of clinically relevant radiotracers [2; 3; 4; 5]. [18F]fluoride generated in a water target as strongly hydrated anion is generally unreactive in nucleophilic substitution scenarios. Therefore, the pre-processing steps are required to isolate [18F]F\({}^{-}\) from an aqueous environment and transform it into a reactive nucleophile species. Early work on the nucleophilic 18F-fluorizations included the removal of the bulk target water (1-3 mL volumes) by thermal distillation in the presence of K\({}_{2}\)CO\({}_{3}\) added to prevent the formation of gaseous [18F]HF. Following it up with azeotropic distillation with acetonitrile allowed for complete removal of residual water, however, the whole procedure was lengthy and generally impractical. Alternatively, separation of the [18F]fluoride from [18O]H\({}_{2}\)O (allowing for recycling of enriched water) on a strong anion-exchange cartridge (AEC) was introduced [6] and was found to be ideally suited for processing 18F\({}^{-}\) with implementation in the commercial automated synthesizers following shortly [7; 8; 9]. The recovery process is composed of two steps: the trapping of radionuclide on the resin and elution of adsorbed radioactivity with an organic-aqueous solution containing a base and a phase-transfer catalyst (PTC), most commonly the combination of K\({}_{2}\)CO\({}_{3}\) and a cryptand (Kryptofix, K\({}_{2\cdot 2\cdot 2}\), Sigma-Aldrich, St. Louis, Missouri, United States). The addition of cryptand enables solubilization of the [18F]fluoride in a polar aprotic solvent and enhances nucleophilicity of the fluoride ion. The first application of K\({}_{2.2\cdot 2}\) as cryptand in a 18F-radioling was reported in 1986 for aliphatic nucleophilic synthesis of a fatty acid [10]; soon after, it was employed in the synthesis of 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) [11], the most commonly used fluorine-18 radiotracer. Following this formative work, a radiolabeling method using azeotropically dried [18F]KF/K\({}_{2.2\cdot 2}\)/K\({}_{2}\)CO\({}_{3}\) was used in the preparation of a great variety of radiotracers for clinical and research applications in both aliphatic and aromatic nucleophilic substitution scenarios [2; 3; 4; 5; 12].