Fluorine-18 is a radionuclide that is commonly used in tracer molecules for positron emission tomography (PET) scans. The isotope can be produced from oxygen-18 by irradiation of oxygen-18-enriched water. However, this produces strongly hydrated ¹⁸F^–, which is unreactive and cannot be directly used as a nucleophile in the synthesis of tracers. The usual approach to “drying” ¹⁸F^– involves time-consuming steps (e.g., repeated distillations) and leads to an overall loss of radioactivity.

Anna Pees, Vrije Universiteit Medical Center, Amsterdam, The Netherlands, and colleagues have developed a faster, simpler process to generate “dry” ¹⁸F^–. The team first trapped the prepared ¹⁸F^– on an anion-exchange resin in a column. The fluoride is then washed from the column using a potassium sulfate solution. The ¹⁸F^–-containing eluent was then added to a solution of bistriflate in dimethylformamide (DMF) to give gaseous [¹⁸F]triflyl fluoride (pictured). This gas can be trapped in a solution of a base (KHCO₃) and a cryptand ligand in acetonitrile to provide “dry”, reactive ¹⁸F^– in radiochemical yields of over 90 %. The process only takes five minutes.

The team confirmed the reactivity of the produced ¹⁸F^– by using it in the synthesis of the PET tracers [¹⁸F]fluoroestradiol (used for breast cancer imaging) and O-2-[¹⁸F]fluoroethyl-L-tyrosine (a radiolabelled amino acid). They achieved good isolated radiochemical yields. According to the researchers, the developed method could be broadly applied to provide ¹⁸F^– for radiofluorination reactions.

• Fast and reliable generation of [¹⁸F]triflyl fluoride, a gaseous [¹⁸F]fluoride source,
  A. Pees, C. Sewing, M. J. W. D. Vosjan, V. Tadino, J. D. M. Herscheid, A. D. Windhorst, D. J. Vugts,
  Chem. Commun. 2018.
  https://doi.org/10.1039/c8cc03206h