⁹⁹ Tc NMR determination of the oxygen isotope content in ¹⁸ O-enriched water

⁹⁹ Tc NMR has been suggested as an original method of evaluating the content of oxygen isotopes in oxygen-18-enriched water, a precursor for the production of radioisotope fluorine-18 used in positron emission tomography. To this end, solutions of NH₄ TcO₄ or NaTcO₄ (up to 0.28 mol/L) with natural abundance of oxygen isotopes in virgin or recycled ¹⁸ O-enriched water have been studied by ⁹⁹ Tc NMR. The method is based on ¹⁶ O/¹⁷ O/¹⁸ O intrinsic isotope effects in the ⁹⁹ Tc NMR chemical shifts, and the statistical distribution of oxygen isotopes in the coordination sphere of TcO₄^- and makes it possible to quantify the composition of enriched water by measuring the relative intensities of the ⁹⁹ Tc NMR signals of the Tc¹⁶ O_4-n¹⁸ O_n^- isotopologues. Because the oxygen exchange between TcO₄^- and enriched water in neutral and alkaline solutions is characterized by slow kinetics, gaseous HCl was bubbled through a solution for a few seconds to achieve the equilibrium distribution of oxygen isotopes in the Tc coordination sphere without distortion of the oxygen composition of the water. Pertechnetate ion was selected as a probe due to its high stability in solutions and the significant ⁹⁹ Tc NMR shift induced by a single ¹⁶ O→¹⁸ O substitution (-0.43 ± 0.01 ppm) in TcO₄^- and spin coupling constant ¹ J(⁹⁹ Tc-¹⁷ O) (131.46 Hz) favourable for the observation of individual signals of Tc¹⁶ O_4-n¹⁸ O_n^- isotopologues.