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

99Tc 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 NH4TcO4 or NaTcO4 (up to 0.28 mol/L) with natural abundance of oxygen isotopes in virgin or recycled 18O‐enriched water have been studied by 99Tc NMR. The method is based on 16O/17O/18O intrinsic isotope effects in the 99Tc 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 99Tc NMR signals of the Tc16O4−n18On− 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 99Tc NMR shift induced by a single 16O→18O substitution (−0.43 ± 0.01 ppm) in TcO4− and spin coupling constant 1J(99Tc–17O) (131.46 Hz) favourable for the observation of individual signals of Tc16O4−n18On− isotopologues. A method is suggested to evaluate the oxygen isotope composition of 18O‐enriched water through measuring the relative intensities of 99Tc NMR signals of the Tc16O4−n18On− isotopologues in equilibrium solutions of pertechnetate salts in enriched water.