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 Tc¹⁶O_4−n¹⁸O_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 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 Tc¹⁶O_4−n¹⁸O_n⁻ isotopologues.