99 T c 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 NH 4 TcO 4 or NaTcO 4 (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 TcO 4 − and makes it possible to quantify the composition of enriched water by measuring the relative intensities of the 99 Tc NMR signals of the Tc 16 O 4− n 18 O n − isotopologues. Because the oxygen exchange between TcO 4 − 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 TcO 4 − and spin coupling constant 1 J ( 99 Tc– 17 O) (131.46 Hz) favourable for the observation of individual signals of Tc 16 O 4− n 18 O n − isotopologues.