On-line interconversion of [¹⁵O]O₂ and [¹⁵O]CO₂ via metal oxide by isotopic exchange

A novel method has been developed for the on-line production of ¹⁵O-labelled gases. The ¹⁵O exchange reactions between O₂ and CO₂ assisted by a metal oxide catalyst were successfully applied to on-line interconversion of [¹⁵O]O₂ and [¹⁵O]CO₂ with Hopcalite II(CuO 40% and MnO₂ 60%). The conversion reactions were optimized as to the reaction temperature, the amount of the catalyst, and the flow rate of a gas added for oxygen exchange. [¹⁵O]O₂ was converted to [¹⁵O]CO₂ in a 80% yield with 0.7 g of Hopcalite II and 100 mL/min of CO₂ at 500°C, and [¹⁵O]CO₂ to [¹⁵O]O₂ in 70% with 100 mL/min of O₂ at 650°C. The radiochemical purities of the ¹⁵O-labelled gases converted under the optimal conditions were high enough for clinical studies using the standard dilution and inhalation procedures.