Isotope preparation method

The present disclosure provides 223Ra of pharmaceutically tolerable purity i.e. comprising no more than 0.05 Bq 227Ac per 1 MBq 223Ra, including where it is produced by a method comprising i) preparing a generator mixture comprising 227Ac, 227Th and 223Ra; ii) loading said generator mixture onto a strong base anion exchange resin; iii) eluting said 223Ra from said strong base anion exchange resin using a first mineral acid in an alcoholic aqueous solution to give a first eluted 223Ra solution; iv) loading the 223Ra of the first eluted 223Ra solution onto a strong acid cation exchange resin; and v) eluting the 223Ra from said strong acid cation exchange resin using a second mineral acid in aqueous solution to provide a second eluted solution.