The present invention provides 223Ra comprising no more than 0.05 Bq 227Ac per 1MBq 223Ra. The 223Ra may be formed or formable 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； andv) eluting the 223Ra from said strong acid cation exchange resin using a second mineral acid in aqueous solution to provide a second eluted solution.
