Quality control of actinium-225 and 225Ac-radiopharmaceuticals: Francium-221 to be or not to be?

Five instruments have been tested for their linearity, sensitivity, accuracy and specificity to actinium-225 quantification using francium-221 and bismuth-213 equilibria. Francium-221 equilibrium was deemed acceptable for TLC RCY% using HPGe and gas-filled proportional counter. Gamma well counting, LSC, and plastic silicon detector required bismuth-213 equilibrium for accurate RCY%. Low content free actinium-225 impurity was accurately reported for all methods except for the silicon detector. Overall, this investigation sheds light on the appropriate analytical methods for ²²⁵Ac-radiopharmaceutical QC considering secular equilibrium.