Separation of rare earth elements

A method for purifying lutetium, the method comprising:

• subliming or distilling ytterbium from a solid composition comprising ytterbium and lutetium in an environment comprising a pressure of 1×10−3 torr or less and at a temperature of 400° C. to less than 700° C. to leave a lutetium composition comprising a higher weight percentage of lutetium than was present in the solid composition;
• chelating the lutetium composition with a chelator to form a chelated lutetium solution comprising both chelated lutetium and ytterbium;
• subjecting the chelated lutetium solution to chromatographic separation;
• collecting a purified, chelated lutetium fraction;
• and
• de-chelating the lutetium to obtain the purified lutetium;
• wherein:
• the purified lutetium comprises Lu-177 that is greater than 99% pure on an isotopic basis;
• and
• the ytterbium is collected in an amount that is 90 wt % to 99.999 wt % of the ytterbium present in the solid composition.