Yttrium-90 used for therapy should be of very high radionuclidic (RN) purity (>99.998%) as the most probable contaminant, strontium-90, is a bone seeker with a maximum permissible body burden of 74 kBq (2 microCi) only. None of the current known methods of RN purity estimations is adequate to reliably measure the 90Sr RN impurity at such low levels. Our aim was to develop a reliable technique to accurately determine the amount of 90Sr in 90Y used for therapy. This new technique combines chelate-based extraction with paper chromatography using paper impregnated with 2-ethylhexyl, 2-ethylhexylphosphonic acid (KSM-17), which is a 90Y-specific chelator. A PC strip impregnated with KSM-17 at the point of spotting is used for chromatography. Upon development with normal saline, 90Sr moves to the solvent front leaving 90Y completely chelated and retained at the point of spotting. The activity at the solvent front (90Sr) is quantified by liquid scintillation counting, and the data are compared with the total applied activity to provide the RN purity of the test solution. The method has a sensitivity of > or =74 kBq (2 microCi) of 90Sr per 37 GBq (1 Ci) of 90Y. This novel, innovative, and simple technique offers a reliable solution to the unanswered problem of estimation of 90Sr content in 90Y used for cancer therapy.

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