Determination of ²²⁶Ra at low levels in environmental, urine, and human bone samples and ²²³Ra in bone biopsy using alpha-spectrometry and metrological traceability to ²²⁹Th/²²⁵Ra or ²²⁶Ra.

<sup>226</sup> Ra is a natural radioelement emitting α and γ radiations. It can be highly concentrated in TENORM materials from the petroleum or fertilizer industries. In Switzerland, <sup>226</sup> Ra is currently a radioactive inheritance problem from the watch industry. Furthermore, <sup>223</sup> Ra is a radium isotope used in nuclear medicine to treat bone metastasis. There exist several methods to measure radium using alpha or gamma spectrometry or using <sup>222</sup> Rn emanation technique. The limitations of these methods are due to the required detection limits and the nature of the samples. When using alpha spectrometry to reach very low detection limits, critical technical hitches often arise because of the difficulties in separating radium from barium, in removing organics eluted from the separating chromatography column, and in plating radium. Moreover, overall chemical recovery of radium is often not reproducible, depending on the studies. Here we propose a method that separates radium from other alkaline-earth cations using cation exchange chromatography and selective complex formation by EDTA and DCTA. Radium is completely free of the <sup>229</sup> Th tracer and its daughter products, particularly <sup>225</sup> Ac. Organics from the column are removed in a further purification step so that radium can be plated with acceptable yields in a HCl/HNO <sub>3</sub> /ethanol solution. We successfully applied the method to soil, water, urine and human bone samples and further extended it to the determination of <sup>223</sup> Ra in a bone biopsy, using <sup>226</sup> Ra as an internal tracer