The main target of this study is the development and the validation of an internal procedure to measure the pg g(-1) levels, of naturally occurring radioisotopes to select ultra low background detector materials. As there are no ultra low-level certified reference materials commercially available, the performance of ICP-MS was compared with the well established ultra low-level gamma-spectroscopy. This work shows also the reliability of very quick ICP-MS semi-quantitative analyses of an environmental matrix such as a geological sample treated according to conventional protocols.