RNAA in metrology: A highly accurate (definitive) method.

The idea of highly accurate (definitive) methods by radiochemical neutron activation analysis (RNAA) is presented and illustrated with several examples of methods worked out in this Laboratory over the past several years. Definitive methods by RNAA are constructed by combining reactor neutron activation with very selective and quantitative post-irradiation separation of the indicator radionuclide by column chromatography followed by gamma-ray spectrometric measurement. All conditions for the determination of the individual element are optimized and uncertainties associated with every step of the analytical procedure are minimized. Even after the method has been thoroughly elaborated and validated through the analysis of appropriate certified reference materials (CRMs), the results obtained in each series of measurements are acknowledged as obtained by definitive method only when a series of previously formulated criteria is simultaneously fulfilled. The examples of definitive methods for the determination of cadmium, cobalt and molybdenum, respectively, in biological materials are presented. Each of these methods has detection limit of the order of ng g(-1) or better, and yields accurate and precise results. The expanded standard uncertainty is of the order of 2.6% for the case of single-element determination (Co) and 3.4-5.2% for the less favourable case (Mo) where there is necessity for simultaneous determination of uranium to correct for interference due to fission reaction. Definitive methods by RNAA may constitute an option or alternative with respect to ID-MS as methods of "guaranteed accuracy" being also a perspective solution in the case of monoisotopic elements, for which ID-MS cannot be used.