Analysis of large samples by neutron activation analysis. Quality assurance aspects

The need for quality assurance in large sample instrumental neutron activation analysis (INAA) requires the development of unconventional methods of quality control. Certified reference materials are not available at the 1-5 kg scale; moreover, inhomogeneities which might affect the accuracy of the real sample analysis would not be reflected in the analysis of a reference material or in-house control sample even when available. Model studies indicate that inhomogeneities with strong gamma ray absorbing properties have the largest effect on the accuracy of the concentrations. The occurrence of these inhomogeneities may be derived from gamma spectrum analysis. Other opportunities for quality assurance are with the calculated estimates of the parameters describing neutron and gamma ray self-attenuation, and eventually through direct assessment after homogenization of the large sample, subsampling and conventional analysis. (author)