Determination of sub-ng g −1 concentrations of thorium and uranium in microelectronic materials by radiochemical neutron activation analysis

Abstract

A radiochemical neutron activation analysis technique for the determination of thorium and uranium in various high-purity microelectronic materials was developed. It is based on the production of 233Pa and 239Np as indicator radionuclides for Th and U, respectively, and their simultaneous separation by anion exchange from 12 M hydrochloric acid followed by elution and 1 M ammonium fluoride. Using the radiotracer technique, separation yields of Np and Pa and 32 other elements were determined. The method was applied to the determination of Th and U at ng g −1 and sub-ng g −1 levels in high-purity quartz, silicon nitride, polyimide and aluminium oxide. Depending on the kind and purity grade of the sample, limits of detection of 10–100 pg g −1 were achieved. Comparison of the results obtained by this method with those obtained by instrumental neutron activation analysis and inductively coupled plasma mass spectrometry showed satisfactory agreement and illustrates the advantages of this technique when ultra-low concentrations of Th and U are to be determined.