Determination of Thorium and Uranium in High-purity Gadolinium Sulfate by ICP-MS after Solid-phase Extraction

Abstract

The solid-phase extraction technique was successfully applied to extract trace uranium and thorium from high-purity gadolinium sulfate to determine the ppt level of those elements by inductively coupled plasma mass spectrometry (ICP-MS). For thorium analysis, the sample was dissolved in hydrochloric acid, adjusted to pH 2 with aqueous ammonia and passed through a column filled with chelate resin having ethylenediaminetriacetic acid as a functional group. For uranium analysis, the sample was dissolved in ultrapure water, adjusted to pH 2 with sulfuric acid and passed through a column filled with anion exchange resin. The adsorbed thorium and uranium were eluted with a hydrochloric acid solution; these eluates were then analyzed by ICP-MS. This method was applied to the high-purity gadolinium sulfates, which was planned for use in a large water Cherenkov detector in Super-Kamiokande in Japan to detect neutrinos. As a result, the proposed methods have achieved limits of quantification of 1 pg g⁻¹, and 4 pg g⁻¹ for thorium and uranium, respectively. Moreover, recoveries of 95 % to 105 % and the removal rate of about 99.97 % or higher for gadolinium in analyte solution were obtained for both elements.