Abstract

A novel and versatile method for the determination of ²³²Th, ²³⁴U, ²³⁵U and ²³⁸U in milling grade ores and intermediates containing high amounts of rare earth elements (REE) was developed. Spectral and non-spectral interferences were eliminated by on-line matrix separation with high performance ion chromatography (HPIC) using 2,6-pyridinedicarboxylic acid (PDCA) as complexing agent. Hyphenation to inductively coupled plasma sector field mass spectrometry (ICP-SFMS) introduced a highly sensitive method of detection. Quantification of uranium and thorium could be established using external calibration and post-column internal standardization. Elemental levels down to concentrations of pg g⁻¹ to ng g⁻¹ in samples containing rare earth elements as the main matrix components could be assessed. The method was validated for ²³⁵U and ²³⁸U by comparison of isotope dilution mass spectrometry (IDMS) results without and with on-line matrix separation using a matrix matching pure cerite carbonate sample containing about 0.5 ng g⁻¹²³⁵U and 65 ng g⁻¹²³⁸U. The results of the external calibration were in good agreement with the IDMS results and overlapped within their uncertainties. ²³²Th was quantified to about 30 ng g⁻¹ (solid material) by on-line HPIC-ICP-SFMS using ²³³U as internal standard. The results were in excellent agreement with those obtained by HPIC-ICP-SFMS using indium as internal standard. Measurement of a series of industrial samples proved the versatility of the method and can give a quick assessment of whether a material is suitable for a further production process or not, since isotope dependent limit values have to be determined.