Direct determination of uranium and thorium in minerals by time-of-flight mass spectrometry with pulsed glow discharge

Abstract

A direct method of uranium and thorium determination in non-conducting geological samples using time-of-flight mass spectrometry with pulsed glow discharge was proposed. The following rock specimens were analysed: metamict zircon, metamict rinkite, metamict samarskite (Y-Fe-niobate), pyrochlore and jacinth. For sample sputtering a combined hollow cathode cell of high purity aluminium or tantalum hollow cathodes was used. Powdered or monolith samples were pressed into the surface of the powdered metal prior to analysis. Model samples (artificial mixtures of oxides) were proposed for calibration; additionally, relative sensitivity factors, internal standardisation and standard additions were employed. For validation, IAEA artificially prepared uranium ore reference material was analysed. For additional validation, the obtained results for real mineral samples were compared to the results of inductively coupled plasma optical emission spectrometry after sample dissolution and semi-quantitative data of energy dispersive X-ray spectrometry. Limits of detection (3σ) for the designed method were 0.3 ppm for uranium and 0.5 ppm for thorium, which is comparable to laser ablation inductively coupled plasma mass spectrometry. The method was also tested for capability to measure isotope ratios for lead and uranium without specific isotope calibration. Acquired isotopic ratios of uranium and lead corresponded to their natural abundances within the experimental error.