High-precision cerium isotope analysis by thermal ionization mass spectrometry using the Ce+ technique

Abstract

The ¹³⁸La–¹³⁸Ce isotope system has been regarded as a useful radiogenic tracer for geochronology studies. Compared to the commonly-used CeO⁺ technique, the measurement of Ce isotope ratios as Ce⁺ is more straightforward and more advantageous, but it is challenging due to the severe isobaric interference of ¹³⁸Ba on ¹³⁸Ce and large variations in relative abundances of all Ce isotopes. In this study, a novel method has been developed for high-precision measurement of Ce isotope ratios by thermal ionization mass spectrometry (TIMS) as Ce⁺. A newly-developed film porous ion emitter (FPIE) was used to enhance the ionization of Ce as Ce⁺ ions. The employment of TaF₅ as an activator significantly suppressed the Ba⁺ isobaric interference signal. ¹⁴⁰Ce was proposed to be an alternative reference Ce isotope as there is no isobaric interference on ¹⁴⁰Ce and complicated peak tailing correction can be avoided. The combinations of diverse amplifiers (10¹⁰ Ω, 10¹¹ Ω, 10¹² Ω and 10¹³ Ω) were used for the measurement of Ce isotope ratios as Ce⁺ and ¹³⁷Ba was monitored simultaneously on a 10¹³ Ω amplifier for ¹³⁸Ba interference correction. The reproducibility of Ce isotope ratios obtained was ca. 10-fold better than the previously published Ce⁺ results and even comparable with that obtained using the more laborious CeO⁺ techniques. This method was further applied for the analysis of reference rock samples and uranium ores of world-wide origin. The analytical results demonstrated that Ce isotope ratios could be a promising signature for the nuclear forensic investigation to identify the source of unknown nuclear materials.