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