Determination of Ce isotopes by TIMS and MC-ICPMS and initiation of a new, homogeneous Ce isotopic reference material

Abstract

Radiogenic 138Ce/136Ce ratios in geological and cosmological materials are a valuable tracer and dating tool in geo- and cosmochemistry. However, the variation in global 138Ce/136Ce ratios is small (e.g., 0.03% in ocean island basalts). In addition, the isobaric interference of 138Ba and the influence of the dominant 140Ce ion beam (88.5%) on 138Ce (ca. 0.251%) during mass spectrometric analysis present an analytical challenge. Hitherto employed methods generally dismiss the influence of the 140Ce low-mass peak-tail on the accuracy of the determined 138Ce/136Ce ratios. Therefore, the reported reproducibility ranges only from 0.05 to 0.004% (2RSD). In this study, TIMS and MC-ICPMS are used to determine 138Ce/136Ce ratios in reference materials. The results show that only the measurement of Ce as an oxide species by TIMS, combined with the accurate monitoring and correction of the background between the acquired ion intensities, can yield accurate and reproducible 138Ce/136C ratios. This method achieves a more than two-fold improvement in reproducibility (0.002%; 2RSD) compared to reported methods. The identification and quantification of individual sources of error resulted in a combined standard uncertainty of 0.002% (2RSE) for the method. A comparison of published data for the CeO2 reference material JMC 304 reveals its isotopic heterogeneity. Therefore, a new reference material has been prepared from ultra-pure Ce metal from Ames Laboratory. It is now available for distribution. An initial characterization of the new reference material yielded a 138Ce/136C ratio of 1.33738 ± 0.000005 (2σmean; N = 35) as a working value.