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 (2σ_mean; N = 35) as a working value.