Radiogenic and stable Ce isotope measurements by thermal ionisation mass spectrometry

Abstract

Techniques for the separation of Cerium (Ce) from silicate matrices and for the analysis of radiogenic (ε¹³⁸Ce) and mass dependent (δ¹⁴²Ce) Ce isotope variations by Thermal Ionisation Mass Spectrometry (TIMS) are presented in this study. We developed a static acquisition method associated with 10¹³ Ω amplifiers that allows (i) the precise determination of the ¹⁴⁰Ce peak tailing effect on the lighter Ce isotopes and (ii) the reduction of the counting time necessary to obtain a high precision isotopic composition. The long-term reproducibility obtained for our Ce reference material (Ce_LMV) on the ¹³⁸Ce/¹⁴²Ce ratio is 0.02257053 ± 0.00000061 (27 ppm, 2 s.d., n = 48). The new Ce_LMV has been calibrated against the commonly used AMES reference material (0.02257426 ± 0.00000068 (30 ppm, 2 s.d., n = 25)). The static cup configuration also allows a more stable determination of the tailing effect from the ¹⁴⁰Ce isotope peak onto ¹³⁶Ce and ¹³⁸Ce. Finally, the 10¹³ Ω amplifiers permit a better determination of the tailing effect during low voltage measurements. A new method to measure mass dependent Ce isotope variations based on the double spike method has also been developed. Uncertainty propagation calculations demonstrate that the best spike mixture is a ¹³⁶Ce–¹³⁸Ce–¹⁴⁰Ce triple spike. We have calibrated an in-house triple spike used to correct for the instrumental mass bias effect and fractionation of Ce isotopes during Ce separation. Numerical simulations demonstrate the effect of potential isobaric interferences from Ba, Nd and La and highlight the need for an efficient chemical separation. Two loading techniques were tested for triple spike measurements (oxide and metal). The best reproducibility for δ¹⁴²Ce is obtained for the metal runs and is about ±0.028‰. Although this technique has been developed for silicate matrices, the mass spectrometry method can be applied to other matrices such as carbonates. The newly calibrated Ce reference material is available to the community upon request.