Radiogenic and stable Ce isotope measurements by thermal ionisation mass spectrometry
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 (CeLMV) on the 138Ce/142Ce ratio is 0.02257053 ± 0.00000061 (27 ppm, 2 s.d., n = 48). The new CeLMV 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.