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 (epsilon Ce-138) and mass dependent (delta(142) Ce) Ce isotope variations by Thermal Ionisation Mass Spectrometry (TIMS) are presented in this study. We developed a static acquisition method associated with 10(13) Omega amplifiers that allows (i) the precise determination of the Ce-140 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 Ce-138/Ce-142 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 Ce-140 isotope peak onto Ce-136 and Ce-138. Finally, the 10(13) Omega 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 Ce-136-Ce-138-Ce-140 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 delta Ce-142 is obtained for the metal runs and is about +/-0.028 parts per thousand. 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.