Evaluation of the Residual Mass Fractionation in High-Precision Cr Isotopic Analysis with TIMS

The Cr isotope ratios of terrestrial and extra-terrestrial materials are emerging as one of the most important tracers in geosciences. Previous studies on Cr isotopic measurements using TIMS have found that there is residual Cr isotopic fractionation between the mass-fractionation-corrected Cr-53/Cr-52 and Cr-54/Cr-52 ratios, which may cause an offset of obtained ratios from the reference values. The residual fractionation was thought to be caused by the evaporation of Cr-oxide species during thermal ionisation, but the mechanism by which this residual fractionation could be reduced remained unclear. Here we revisit the issue of residual fractionation and propose that this problem can be alleviated by utilising W filaments instead of conventionally used Re filaments for Cr ionisation. Using W filaments, the formation of CrO+ was suppressed during heating as the filament temperature was similar to 100 degrees C lower than when Re filaments were used. In repeated measurement of a carbonaceous chondrite, the intermediate precisions of Cr-53/Cr-52 and Cr-54/Cr-52 ratios in the W filament runs were two to three times better than those of the Re filament runs. Therefore, the new finding of this study will be of key importance for future studies of Cr isotopes for terrestrial and extra-terrestrial materials.

Automated summary

The Cr isotope ratios of terrestrial and extra-terrestrial materials are important tracers in geosciences. Previous studies found residual Cr isotopic fractionation in measurements, but the mechanism to reduce this fractionation remained unclear. This study proposes the use of W filaments instead of Re filaments for Cr ionisation to alleviate the residual fractionation, resulting in improved precision in Cr isotope ratios. This finding is of key importance for future studies of terrestrial and extra-terrestrial materials.