Effects of Faraday cup deterioration on Sr and Cr isotope analyses by thermal ionization mass spectrometry

By comparing data from an extensive set of Sr and Cr isotope measurements performed on two different thermal ionization mass spectrometers (TIMS), using three sets of Faraday cups with different usage histories, we assess the effects of Faraday cup deterioration on high-precision isotope measurements by TIMS. We find that dynamic ⁸⁴Sr/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr measurements provide stable and reproducible results over the entire 56 months of this study, regardless of which set of Faraday cups is used. By contrast, static ⁸⁴Sr/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr measurements lead to deviant results, drifts over time, and in general exhibit larger scatter. For the most part, these differences can be attributed to changing Faraday cup efficiencies. For the instruments of this study we find that the center cup is most affected, consistent with this cup often receiving the highest ion beam intensities during measurements conducted in our laboratory. For Cr isotopes, we find that the correlation between mass fractionation-corrected ⁵³Cr/⁵²Cr and ⁵⁴Cr/⁵²Cr ratios observed for static measurements in several prior studies also reflects different Faraday cup efficiencies. Again, the changing efficiency of predominantly the center cup can account for the observed drift and correlation in ⁵³Cr/⁵²Cr and ⁵⁴Cr/⁵²Cr. Multi-static Cr isotope measurements reduce this drift, but still result in a residual correlation between the two ratios, suggesting this correlation in part also reflects unaccounted mass fractionation effects.