Evaluation of the residual mass fractionation in high-precision Cr isotopic analysis with TIMS

Abstract

Remarkable progress has been made in cosmochemical studies using precise Cr isotope analysis. In Cr isotope measurements by TIMS, the ⁵³Cr/⁵²Cr and ⁵⁴Cr/⁵²Cr ratios are obtained by correcting for mass fractionation using a constant ⁵⁰Cr/⁵²Cr ratio. However, a linear correlation has been observed between the ⁵³Cr/⁵²Cr and ⁵⁴Cr/⁵²Cr ratios after mass fractionation correction, with a slope of mu⁵⁴Cr/mu⁵³Cr = ~2. In this study, the residual isotopic fractionation was evaluated by repeated measurements of Cr standards using TIMS. Data were acquired by the 3-line multistatic method and compared with analyses by Re and W filaments. The residual isotope fractionation was confirmed in both analyses, but the slopes of the correlations were different: mu⁵⁴Cr/mu⁵³Cr = 1.93 for Re filament and 1.08 for W filament. The ⁵³Cr/⁵²Cr and ⁵⁴Cr/⁵²Cr ratios were several times more precise for W than for Re filament measurements. This difference is attributed to the fact that the Re filament temperature during analysis was about 100 ^oC higher than that of W, resulting in the formation of Cr oxides. In Cr isotope analysis by TIMS, it is important to use W filaments and operate at a lower temperature to suppress oxide formation as much as possible in order to improve the precision.