Precise and Accurate Mass-Independent Chromium Isotope Measurement by Total Evaporation Mode on Thermal Ionization Mass Spectrometry (TE-TIMS) at 200 Ng Level

Mass-independent chromium (Cr) isotope ratios measurements have been widely used in dating early solar system events and tracing the genetic relationships between different solar system, as well as terrestrial materials. Current analytical techniques need relatively large sample sizes, and this limits the application of Cr isotopes on precious mission return samples and some Cr-poor samples (e.g., BCR-2 with Cr contents of similar to 15 ppm). This paper reports a novel analytical method for mass-independent Cr isotopic ratios measurements using a Thermal Ionization Mass Spectrometer (TIMS) operated in total evaporation (TE) mode. A three-step cation column (AG 50W-X12 resin, 200-400 mesh) chemistry is used to purify Cr in various samples, including chondrites, basalts and peridotites, and the Cr yield is better than 92%. Residual organics from column resins is detrimental to Cr ionization on filaments, we demonstrate that this can be effectively removed by treating samples with H2O2 at 40 degrees C on hotplate. Single Cr measurements on rhenium filaments consume 15 to 20 ng of Cr and sustain Cr-52 of similar to 10V for 10 to 20 minutes. Generally, for one sample of 200 ng Cr, 10-15 ng repeated measurements can be made and 2-standard error precisions of similar to 0.05 and similar to 0.10 for epsilon Cr-53 and epsilon Cr-54*, respectively can be achieved. The reproducibility (the 2standard deviation) for epsilon Cr-53 and epsilon Cr-54 is 0.05 and 0.07, respectively, tested by multiple measurements for DTS-2b (USGS dunite) and NWA 7734 (ordinary chondrite). Concentration and doping test (mainly for Fe) have also been conducted. These tests show that the Fe/Cr must be < 5 % and the measured Fe-56/Cr-52<1 % in order to achieve accurate epsilon Cr-54. Finally, the epsilon Cr-53 and epsilon Cr-54 values measured by normal method on TIMS are systematically higher than those measured by total evaporation method on TIMS, which is potentially caused by non-kinetic mass-dependent fractionation during Cr evaporation and ionization on TIMS. The TE method could reduce this effect that may cause inaccurate mass-independent Cr isotope data on TIMS.