Determination of chromium isotopic composition in various geological material by thermal ionization mass spectrometry

We present a new method for the chemical purification and the measurement of the isotopic composition of Cr by Thermal-Ionization Mass Spectrometry in various geological materials. The separation and purification protocol was adapted to match a wide range of geological and cosmochemical matrices and was tested with terrestrial reference materials, uranium-rich samples and meteorites. The total Cr yield was at least 86% and the Cr/U ratio went from 7.84 x 10(-5) to 5.72 x 10(3) after purification for uranium-rich materials. It also permitted an efficient removal of matrix elements such as Na and Mg and interfering elements such as Fe, Ti and V. Cr isotopic composition was measured using multistatic and multidynamic modes. The multidynamic mode displayed no improvement of the external reproducibility and internal precision compared with the multistatic mode, in contrast with what has been found with other isotope systems. This method gave a reproducibility of 5 ppm and 13 ppm (2 s.d.) for the Cr-53/Cr-52 and Cr-54/Cr-52 ratios respectively, over a period of several months, which represents an improvement of the long-term reproducibility compared with the other published methods. The accuracy of the protocol was tested by measuring terrestrial reference materials (AGV-1, AGV-2, BHVO-1 and BHVO-2) as well as extra-terrestrial samples (Bouvante, Stannern and Waconda), displaying good agreement with published data. This method is thus suited to detect any mass-independent fractionation or nucleosynthetic anomalies in geological materials. (C) 2022 Elsevier B.V. All rights reserved.