High‐Precision Measurement of Stable Cr Isotopes in Geological Reference Materials by a Double‐Spike TIMS Method

Chromium (Cr) isotopes have been widely used in various fields of Earth and planetary sciences. However, high-precision measurements of Cr stable isotope ratios are still challenged by difficulties in purifying Cr and organic matter interference from resin using double-spike thermal ionisation mass spectrometry. In this study, an improved and easily operated two-column chemical separation procedure using AG50W-X12 (200-400 mesh) resin is introduced. This resin has a higher cross-linking density than AG50W-X8, and this higher density generates better separation efficiency and higher saturation. Organic matter from the resin is a common cause of inhibition of the emission of Cr during analysis by TIMS. Here, perchloric and nitric acids were utilised to eliminate organic matter interference. The Cr isotope ratios of samples with lower Cr contents could be measured precisely by TIMS. The long-term intermediate measurement precision of delta Cr-53/52(NIST SRM 979) for BHVO-2 is better than +/- 0.031 parts per thousand (2s) over one year. Replicated digestions and measurements of geological reference materials (OKUM, MUH-1, JP-1, BHVO-1, BHVO-2, AGV-2 and GSP-2) yield delta Cr-53/52(NIST SRM 979) results ranging from -0.129 parts per thousand to -0.032 parts per thousand. The Cr isotope ratios of geological reference materials are consistent with the delta Cr-53/52(NIST SRM 979) values reported by previous studies, and the measurement uncertainty (+/- 0.031 parts per thousand, 2s) is significantly improved.