Use of Ga for mass bias correction for the accurate determination of copper isotope ratio in the NIST SRM 3114 Cu standard and geological samples by MC-ICPMS

Abstract

The absolute determination of the Cu isotope ratio in NIST SRM 3114 based on a regression mass bias correction model is performed for the first time with NIST SRM 944 Ga as the calibrant. A value of 0.4471 ± 0.0013 (2SD, n = 37) for the ⁶⁵Cu/⁶³Cu ratio was obtained with a value of +0.18 ± 0.04‰ (2SD, n = 5) for δ⁶⁵Cu relative to NIST 976. The availability of the NIST SRM 3114 material, now with the absolute value of the ⁶⁵Cu/⁶³Cu ratio and a δ⁶⁵Cu value relative to NIST 976 makes it suitable as a new candidate reference material for Cu isotope studies. In addition, a protocol is described for the accurate and precise determination of δ⁶⁵Cu values of geological reference materials. Purification of Cu from the sample matrix was performed using the AG MP-1M Bio-Rad resin. The column recovery for geological samples was found to be 100 ± 2% (2SD, n = 15). A modified method of standard-sample bracketing with internal normalization for mass bias correction was employed by adding natural Ga to both the sample and the solution of NIST SRM 3114, which was used as the bracketing standard. An absolute value of 0.4470 ± 0.0013 (2SD, n = 37) for ⁶⁵Cu/⁶³Cu quantified in this study was used to calibrate the ⁶⁹Ga/⁷¹Ga ratio in the two adjacent bracketing standards of SRM 3114, their average value of ⁶⁹Ga/⁷¹Ga was then used to correct the ⁶⁵Cu/⁶³Cu ratio in the sample. Measured δ⁶⁵Cu values of 0.18 ± 0.04‰ (2SD, n = 20), 0.13 ± 0.04‰ (2SD, n = 9), 0.08 ± 0.03‰ (2SD, n = 6), 0.01 ± 0.06 (2SD, n = 4) and 0.26 ± 0.04‰ (2SD, n = 7) were obtained for five geological reference materials of BCR-2, BHVO-2, AGV-2, BIR-1a, and GSP-2, respectively, in agreement with values obtained in previous studies.