Calibrating NIST SRM 683 as a new international reference standard for Zn isotopes

Abstract

Zinc isotopes have been increasingly applied in cosmochemistry, geochemistry, and environmental sciences. A direct comparison of data obtained in different laboratories is a prerequisite to these applications. The JMC 3-0749L Zn solution is the international primary standard for Zn isotopes, but it is nearly exhausted and no longer available. Thus, it is necessary to calibrate a new reference material for Zn isotope analysis. Herein, we measured the Zn isotope composition and examined the homogeneity of the Standard Reference Material 683 of the National Institute of Standard and Technology (NIST SRM 683, a pure Zn metal nugget of 140 grams) with the double-spike method using Multiple Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS). Five metal nuggets of NIST SRM 683 were intensively sampled by micro-drilling, and they exhibited δ⁶⁶Zn_JMC-Lyon of 0.13 ± 0.06‰ (here and throughout the text, errors are given as two-standard deviation (2SD), N = 32), 0.13 ± 0.04‰ (N = 31), 0.13 ± 0.04‰ (N = 30), 0.13 ± 0.04‰ (N = 30), and 0.12 ± 0.06‰ (N = 30). These values were identical to those of two Zn metal nuggets (0.11 ± 0.02‰ and 0.12 ± 0.02‰) previously reported, which confirmed the accuracy of our data and the isotopic homogeneity of NIST SRM 683. We also digested one whole nugget with δ⁶⁶Zn_JMC-Lyon of 0.12‰ ± 0.04‰ (N = 44), which was identical to that of the drilling samples. Our results indicated that within our analytical precision of 0.04, NIST SRM 683 is homogeneous in Zn isotopic compositions with δ⁶⁶Zn_JMC-Lyon of 0.12 ± 0.04‰ (N = 295) at the 95% confidence level, which is close to the Zn isotope value of bulk silicate Earth. Therefore, we believe that NIST SRM 683 is a good candidate for the next primary standard for Zn isotopes.