AUTHOR=Yang Zhaoping , Jackson Simon Edward , Skulski Thomas 

TITLE=Characterization of Four Copper Materials for Application as Reference Materials for High Precision Copper Isotope Analysis by Laser Ablation Inductively Coupled Plasma Multi-Collector Mass Spectrometry

JOURNAL=Frontiers in Chemistry

VOLUME=9

YEAR=2021

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2021.617205

DOI=10.3389/fchem.2021.617205

ISSN=2296-2646

ABSTRACT=<p>Laser ablation inductively coupled plasma multi-collector mass spectrometry (LA-MC-ICP-MS) allows rapid, <italic>in situ,</italic> highly precise measurements of Cu isotope ratios of native Cu and Cu-bearing minerals. However, the National Institute of Standards and Technology Cu-metal isotope standard NIST SRM976 that is commonly used to calibrate LA-MC-ICP-MS Cu isotope measurements of native Cu is no longer available. We have investigated the suitability of four Cu metal materials, SSC-1, SSC-3 and SSC-4 (cathode Cu metal rods) and CUPD-1 (Cu anode sawings), originally developed by the Canada Centre for Mineral and Energy Technology (CANMET) as certified reference materials for trace element analysis, as Cu isotope reference materials for LA-MC-ICP-MS analysis and solution nebulization (SN) of Cu. The Cu isotopic composition and homogeneity of these four materials were characterised by SN- and LA-MC-ICP-MS, and are reported for the first time. The bulk Cu isotopic compositions, expressed as δ<sup>65</sup>Cu<sub>SRM976</sub> in per mil (‰) relative to NIST SRM976 with combined uncertainties (<italic>U</italic>, <italic>k</italic> = 2), of SSC-1, SSC-3 and SSC-4, determined utilizing SN-MC-ICP-MS, are identical within analytical uncertainty at 0.03 ± 0.07‰ (<italic>n</italic> = 29), 0.04 ± 0.04‰ (<italic>n</italic> = 28), and 0.05 ± 0.08‰ (<italic>n</italic> = 29), respectively; the composition of CUPD-1 is 2.14 ± 0.08‰ (<italic>n</italic> = 28). The compositions are 0.01 ± 0.07‰ (<italic>n</italic> = 29), 0.04 ± 0.06‰ (<italic>n</italic> = 29), 0.03 ± 0.06‰ (<italic>n</italic> = 28) and 2.15 ± 0.06‰ (<italic>n</italic> = 28), respectively, relative to the European Reference Material ERM®-AE633 Cu isotope standard. The Cu isotope homogeneity of the four new reference materials was assessed by determining whether multiple individual <italic>in situ</italic> Cu isotope measurements made by LA-MC-ICP-MS analysis (43 µm spot size), using each of the other three reference materials as a calibrator, approximate a single normal distribution. We also investigate whether there are statistically significant differences between the mean δ<sup>65</sup>Cu values of three independent data sets for each of the Cu isotope reference materials using one-way analysis of variance (ANOVA). Normality tests (graphical assessment of normal distribution quantile-quantile plots, and the Shapiro-Wilk, Jarque-Bera and reduced chi-squared statistic tests) show that: 1) the Cu isotope data acquired on SSC-1, SSC-3, SSC-4 and CUPD-1 do not depart significantly from a normal distribution, 2) the scatter of the Cu isotope data is due to analytical uncertainty with 95% confidence, and 3) there are no other significant sources of scatter; e.g. heterogeneity of the reference materials. The results of one-way ANOVA reveal that the mean difference of the δ<sup>65</sup>Cu value for each of the reference materials SSC-1, SSC-3, SSC-4 and CUPD-1 is statistically not significant at the 0.05 level. The mean δ<sup>65</sup>Cu<sub>SRM976</sub> values with combined uncertainties (<italic>U</italic>, <italic>k</italic> = 2) of SSC-1, SSC-3, SSC-4 and CUPD-1, determined by LA-MC-ICP-MS using each of the other three reference materials as a calibration standard, are 0.03 ± 0.09‰ (<italic>n</italic> = 132), 0.05 ± 0.09‰ (<italic>n</italic> = 154), 0.03 ± 0.09‰ (<italic>n</italic> = 144) and 2.14 ± 0.10‰ (<italic>n</italic> = 106), respectively. These values are in agreement with those determined by SN-MC-ICP-MS analysis at the 95% confidence level and have excellent precision (2 s.d. ≤ 0.10‰). These results suggest that SSC-1, SSC-3, SSC-4 and CUPD-1 can be considered isotopically homogeneous at a spatial resolution of 43 μm, and they are suitable reference materials for calibration and quality control of <italic>in situ</italic> and solution nebulization Cu isotope analyses of Cu.</p>