Accurate determination of Cu isotope compositions in Cu-bearing minerals using microdrilling and MC-ICP-MS

Abstract

In this study, we provide a microsampling method that determines the Cu isotopic composition of copper-bearing minerals using the microdrill sampling and MC-ICP-MS measurement. Eleven different natural copper minerals were sampled with microdrill and analyzed using MC-ICP-MS after digestion for half an hour on a 140 °C hot plate without chromatography. The Cu isotope results were calibrated using the standard sample bracketing combined with NIST 994 Ga as the internal standard. Cu isotope compositions of the same sample processed with chromatography were also determined to verify the accuracy of this method. Results have shown that δ65Cu of all sample solutions with or without chromatography are overlapped and the deviations between them are less than 0.04‰, indicating this method is applicable to most natural copper-bearing minerals. All the microdrill-sampled copper minerals were directly digested without chemical purification, thus potential matrix elements, including metal cations and some anions, still existed in the solutions. The matrix effects of metal cations and anions were systematically investigated in this study. Results have shown that the δ65Cu deviations are less than 0.05‰ within the uncertainty when the concentration ratios of matrix elements Zn, Fe, Ni, Al, Sb, and Co/Cu + S were less than 2, Ti/Cu + S ≤ 0.8, and Mn/Cu + S ≤ 0.5. This method has the merits of saving time and reagents, low background, good accuracy and reliability as well as improved spatial resolution. It is easier to achieve in most isotope laboratories compared to other microsampling methods such as laser ablation method.