In situ determination of Cu isotope ratios in copper-rich materials by NIR femtosecond LA-MC-ICP-MS

Abstract

We have developed a new method for in situ measurement of Cu isotopic compositions of pure Cu metal and Cu-rich minerals using a near-infrared (NIR) femtosecond laser ablation (LA) system with a laser spot diameter of 15 μm, combined with multiple collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). The Cu-rich minerals include native copper (Cu), cuprite (Cu₂O), chalcocite (Cu₂S) and chalcopyrite (CuFeS₂). Time resolved analysis (TRA) was utilized for the acquisition of each individual analysis, and a standard-sample-standard bracketing technique was applied to correct the instrumental mass fractionation and drift. To evaluate the precision and accuracy of our laser ablation technique, Cu isotopic compositions of the same sample materials were also measured by conventional solution MC-ICP-MS using the same MC-ICP-MS system. The long-term analytical reproducibility of Cu isotopic ratio measurements of pure copper metal sample over a 6 month period using laser ablation analysis was 0.05‰ (2SD), which is comparable to that obtained by conventional solution MC-ICP-MS analysis (0.05‰, 2SD). The copper isotopic compositions measured relative to NIST-SRM 976 Cu standard (pure Cu metal) both by laser and solution MC-ICP-MS techniques are in excellent agreement for pure Cu metal and native copper samples. Copper isotope ratios of cuprite samples could be determined using NIST-SRM 976 Cu standard, while matrix-matched standards are required for reliable in situCu isotope analysis of chalcocite and chalcopyrite samples. Repeated measurements of Cu isotope ratios for all samples provide an analytical precision of better than 0.14‰ (2SD). The femtosecond LA-MC-ICP-MS method provides a rapid and effective approach to determine the Cu isotopic composition of pure copper metal and typical copper ore minerals at analytical scales down to 15 μm, with comparable precision and accuracy to that attainable with conventional solution nebulization techniques, without requiring time-consuming chemical separation procedures.