Accurate analysis of Cu isotopes by fs-LA-MC-ICP-MS with non-matrix-matched calibration

The copper isotopic compositions of 12 copper-rich minerals(including native copper,sulfides,carbonates,oxides,and copper chloride)have been determined using a 206 nm ultraviolet femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry(UV-fs-LA-MC-ICP-MS).A pure copper wire NWU-Cu-B and a natural chalcopyrite TC1725 were used as bracketing standards for calibration.Reliable and precise(2SD<0.07‰)δ^(65) Cu values can be obtained using matrix-matched standards under dry plasma condition and calibrated by the standard-sample bracketing method(SSB).However,theδ^(65) Cu values calibrated by non-matrix-matched standards were seriously affected by matrix effect,with a deviation of up to 1.42‰.Therefore,matrix-matched standards are necessary for reliable in situ Cu isotope ratio measurement.Although the analytical precision(2SD)is slightly improved,the use of Ga as an internal standard combined with the SSB correction does not reduce the deviation caused by the matrix effect.However,the matrix effect can be significantly suppressed by adding 8.6μL min^(–1) water into the carrier gas.The matrix-inducedδ^(65) Cu deviation of the TC1725 calibrated against the pure copper NWU-Cu-B was reduced from 0.99‰in dry plasma mode to 0.03‰in wet plasma mode and achieved a long-term reproducibility of 0.10‰(2SD).For the Cu isotopic compositions of 12 natural copper-rich minerals determined under wet plasma mode,the deviation ofδ^(65) Cu was less than 0.13‰if the mineral is homogeneous.These results indicate that the non-matrixmatched standardization can be achieved by fs-LA-MC-ICP-MS under wet plasma condition,whether using chalcopyrite or pure copper as the external bracketing standards.