Ultra-trace Cu isotope ratio measurements via multi-collector ICP-mass spectrometry using Ga as internal standard: an approach applicable to micro-samples.

The capabilities of Cu isotope ratio measurements are often restricted by the small volumes of sample available and/or their low Cu concentration. In this work, an analytical approach was developed for performing Cu isotopic analysis via multi-collector ICP-mass spectrometry (MC-ICP-MS) at ultra-trace level using Ga as an internal standard for mass bias correction. The minimum concentration of Cu required for accurate and precise isotope ratio measurements was established to be 20 μg L with wet plasma conditions and 5 μg L with dry plasma conditions. The use of Ga as an internal standard for mass bias correction provided several advantages compared to Ni, i.e. improved internal precision on δCu values and lower blank levels. Ga can also be used at a 4-fold lower concentration level than Ni. However, in wet plasma conditions, the signals of ArOH and ArNO interfered with the signals of Ga and Ga, respectively, while in dry plasma conditions, realized by the use of a desolvation unit, Ga suffered from spectral interference from ArNH. These interferences were resolved by using medium mass resolution. For validation purposes, the approach was applied to commercially available blood and serum samples. The δCu values for the samples measured at a concentration level of 5 μg L Cu and 5 μg L Ga using dry plasma conditions were in good agreement with those obtained for isotope ratio measurements at the "standard" concentration level of 200 μg L Cu and 200 μg L Ni using wet plasma conditions. In addition, the δCu values obtained for micro-samples of serum/blood (volume of 100 µL) were in good agreement with the corresponding ones obtained using the "standard" volume for isotopic analysis (500 μL).