Effects of Sample Size on Cu and Zn Isotope Ratio Measurements

Using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to measure copper and zinc isotopes, the sample weighting can influence the measurements of copper and zinc isotopic ratios. Different amounts of copper and zinc laboratory reference solutions (CAGS-Cu, CAGS-Zn) were loaded and passed through a column of ion exchange resin. The sample-standard bracketing technique was applied for the measurements of Cu and Zn. The measured data were corrected using certified values of laboratory reference solutions. The results showed δ⁶⁶Cu and δ⁶⁶Zn deviate from true values for small sample weighting of copper and zinc; conversely, the isotopic value tends to the true values for large sample weighting. Procedure blank and isobaric interferences can be ignored for the isotopic values, which seems to indicate that the problem lies with the matrix effect from the Ion Exchange Resin and reagents. Therefore, using values for sample weighting of <4 μg for copper and <1 μg for zinc, accurate isotopic ratios can be guaranteed under these experimental conditions.