High-precision Cu and Zn isotope analysis by plasma source mass spectrometry: Part 2. Correcting for mass discrimination effects

Two approaches to correct for mass discrimination effects associated with Cu and Zn isotopic measurements on two different MC-ICP-MS instruments (a Micromass IsoProbe and a VG Axiom) have been compared and assessed in detail: (1) sample-standard bracketing (SSB), and (2) the ‘empirical external normalisation’ (EEN) whereby a second element is used to simultaneously correct for mass discrimination. This has provided new insights into the mass discrimination behaviours of Cu and Zn under varying instrumental set-ups, and has allowed improvements to be made to the existing correction procedures. With the SSB approach, mass bias stability is a prerequisite, and matrix components must be removed from the analyte to avoid matrix-related mass discrimination effects. By comparison, the EEN approach requires a degree of mass bias instability, and automatically corrects for matrix-related mass discrimination effects. The EEN correction may therefore appear more robust. However, while the EEN correction yields high-precision ⁶⁵Cu/⁶³Cu and ⁶⁶Zn/⁶⁴Zn data, an as yet unidentified source of systematic drift in the ⁶⁷Zn and ⁶⁸Zn signals through time hinders analyses of ratios incorporating these isotopes. Using the EEN correction where analyte and spike ratios were measured sequentially within a peak-switching protocol led to a three-fold deterioration in precision relative to static measurements. This is consistent with mass bias drift on the scale of a single five-second-measurement integration. For relative ⁶⁵Cu/⁶³Cu and ⁶⁶Zn/⁶⁴Zn ratio measurements, the SSB and EEN corrections give long-term reproducibilities of less then ±0.07‰ (2SD) for pure Cu and Zn reagents. This is sufficient for resolving mass-dependent isotopic variability in natural and anthropogenic materials.