High-precision Cu and Zn isotope analysis by plasma source mass spectrometry

Abstract

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 65 Cu/ 63 Cu and 66 Zn/ 64 Zn data, an as yet unidentified source of systematic drift in the 67 Zn and 68 Zn 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 65 Cu/ 63 Cu and 66 Zn/ 64 Zn 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.

Copper and tin isotopic analysis of ancient bronzes for archaeological investigation: development and validation of a suitable analytical methodology

“…For Cu isotopic analysis, two elements are commonly used for this purpose, Zn [5,[43][44][45][46][47][48] and Ni [49][50][51][52][53][54]. According to the literature, both elements seem to be well-suited for mass bias correction affecting the Cu isotope ratio, although marginal precision improvements have been recorded when Ni is used [49][50][51][52][53][54].…”

Assessment of international reference materials for isotope-ratio analysis (IUPAC Technical Report)

“…In addition, isobaric interferences need to be evaluated and corrected for accordingly. For all techniques, the chemical preparation prior to the actual measurement can be laborious and may require treatments from sample digestion to matrix separation (e.g., via ion exchange) [25][26][27][28]. If possible, the reference materials are subjected to the same procedures, thus adhering to the IT principle [13,14].…”

A simple combined sample–standard bracketing and inter-element correction procedure for accurate mass bias correction and precise Zn and Cu isotope ratio measurements

“…11,15,16 In 2004 Mason et al 4 developed the so-called modified sample-standard bracketing technique (m-SSB) to account for changes in mass bias that are not adequately quantified by the d-SSB. The m-SSB technique is a combined samplestandard bracketing and inter-element correction procedure, whereby samples and standards are doped and the d-values (deviation of the isotope ratio of the sample relative to that of a reference standard expressed as parts per mil, see below) calculated for the dopant are subtracted from the measured dvalues of the analyte, using the assumption that f Cu E f Zn .…”

“…2 Mass bias varies significantly on a temporal scale of seconds to days 3,4 and incorporates instrumental mass discrimination and nonspectral matrix effects. The causes of this phenomenon are not fully understood, but probably arise from a combination of supersonic expansion of the neutral plasma into the vacuum between the sample and skimmer cones 5 and space-charge effects in the wake of the skimmer cone.…”