Anomalous isotopic shifts associated with organic resin residues during cadmium isotopic analysis by double spike MC-ICPMS

Previous studies have reported that matrix effects during analysis by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS), leading to highly anomalous isotopic results, can be derived from ion exchange separation procedures, as resin-derived organics are stripped from the column together with the element of interest. In low concentration samples, where matrix to analyte ratios are high, these artifacts cannot be corrected for using sample-standard bracketing or external normalization techniques to monitor instrumental mass fractionation. It has been suggested that such artifacts can be overcome by implementing a double spike for the correction of instrumental mass fractionation. We present new Cd isotopic results showing that although a double spike reliably corrects for instrumental mass fractionation during sample analysis, the effects of these resin-derived organics introduce very anomalous shifts in bracketing standards that have not been chemically processed using ion exchange resins. These inaccurate results appear to be the consequence of one or more polyatomic interference/s or mass independent isotopic fractionation as opposed to the previously proposed matrix effects. These analytical artifacts can be completely alleviated by the oxidation of samples with solutions of either H₂O₂/HNO₃ or HClO₄/HNO₃ prior to analysis. Although the present results have direct relevance for Cd analysis, it is probable that other isotopic systems are affected by similar resin-derived organics. The increased sensitivity of the next generation of MC-ICPMS instruments means that progressively smaller sample sizes are accessible for analysis, leading to a larger proportion of resin-derived matrix in many samples, such that oxidation of samples will prove to be essential to circumvent the issue of inaccurate isotopic measurement.