Low Uncertainty Reverse Isotope Dilution ICP-MS Applied to Certifying an Isotopically Enriched Cd Candidate Reference Material: A Case Study

An analytical method is presented based on reverse isotope dilution single detector inductively coupled plasma magnetic sector mass spectrometry (ID-ICP-SMS) and applied to the specific case of the certification of a 111Cd enriched candidate Cd spike calibration material (nominal mass fraction 10 mg kg-1 in 5 % HNO3 solution). Uncertainty propagation was used as a tool for both determining the analytical approach and validating it. The robustness of close to ‘exact matching’ reverse IDMS to correction of measured isotope intensities for multiplicative (mass discrimination) and (semi)additive effects (dead time, instrumental background and isobaric interference) is discussed. The very low experimental relative standard deviation (0.08 %) of eight replicate determinations indicates the likeliness that all significant sources of uncertainty are taken into account in the low final combined uncertainty statement (Uc = 0.35 %, k = 2). IRMM-621 was used as comparator. Uncertainties on IUPAC isotopic abundances of natural 111Cd and 112Cd, for the natural Cd solution involved between the two enriched materials, formed nearly 60 % of Uc. The repeatability of the isotope ratio measurements contributed less than 10 %. Correction for procedural blank necessitated calculations somewhat unusual (potential contamination of an enriched material with natural Cd). The procedure also involved a quadrupole based ICP-MS judged to be appropriate for the characterisation of the isotopic composition. For comparison purposes, direct IDMS results are simulated using identical experimental input data. Finally, a significant background signal in the 106-116 mass region, observed only with the magnetic sector instrument, was attributed to argon based isobaric interferences.JRC.D.4-Isotope measurement