Low uncertainty reverse isotope dilution ICP-MS applied to certifying an isotopically enriched Cd candidate reference material: A case study

Ivan Trel 0 1 Christophe R. Qutel 0 1 0 Published online March 11, 2005 Address reprint requests to Dr. C. R. Qutel, European Commission Joint Research Center, Institute for Reference Materials and Measurements , 111 Retieseweg, B-2440 Geel, Belgium 1 European Commission Joint Research Center, Institute for Reference Materials and Measurements , Geel, Belgium 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 of the mean (0.08%) of eight replicate determinations indicated that all significant sources of uncertainty had probably been taken into account for the estimation of the final combined uncertainty statement (Uc 0.17%, k 1). IRMM-621 was used as comparator. Uncertainties on IUPAC isotopic abundances of 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 somewhat unusual calculations (potential contamination of an enriched material with natural Cd). The procedure also involved a quadrupole based ICP-MS judged to be appropriate for the characterization 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. (J Am Soc Mass Spectrom 2005, 16, 708 -716) 2005 American Society for Mass Spectrometry - Iognized as having the potential to be a primary ratio sotope dilution mass spectrometry (IDMS) was recmethod of measurement [1]. When applied correctly, it can be of practical use in establishing traceability to the SI unit system [2] and accurate results with sufficiently small uncertainties can be achieved. The advantages and disadvantages of the IDMS method have been discussed [3, 4] since its invention nearly 50 years ago. One of the significant advantages over other approaches is that the analyte recovery does not have to be quantitative, providing that isotopic equilibration has been achieved between all of the analyte and added spike material. Commonly there is the reverse IDMS (or two-way IDMS) whereby the spike material is calibrated against a well-characterized assay material. The alternative is direct IDMS (or one-way IDMS) which is faster but requires a spike material already reliably certified once for all for the element content and isotopic composition. Direct IDMS has the potential to lead to relative combined uncertainties on the amount content determination of 1% (k 2) even with quadrupole ICP-MS (inductively coupled plasma mass spectrometry) instrumentation [5]. In many cases this is entirely fit for the purpose and a better commercial availability of certified spike materials, carrying SI traceable values with demonstrated small uncertainty, should help the use of direct IDMS to spread. Merck (Darmstadt, Germany) launched the production of new sets of isotopically enriched materials that enable uncertainty targets of 2% (k 2) or better for ID based methods, when correctly applied. A joint project with the Institute for Reference Materials and Measurements (IRMM) [6] was set up to certify isotopically enriched spike calibration solutions. Within that project, a 53Cr enriched solution was recently characterized [7]. It was also decided to certify the isotopic composition and the amount content of a 111Cd enriched solution because of the important demand on measurement of the Cd content by IDMS in a wide range of sample matrices [8 12]. There was no suitable natural Cd certified reference material in our possession that would have allowed us to implement direct IDMS for this certification. Rather than preparing and characterizing one from high purity Cd metal, we developed a reverse ID-ICP-MS procedure, using as a constraint a target relative expanded uncertainty of 0.51% (k 2) on the Cd content. Our paper, through the description of the way we carried out this project, examines the merits and limitations of close to exact matching reverse IDMS involving two isotopically enriched materials (the 111Cd enriched IRMM-621 against the Merck candidate Reference Material). Our experimental results allowed us to investigate in particular the robustness of the correction of measured isotope intensities for multiplicative (mass discrimination) and (semi)additive effects (dead time, instrumental background, and isobaric interference), as well as the potential for low final combined uncertainties, achievable with single detector ICP-MS and not necessarily with more complex instrumentation (multiple detector ICP-MS or thermal ionisation mass spectrometry, TIMS). This work was originally presented at the 2003 European Winter Conference [13]. Instrumentation The measurements were of two kinds and were performed on two different ICP-MS instruments. An Element2 (double focusing single detector) was employed at low mass resolution for the measurement of the IDMS blend solutions. The determination of the isotopic composition (on nonspiked samples) was performed on a quadrupole based ICP mass spectrometer Elan 6000 (Perkin-Elmer Sciex, Ontario, Canada). Both instruments were operated with a noncooled quartz minicyclonic spray chamber (P/N 809-0188, Cinnabar, Glass Expansion, West Melbourne, Australia), fitted with a MicroFlow ( 50 L min 1) PFA nebulizer (no. 50-1142, CPI International, Amsterdam, The Netherlands) for the Element2, and with a forced low-flow concentric glass nebulizer (P/N 1110720, MicroMist, Glass Expansion, West Melbourne, Australia) for the Elan 6000. All the measurements were carried out in the pulse counting mode, and the dead time effects [14] were corrected following specific methods described elsewhere [15] (using Method 2 for the Element2 and Method 4 for the Elan 6000). Dead time values of 14 4 ns (k 2) and 57 10 ns (k 2) were found for the Element2 and the Elan 6000, respectively. Reagents, Reference Materials, and Labware A Milli-Q system (Millipore, Bedford, MA) was used for the production of ultra-pure water. All dilutions were prepared using ultra pure, concentrated nitric acid (Ultrex, J.T. Baker, Phillipsburg, NJ). The 111Cd enriched ca (...truncated)