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

Abstract

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 111 Cd enriched candidate Cd spike calibration material (nominal mass fraction 10 mg kg Ϫ1 in 5% HNO 3 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 (U c ϭ 0.17%, k ϭ 1). IRMM-621 was used as comparator. Uncertainties on IUPAC isotopic abundances of 111 Cd and 112 Cd, for the natural Cd solution involved between the two enriched materials, formed nearly 60% of U c . 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. I sotope dilution mass spectrometry (IDMS) was recognized as having the potential to be a primary ratio method 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 ϳ1% (k ϭ 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 bett...

Measurement uncertainty in single, double and triple isotope dilution mass spectrometry

“…Single IDMS results in a relative expanded uncertainty (k = 2) of 1.4 % falling in the typical range between 1 and 2 % which is known for single IDMS applications. [3,4,8] With double IDMS a relative expanded uncertainty of 0.083 % was obtained which was at the lower end of typical uncertainties (0.1 to 0.5 %). Triple IDMS was applied for the first time to elemental analysis and therefore no typical uncertainties are known.…”

“…[1,2] Although frequently applied as analytical reference procedure and used in metrology in chemistry, only a few publications deal with IDMS measurement uncertainty [3][4][5][6][7][8] and even fewer with the differences in measurement uncertainty between different IDMS approaches. [6][7][8] However, the advantages and disadvantages of different calibration strategies, namely single, double and triple IDMS, have never been compared. In particular, triple IDMS was once applied to the quantification of CO 2 , but never previously to elemental analysis.…”

Characterisation of reference materials by isotope dilution mass spectrometry

“…The advantage of double IDMS over single IDMS, as already explained, has been verified by Tresˇl et al 128 They performed double IDMS for characterising an unknown 111 Cd-spike by using IRMM-621 (a certified 111 Cd-spike) as primary assay. The expanded uncertainty for double IDMS was 0.35%, whereas the expanded uncertainty for the simulated single IDMS resulted in 0.65%.…”

Two-dimensional heart-cut LC-LC improves accuracy of exact-matching double isotope dilution mass spectrometry measurements of aflatoxin B1 in cereal-based baby food, maize, and maize-based feed

“…As primary ratio method, it provides full traceability with shortest possible connection to SI units. This is of particular interest for the production of certified reference materials [ 13 ] or for assigning a value [ 14 ] to materials used in a proficiency testing scheme [ 15 – 17 ].…”