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Eingeladener Vortrag
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The calibration of isotope ratio measurements is an ongoing challenge since instrumental isotope fractionation (IIF) has been detected in mass spectrometry (MS). There is a variety of approaches which either bypass IIF such as delta measurements or refer to reference materials (RMs) and thus shifting the problem of calibration to somebody else: the RM producer. For certifying isotope RMs with absolute isotope ratios only a few approaches are available, namely the isotope mixture approach, the double spike approach, the mass bias regression model and total evaporation in TIMS. All of them require either enriched isotopes, isotope RMs of another element or an RM for correcting residual error. As the enriched isotopes required for the isotope mixture and the double spike approach need to be fully characterized beforehand, all mentioned calibration approaches require a standard.
Here, a new and standard-free calibration approach for obtaining absolute isotope ratios of multi-isotopic elements has been developed. The underlying principle is that each MS suffers from IIF and thus yields a specific isotope fractionation line in a three-isotope diagram. When applying a second MS featuring a different ionization mechanism, we obtain a second isotope fractionation line with a different slope in the same three-isotope diagram. In both cases the absolute isotope ratios range somewhere on the isotope fractionation line. Consequentially, the intersect of both lines yield the absolute isotope ratios of the measured sample. This theory has been tested by measuring Cd and Pb isotope ratios of suitable isotope RMs with a TIMS and an ICP-MS, both equipped with multi-collector array. During the measurements the ionization conditions were changed such that different extent of the isotope fractionation has been achieved. With the resulting data set the theory described above could be verified. The obtained absolute isotope ratios were metrologically compatible with the certified isotope ratios. The remaining average bias of -5 ‰ can be reduced with further improvements. The calibration approach is universal and can be applied to any multi-isotopic element and it is not limited by the type of the mass spectrometer.
Isotope reference materials
(2022)
The variation of isotope ratios is increasingly used to unravel natural and technical questions. In the past, the investigation and interpretation of such variations was the field of a limited number of experts. With new upcoming techniques and research topics in the last decades, such as provenance or food authenticity studies, the number of published isotope data strongly increased. Instrumental developments such as the enhancement of inductively coupled plasma mass spectrometers (ICP-MS) from an instrument for simple quantitative analysis to highly sophisticated isotope ratio machines influenced this process significantly. While in former times only experts in mass spectrometry were able to produce reliable isotope data, nowadays many laboratories, never been in touch with mass spectrometry before, produce isotope data with an ICP-MS. Isotope reference materials (iCRM) are indispensable to enable a reliable method validation or in rare cases even SI-traceability. The fast development and the broad availability of ICP-MS also lead to an expansion of the classical research areas and new elements are under investigation. Irrespective of the investigated element or the knowledge of the user all isotope ratio applications require reference materials either for correction of instrumental isotope fractionation, for method validation or to provide a common accepted basis as needed for delta measurements. This presentation will outline the basic principles and illustrate the urgent need for new iCRMs. Consequently, the production and certification of iCRMs will be discussed and illustrated by examples of already completed certification projects. Finally, plans for future iCRMs to be produced at BAM will be presented.
Determination of absolute (SI‐traceable) isotope ratios: The use of Gravimetric Isotope Mixtures
(2023)
The presentation is brief overview on how to use gravimetric isotope mixtures to determine SI traceable isotope ratios. There is no mass spectrometer on earth that directly measures isotope ratios. Mass spectrometers will always measure signal intensity ratios instead. The actual problem is that the measured intensity ratios differ more or less from the isotope ratios. The difference can be up to more than 10 % in case of lithium while it‘s below 1 % in case of the heavier elements like lead or uranium. Consequently, the signal intensity ratios are expressed for example in V/V depending on the type of mass spectrometer you are using, while the isotope ratios are expressed in mol/mol. This phenomenon is called Instrumental Isotopic Fractionation (or short IIF) but the more common name is still mass bias (even though this name is not entirely correct). To convert the measured into the isotope ratio usually a simple multiplication with a so-called correction (or short K) factor is done. Therefore, the problem is to determine the K factor. In absence of isotope reference materials the golden route is via gravimetric isotope mixtures, which will be explained within the presentation.
The continuous improvement of analytical procedures using multi-collector technologies in ICP-mass spectrometry has led to an increased demand for isotope standards with improved homogeneity and reduced measurement uncertainty. For magnesium, this has led to a variety of available standards with different quality levels ranging from artefact standards to isotope reference materials certified for absolute isotope ratios. This required an intercalibration of all standards and reference materials, which we present in this interlaboratory comparison study. The materials Cambridge1, DSM3, ERMAE143, ERM-AE144, ERM-AE145, IRMM-009 and NIST SRM 980 were cross-calibrated with expanded measurement uncertainties (95% confidence level) of less than 0.030‰ for the δ25/24Mg values and less than 0.037‰ for the δ26/24Mg values. Thus, comparability of all magnesium isotope delta (δ) measurements based on these standards and reference materials is established. Further, ERM-AE143 anchors all magnesium δ-scales to absolute isotope ratios and therefore establishes SI traceability, here traceability to the SI base unit mole. This applies especially to the DSM3 scale, which is proposed to be maintained. With ERM-AE144 and ERM-AE145, which are product and educt of a sublimation-condensation process, for the first time a set of isotope reference materials is available with a published value for the apparent triple isotope fractionation exponent θapp, the fractionation relationship ln α(25/24Mg)/ln α(26/24Mg).
The determination of the mass fractions of bromide, sulfate, and lead as well as the isotopic composition of the lead (expressed as the molar mass and the amount fractions of all four stable lead isotopes) in an aqueous solution of sodium chloride with a mass fraction of 0.15 g/g was the subject of this comparison. Even though the mass fractions ranged from 3 μg/g (bromide) to 50 ng/g (lead), almost all results reported agreed with the according KCRVs.
An introduction into absolute isotope ratios is given, with application fields ranging from fundamental science to geochronology and forensics. This is followed by a proposal for developing new calibration approaches for obtaining absolute isotope ratios at unrivaled uncertainty levels. This new developments will set the basis for improvements in all scientific fields.
An international comparison study on the accurate determination of the molar mass M(Si) of silicon artificially enriched in 28Si (x(28Si) > 0.9999 mol mol−1) has been completed. The measurements were part of the high level CCQM-P160 pilot study assessing the ability of National Metrology Institutes (NMIs) and Designated Institutes (DIs) to make such measurements at the lowest possible levels of measurement uncertainty and to identify possible difficulties when measuring this kind of sample. This study supports the molar mass measurements critical to disseminating the silicon route to realizing the new definitions for the kilogram and the mole. Measurements were also made by one external research institute and an external company. The different institutes were free to choose their experimental (mass spectrometric) set-ups and equipment, thereby enabling also the comparison of different techniques. The investigated material was a chemically pure, polycrystalline silicon material. The subsequent modified single crystalline secondary product of this material was intended for the production of silicon which was used for two additional spheres in the context of the redetermination of the Avogadro constant NA, required for the revision of the International System of Units (SI) via fundamental constants which came into force from May 2019. The CCQM pilot study was organized by Physikalisch-Technische Bundesanstalt (PTB). Aqueous silicon solutions were shipped to all participating institutions. The data analysis as well as the uncertainty modelling and calculation of the results was predefined. The participants were provided with an uncertainty budget as a GUM Workbench® file as well as a free software license for the duration of the comparison. The agreement of the values of the molar mass (M(Si) = 27.976 942 577 g mol−1) was excellent with ten out of 11 results reported within the range of relative uncertainty of 1 × 10−8 required
for the revision of the SI.