The comparability of the determination of the molar mass of silicon highly enriched in ²⁸Si: results of the CCQM-P160 interlaboratory comparison and additional external measurements

Abstract

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.

Combining Isotope Dilution and Standard Addition—Elemental Analysis in Complex Samples

“…In the first test system, the sample solutions, x, consisted of aqueous TMAH ( w (TMAH) = 0.0006 g/g) as the matrix with traces of the analyte: silicon with natural isotopic composition. The sample solutions were taken from a study described in [ 29 ]. There, the aqueous TMAH served as a blank solution for the measurement of isotope ratios of enriched silicon samples.…”

“…Briefly, the TMAH sample solutions were prepared by dissolving highly concentrated ( w (TMAH) = 0.25 g/g), electronic grade (99.9999%) TMAH from Alfa Aesar™ (Thermo Fisher (Kandel) GmbH, Germany) in highly purified water (resistivity = 18 MΩ cm), the latter prepared using a commercial water purification system (Merck Millipore™, Burlington, MA, USA). All solutions were prepared and stored in precleaned labware (perfluoroalkoxy copolymer PFA) according to a protocol given in [ 29 ]. The spike solutions, y, originated from chemically pure silicon single crystals highly enriched in 30 Si dissolved in aqueous TMAH, whereas reference material z was prepared from well-characterized silicon crystals (material code: WASO04) with known natural-like isotopic composition [ 30 ].…”

Molar mass measurement of a 28Si-enriched silicon crystal with high precision secondary ion mass spectrometry (SIMS)

“…In order to further increase the accuracy of the measured molar mass, ultra-enriched 28 Si crystals (with much lower 29 Si and 30 Si abundance compared to the AVO28 material) were produced, and an international comparison study to determine the molar mass was organized by Physikalisch-Technische Bundesanstalt (PTB, Germany). 6 Two techniques have been used to measure isotopic compositions: the traditional gas isotope ratio measurement, and, the 'virtual element' isotope dilution mass spectrometry (VE-IDMS). These two techniques produced internally consistent results within each technique respectively, 4,7,8 while large discrepancies were found between the different approaches.…”

Certification of a Natural Silicon Reference Material with SI‐Traceable Isotopic Composition

“…2011, Rienitz et␣al . 2020). Because large variations of Si isotopic composition in normal terrestrial materials were observed and thus the uncertainties of the Si isotopic composition and its atomic weight could not be reduced, the Commission on Isotopic Abundances and Atomic Weight of the International Union of Pure and Applied Chemistry (IUPAC) has changed the isotope abundances of Si to the interval [0.92191, 0.92318] ( 28 Si), the interval [0.04645, 0.04699] ( 29 Si) and the interval [0.03037, 0.03110] ( 30 Si), respectively, and the corresponding atomic weight to the interval [28.084, 28.086] (Wieser et␣al .…”

“…As one of the important sources of oceanic silicon, much attention has been attracted by dissolved Si, which greatly affects the growth of diatoms in coastal areas, contributing to global carbon fixationan important process proved to be affected by weathering and biological factors (Tréguer and Rocha 2013, Weiss et al 2015, Frings et al 2016, Zhang et al 2020. What is more, as one of the two ways to redefine the SI (International System of Units) unit of mass, the kilogram, silicon isotope ratio measurement has played an important role in the determination of the Avogadro constant (De Bièvre et al 1995, Rienitz et al 2020. Because large variations of Si isotopic composition in normal terrestrial materials were observed and thus the uncertainties of the Si isotopic composition and its atomic weight could not be reduced, the Commission on Isotopic Abundances and Atomic Weight of the International Union of Pure and Applied Chemistry (IUPAC) has changed the isotope abundances of Si to the interval [0.92191, 0.92318] ( 28 Si), the interval [0.04645, 0.04699] ( 29 Si) and the interval [0.03037, 0.03110] ( 30 Si), respectively, and the corresponding atomic weight to the interval [28.084, 28.086] (Wieser et al 2013, Meija et al 2016.…”