Novel concept for the mass spectrometric determination of absolute isotopic abundances with improved measurement uncertainty: Part 3—Molar mass of silicon highly enriched in 28Si

A novel method of isotope amount ratio measurements using state-of-the-art techniques of a double focusing sector field mass spectrometer combined with isotope dilution mass spectrometry (IDMS) has been applied connecting analytical chemistry with metrology in chemistry aiming at the determination of the Avogadro constant (NA). The molar mass M(“Si28”) and the corresponding isotopic composition of an artificial silicon crystal material highly enriched in the 28Si isotope has been measured for the first time using a combination of a modified IDMS- and a multicollector-ICP-mass spectrometer (MC-ICP-MS) technique. A value M(“Si28”) = 27.97697027(23) g/mol has been determined. This corresponds to a relative uncertainty urel = 8.2 × 10−9 (k = 1). From this silicon crystal material two 1 kg spheres were manufactured which are used by the International Avogadro Coordination (IAC) in order to reassess (NA) with an associated relative measurement uncertainty urel(NA) ≤1 × 10−8. The experiment presented here is the advancement and completion of parts 1 and 2 of this series of papers, describing the theoretical and general experimental applicability of the novel method. The current work summarizes the experimental findings aiming at the determination of the molar mass of the “Si28” material with the lowest uncertainty possible so far. The experimental prerequisites and bottlenecks for examining this highly enriched silicon material as well as experimental proofs for the verification of the presented results are described in detail. The experimental results are supplemented by an uncertainty budget according to the Guide to the Expression of Uncertainty in Measurement (GUM).

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► Measurement of the molar mass of silicon highly enriched in 28Si.
► Molar mass enables new determination of Avogadro constant with lowest uncertainty so far.
► Extended experimental proof of applicability of MC-ICP-MS and IDMS for molar mass determination.
► Measurement uncertainty of the molar mass according to GUM.