Molar mass of silicon highly enriched in28Si determined by IDMS

Abstract

The molar mass of a new silicon crystal material highly enriched in 28 Si (‘Si28’, x ( 28 Si) >99.99%) has been measured for the first time using a combination of a modified isotope dilution mass spectrometry (IDMS) technique and a high resolution multicollector-ICP-mass spectrometer. This work is related to the redetermination of the Avogadro constant N A with an intended relative measurement uncertainty u rel ( N A ) ⩽ 2 × 10 −8 . The corresponding experimental investigations of the International Avogadro Coordination (IAC) were performed using this novel ‘Si28’ material. One prerequisite of the redetermination of N A is the determination of the isotopic composition and thus molar mass of ‘Si28’ with u rel ( M (‘Si28’)) ⩽ 1 × 10 −8 . At PTB, a molar mass M (‘Si28’) = 27.976 970 27(23) g mol −1 has been determined with an associated relative uncertainty u rel ( M (‘Si28’)) = 8.2 × 10 −9 , opening the opportunity to reach the target uncertainty of N A .

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