Measurement of the molar mass of the 28Si-enriched silicon crystal (AVO28) with HR-ICP-MS

Abstract

Accurate molar mass measurement of the highly ²⁸Si-enriched crystal (AVO28) plays a crucial role in the accurate determination of the Avogadro constant and thus the new definition of the kilogram. At the National Institute of Metrology (NIM), a high resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) combined with the ‘virtual element’ isotope dilution mass spectrometry (VE-IDMS) method was used to determine the molar mass of the AVO28 material. The key advantage of this approach is that it can separate interferents, such as ²⁸Si¹H, completely from silicon ions when using medium mass resolution mode (M/ΔM ∼ 5000). During the sample preparation, AVO28, WASO 04, and enriched ²⁹Si and ³⁰Si materials were dissolved with 25% (mass fraction) tetramethylammonium hydroxide (TMAH), and the amounts of TMAH in prepared sample solutions were finally diluted to 0.025% to maximally reduce the potential interference of nitrogen or carbon based polyatomic species on silicon isotopes. Dead time correction and mass bias correction were also studied and performed. As a result, an average molar mass of four pieces of the AVO28 material was determined to be 27.97696905 g mol⁻¹ with an associated relative standard uncertainty of 9.4 × 10⁻⁹ (k = 1). This result is in between the reported results of the Physikalisch-Technische Bundesanstalt (PTB, Germany), the National Metrology Institute of Japan (NMIJ, Japan), the National Institute of Standards and Technology (NIST, USA) and the National Research Council (NRC, Canada), with its uncertainty level very close to that of PTB and NRC.