Improved Protocol for Isotope Ratio Measurements of a New Silicon Precursor Crystal Extremely Enriched in 28Si while Strongly Depleted in 30Si

An improved measurement protocol for the mass spectrometric determination of the isotopic composition and molar mass of a new polycrystalline silicon precursor material (Si28-34Pr7) highly enriched in 28Si, depleted in 29Si as well as strongly depleted in 30Si is presented using a combination of wet chemistry sample preparation and high resolution multicollector-inductively coupled plasma mass spectrometry (MC-ICP-MS). The silicon material is intended for the production of a single crystal for the pool of spheres highly enriched in 28Si used as metrological national standards for the realization and dissemination of the kilogram and the mole. It shows the largest ratio x(29Si)/x(30Si) ≈ 180 mol/mol so far in comparison with the already available crystals, implying a challenge in both detection and determination with lowest possible uncertainty. The ratio found in natural Si equals approximately 1.5. The reported molar mass determination as part of the X-ray crystal density (XRCD) method was performed using the state-of-the art Virtual-Element Isotope Dilution Mass Spectrometry (VE-IDMS) method. Application and comparison of 1011 Ω and 1013 Ω resistors in the feedback loop of the Faraday cup amplifiers resulted in an average molar mass M(Si28-34Pr7) = 27.976 951 075(62) g mol-1 with uc,rel(M(Si28-34Pr7)) = 2.2 × 10-9. An enrichment of x(28Si) = 0.999 975 585(59) mol/mol accompanied by a depletion with x(30Si) = 1.359(87) × 10-7 mol/mol was determined, covering a range of 7 orders of magnitude. Data analysis and uncertainty calculation were performed according to Guide to the Expression of Uncertainty in Measurement (GUM).