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

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 u_c,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).