Comparison of the Isotopic Composition of Silicon Crystals Highly Enriched in 28Si

The isotopic composition and molar mass M of silicon in a new crystal (code: Si28-33Pr11) measured by isotope ratio mass spectrometry using a high-resolution multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS) is presented using the virtual-element isotope dilution mass spectrometry (VE-IDMS) method. For this new crystal, M = 27.976 950 48 (16) g/mol was determined with urel(M) = 5.7 &times
10&minus
9. The &ldquo
X-ray-crystal-density (XRCD) method&rdquo
one of the primary methods for realizing and disseminating the SI units kilogram and mole in the recently revised SI, is based on &ldquo
counting&rdquo
silicon atoms in silicon single crystal spheres. One of the key quantities is the isotopic composition&mdash
expressed by the molar mass M&mdash
of the three stable isotopes 28Si, 29Si, and 30Si in the material highly enriched in 28Si. M was determined with lowest possible uncertainty using latest improvements of the experimental techniques. All uncertainties were estimated according to the &ldquo
Guide to the expression of uncertainty in measurement, GUM&rdquo
The results of the new crystal are discussed and compared with the four previously available crystals, establishing a worldwide limited pool of primary reference spheres of highest metrological quality.