Abstract
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 × 10−9. The “X-ray-crystal-density (XRCD) method”, one of the primary methods for realizing and disseminating the SI units kilogram and mole in the recently revised SI, is based on “counting” silicon atoms in silicon single crystal spheres. One of the key quantities is the isotopic composition—expressed by the molar mass M—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 “Guide to the expression of uncertainty in measurement, GUM”. 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.
Highlights
The choice of silicon with the three stable natural isotopes Si, Si, and Si as an element which is ideally suitable for the realization and dissemination of the SI base units kilogram and mole dates back to its first comprehensive use for the X-ray-crystal-density (XRCD) method in the 1970s, first applied by the National Bureau of Standards ( National Institute of Standards and Technology, NIST) [1,2].During the 1980s up to the beginning of the 2000s, first silicon with a natural isotopic abundance has been used for the production of single crystals in a macroscopic manner ready to produce perfectly round spheres with a mass of approximately one kilogram each [3]
The broader background of the research—focused on the isotopic composition and molar mass M of the silicon material—was the revision of the Si units defined by fixed fundamental physical constants in 2019 and the scientific methods used therein [7,8]
Crystals 2020, 10, 500 used in the XRCD method, because when taking silicon with natural isotopic composition, the relative uncertainty associated with the molar mass M, a key quantity in this procedure, reaches a lower limit in the 10−7 range, at least one order of magnitude too high for a respective small uncertainty of NA and a precondition of the revision of the SI at that time
Summary
Crystals 2020, 10, 500 used in the XRCD method, because when taking silicon with natural isotopic composition, the relative uncertainty associated with the molar mass M, a key quantity in this procedure, reaches a lower limit in the 10−7 range, at least one order of magnitude too high for a respective small uncertainty of NA and a precondition of the revision of the SI at that time. NA has a fixed numerical value NA = 6.022 140 76 × 1023 mol−1 without an associated uncertainty by definition This in turn shows, that a silicon sphere characterized by the XRCD method (second term in Equation (1)) can serve as a primary mass standard which is expressed by. We report on the isotopic composition x(i Si) and molar mass M of another new silicon single crystal highly enriched in 28 Si with the code Si28-33Pr11.
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