The X-ray crystal density method uses silicon spheres highly enriched in ²⁸Si as a primary method for the dissemination of the SI base unit kilogram yielding smallest possible uncertainties associated with the mass m within a few parts in 10⁻⁸. This study compares different available and newly developed analytical methods and their results for the determination of the molar mass M of silicon highly enriched in ²⁸Si (M_e) and of silicon (M_x) with an almost natural isotopic distribution. While for M_e relative uncertainties u_rel(M_e) in the lower 10⁻⁹ range are obtained routinely, it was not possible to fall below a value of u_rel(M_x) < 4 × 10⁻⁶ in the case of natural silicon, which is approximately three orders of magnitude larger. The application of the state-of-the-art isotope ratio mass spectrometry accompanied with sophisticated thoroughly investigated methods suggests an intrinsic cause for the large uncertainty associated with the molar mass of natural silicon compared to the enriched material.

X射线晶体密度法使用高度富集²⁸ Si的硅球作为散发SI基本单位千克的主要方法，从而在10 ^-8的几部分内产生与质量m相关的最小可能不确定性。这项研究比较了不同的可用分析方法和最新开发的分析方法，以及他们的结果，这些结果用于测定高度富集²⁸ Si（M _e）的硅和几乎具有自然同位素分布的硅（M _x）的摩尔质量M。虽然中号_è相对不确定性ù _REL（中号_Ë）通常获得在较低的10 ^-9范围内），在天然硅的情况下，不可能降至u _rel（M _x）<4×10 ^-6的值以下，该值大约大三个数量级。与富集的材料相比，最先进的同位素比率质谱法的应用以及复杂的经过深入研究的方法表明，天然硅的摩尔质量存在较大不确定性的内在原因。