New Silicon Crystals for a Redefined Kilogram and Mole: Isotopic Composition of the First Two Crystals

The isotopic compositions (molar mass, M) of the first and new silicon crystals highly enriched in $^{28}\text{Si}(> 99.99\ \%)$ used for the determination of the Avogadro constant were measured using a high resolution multicollector ICP mass spectrometer and applying a modified isotope dilution mass spectrometry technique. Relative uncertainties $u_{\text{rel}}(M) were obtained: a reduction by almost three orders of magnitude in ten years. This study compares the homogeneities in M(Si) of the first “AVO28” crystal and the first crystal (Si28-23Pr11) of the “Kilogram-2” project.