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
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
_rel
(M) <; 1 x 10
^-9
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.