On the molar mass of silicon for a new Avogadro constant

An overview of the present results and status of the molar mass of a silicon crystal highly enriched in
²⁸
Si used for a new determination of the Avogadro constant is given with special emphasis on the cooperation between NIST, NRC, and PTB currently working on this topic. Using an experimental combination of multicollector ICP and isotope dilution mass spectrometry, a new approach developed by PTB for the measurement of the isotopic composition has been applied. It focuses on the isotope ratio treatment of
³⁰
Si/
²⁹
Si mainly, avoiding the more uncertain measurement of ratios related to the main isotope
²⁸
Si.