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.