NEW VALUES FOR SILICON REFERENCE MATERIALS, CERTIFIED FOR ISOTOPE ABUNDANCE RATIOS

New isotope abundance and relative atomic mass (atomic weight) values- with low, hitherto unattained uncertainty-are reported for two previou sly described silicon reference materials using a well-known method wi th an improved isotope-ratio mass spectrometer. These new values are d irectly traceable to the SI, more specifically to the unit for amount of substance, the mole, and independent of the SI unit of mass and of the Avogadro constant. Besides the residual mass-spectrometric uncerta inties, these new values depend in effect only on a recently published direct comparison of the cyclotron frequency in a Penning trap of Si- 28+ with that of C-12+.