Letter to the Editor: 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 previously described silicon reference materials using a well-known method with an improved isotope-ratio mass spectrometer. These new values are directly 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 uncertainties, these new values depend in effect only on a recently published direct comparison of the cyclotron frequency in a Penning trap of (28)Si(+) with that of (12)C(+).