Absolute Isotopic Abundance Ratios and the Atomic Weight of a Reference Sample of Silicon.

Abstract

Absolute values have been obtained for the isotopic abundance ratios of a reference sample of silicon using electron impact mass spectrometry. Samples of known isotopic composition prepared from nearly isotopically pure separated silicon isotopes were used to calibrate the mass spectrometers. The resulting absolute ²⁸Si/³⁰Si ratio=29.74320±0.00747 and the ²⁹Si/³⁰Si ratio=1.50598±0.00086 which yield atom percents of ²⁸Si = 92.22933± 0.00155, ²⁹Si = 4.66982 ± 0.00124 and ³⁰Si = 3.10085 ± 0.00074. The atomic weight calculated from this isotopic composition is 28.085526 0.000056. The indicated uncertainties are overall limits of error based on 95 percent confidence limits for the means and allowances for the effects of known sources of possible systematic error. A study of natural ²⁸Si/³⁰Si ratio variations reported in the literature extends the estimated uncertainty in the atomic weight of natural silicon to ±0.00039.