ABSOLUTE ISOTOPIC ABUNDANCE RATIOS AND THE ATOMIC WEIGHT OF A REFERENCE SAMPLE OF SILICON.

Abstract

Absolute values were 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 **2**8Si/**3**0Si ratio equals 29. 74320 plus or minus 0. 00747 and the **2**9Si/**3**0Si ratio equals 1. 50598 plus or minus 0. 00086 which yield atom percents of **2**8Si equals 92. 22933 plus or minus 0. 00155, **2**9Si equals 4. 66982 plus or minus 0. 00124 and **3**0Si equals 3. 10085 plus or minus 0. 00074. The atomic weight calculated from this isotopic composition is 28. 085526 plus or minus 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 **2**8Si/**3**0Si ratio variations reported in the literature extends the estimated uncertainty in the atomic weight of natural silicon to plus or minus 0. 00039.