Absolute Isotopic Abundance Ratio and Atomic Weight of a Reference Sample of Rhenium.

An absolute value has been obtained for the isotopic abundance ratio of a reference sample of rhenium, using surface emission mass spectrometry. Samples of known isotopic composition, prepared from nearly isotopically pure separated rhenium isotopes, were used to calibrate the mass spectrometers. The resulting absolute 185Ke/187Re ratio is 0.59738±0.00039, which yields atom percents of 185Re = 37.398±0.016 and 187Re = 62.602±0.016. The atomic weight calculated from this isotopic composition is 186.20679±0.00031. The indicated uncertainties are overall limits of error based on 95 percent confidence limits for the mean and allowances for the effects of known sources of possible systematic error.