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 se parat ed rhenium isotopes , were used to calibrate the mass spectrometers. The resulting absolute '85 Re/'7 Re ratio is 0.59738 0.00039, which yields atom per cents of ' 8s Re=37 .3980.016 and '87Re=62.6020.016. The atomi c weight calculated from this isotopi c c omposition is 186.20679 0.00031. The indi cated uncertainties are overall limits of e rror based on 95 perce nt confidence limits for th e mean and allowances for the effe cts of known sources of possible systematic error.