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

An absolute value for the isotonic abundance ratio of a reference sample of copper has been obtained by thermal emission mass spectrometry. Samples of known abundance ratio, prepared from nearly pure separated copper isotopes, were used to calibrate the mass spectrometers. The results yield an absolute value of Cu63/Cu65=2.2440 ±0.0021 and an atomic weight of 63.54555 ± 0.00040 (C12=12). The indicated uncertainties are overall limits of error based on 95 percent confidence limits for the mean and on allowances for the effects of known sources of possible systematic error. A companion study of the Cu63/Cu65 ratio in natural materials has shown that there is significant variation in some rare secondary minerals.