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

Solid sample, thermal ionization, mass spectrometry has been used to obtain absolute values for the isotopic abundance ratios of a reference sample of potassium. Standards of known isotopic composition, prepared by gravimetrically mixing nearly isotopically and chemically pure separated isotopes of, 39K and 41K, were used for calibration. The absolute isotopic abundance ratios are : 39K/41K = 13.8566 ± 0.0063 and 40K/41K = 0.0017343 ± 0.0000061 which yield atom percent compositions of 39K=93.2581 ± 0.0029. 40K = 0.01167 ± 0.00004, and 41K = 6.7302 ± 0.0029. The calculated atomic weight for potassium is 39.098304 ± 0.000058. The indicated uncertainties are overall limits of error which are the sum of the uncertainty components for ratio determinations and the components covering the effects of known sources of possible systematic error.