A Redetermination of the Relative Abundances of the Isotopes of Carbon, Nitrogen, Oxygen, Argon, and Potassium

Essentially pure samples of ${\mathrm{A}}^{36}$ and ${\mathrm{A}}^{40}$ have been produced by thermal diffusion and used for the preparation of synthetic argon isotope mixtures whose ${\mathrm{A}}^{36}$/${\mathrm{A}}^{40}$ isotope abundance ratios were very accurately determined. The mixtures were then employed for determining the mass discriminating effects in two mass spectrometers. Carbon, nitrogen, oxygen, argon, and potassium were investigated and new values given for the relative abundances of the isotopes. With atmospheric oxygen as a standard, the conversion factor between the physical and chemical atomic weight scales is 1.0002783±5. The percentage abundance of ${\mathrm{K}}^{40}$ in potassium is found to be 0.0119±0.0001 percent, a figure of special interest in geophysical problems, and approximately ten percent higher than the present accepted value.