A REDETERMINATION OF THE ISOTOPIC ABUNDANCES OF ATMOSPHERIC AR

Abstract

Atmospheric argon measured on a dynamically operated mass spectrometer with an ion source magnet, indicated systematically larger 40Ar/36Ar ratios compared to the generally accepted value of Nier [Nier A.O., 1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Phys. Rev. 77, 789-793], 295.5 ? 0.5, which has served as the standard for all isotopic measurements in geochemistry and cosmochemistry. Gravimetrically prepared mixtures of highly enriched 36Ar and 40Ar were utilized to redetermine the isotopic abundances of atmospheric Ar, using a dynamically operated isotope ratio mass spectrometer with minor modifications and special gas handling techniques to avoid fractionation. A new ratio 40Ar/36Ar = 298.56 ? 0.31 was obtained with a precision of 0.1%, approximately 1% higher than the previously accepted value. Combined with the 38Ar/36Ar (0.1885 ? 0.0003) measured with a VG5400 noble gas mass spectrometer in static operation, the percent abundances of 36Ar, 38Ar, and 40Ar were determined to be 0.3336 ? 0.0004, 0.0629 ? 0.0001, and 99.6035 ? 0.0004, respectively. We calculate an atomic mass of Ar of 39.9478 ? 0.0002. Accurate Ar isotopic abundances are relevant in numerous applications, as the calibration of the mass spectrometer discrimination.