Abstract
Recent improvements in the precision of mass spectrometric measurements have reduced the uncertainty of K–Ar and 39Ar–40Ar ages measured on geological materials. Now the major sources of uncertainty are the uncertainties on the 40K decay constant and the absolute abundance of 40K. In order to improve on this situation we determined the abundance of the 40K isotope in terrestrial standards.A ThermoFischer Triton+ thermal ionization mass spectrometer was used for K isotope ratio measurements of the NIST K standard reference materials SRM 918b and SRM 985. Ion beams were measured in Faraday cups with amplifiers equipped with 1010, 1011 and 1012Ω resistors. Three measurement protocols were used: (A) dynamic measurement with in-run fractionation correction by normalization to the IUPAC recommended isotope ratio 41K/39K=0.072 1677; (B) total evaporation; (C) a modified total evaporation with interblock baseline measurements. Different measurement protocols were combined with different loading procedures. The best results were obtained by loading samples on single oxidized tantalum filaments with 0.1M H3PO4. The total ion yields (ionization+transmission) were tested for the evaporation procedures (B) and (C) and ranged up to 48%.The resulting best estimate for the 40K/39K ratio is 0.000125116±57 (2σ), corresponding to an isotopic abundance 40K/K=(1.1668±8)×10−4.
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