High precision determination of the terrestrial 40K abundance

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 K-40 decay constant and the absolute abundance of K-40. In order to improve on this situation we determined the abundance of the K-40 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 10(10), 10(11) and 10(12) Omega resistors. Three measurement protocols were used: (A) dynamic measurement with in-run fractionation correction by normalization to the IUPAC recommended isotope ratio K-41/K-39 = 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 0.1 M 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 K-40/K-39 ratio is 0.000125116 +/- 57 (2 sigma), corresponding to an isotopic abundance K-40/K = (1.1668 +/- 8) x 10(-4). (C) 2013 Elsevier Ltd. All rights reserved. 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 K-40 decay constant and the absolute abundance of K-40. In order to improve on this situation we determined the abundance of the K-40 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 10(10), 10(11) and 10(12) Omega resistors. Three measurement protocols were used: (A) dynamic measurement with in-run fractionation correction by normalization to the IUPAC recommended isotope ratio K-41/K-39 = 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 0.1 M 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 K-40/K-39 ratio is 0.000125116 +/- 57 (2 sigma), corresponding to an isotopic abundance K-40/K = (1.1668 +/- 8) x 10(-4). (C) 2013 Elsevier Ltd. All rights reserved. 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 K-40 decay constant and the absolute abundance of K-40. In order to improve on this situation we determined the abundance of the K-40 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 10(10), 10(11) and 10(12) Omega resistors. Three measurement protocols were used: (A) dynamic measurement with in-run fractionation correction by normalization to the IUPAC recommended isotope ratio K-41/K-39 = 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 0.1 M 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 K-40/K-39 ratio is 0.000125116 +/- 57 (2 sigma), corresponding to an isotopic abundance K-40/K = (1.1668 +/- 8) x 10(-4). (C) 2013 Elsevier Ltd. All rights reserved.