Isotopic analysis of potassium by total evaporation and incipient emission thermal ionisation mass spectrometry

Abstract

The isotopic compositions of the natural potassium chloride sample NIST SRM 999b, and the isotopically enriched material GSC K-40 that is used in an ongoing study of the half-life of 40K by decay counting, were determined using total evaporation thermal ionisation mass spectrometry (TE-TIMS) and incipient emission thermal ionisation mass spectrometry (IE-TIMS). Both methods were comprehensively calibrated for precision and accuracy by multiple analyses against gravimetrically prepared synthetic mixtures of enriched 39K and 41K. These mixtures of enriched isotopic materials characterised for purity and stoichiometry, have 41K/39K with uncertainty of ca. 0.02%, and can be used for calibration of either TIMS or MC-ICPMS (multi-collector inductively coupled plasma mass spectrometry) measurements of K isotopic composition independently of the currently accepted IUPAC value. The TE-TIMS and IE-TIMS analyses of potassium using some, but not all, combinations of loading medium, filament configuration, and data processing methods can yield accurate “absolute” isotopic compositions without standardisation. The 41K/39K ratio in NIST SRM 999b determined in this study with TE-TIMS and IE-TIMS has the value of 41K/39K = 0.072247 ± 0.000086, 95% confidence level), which is consistent within 95% confidence limits with the isotopic composition of this material derived from the calibration of Garner et al. (1975), which serves as the basis of the current IUPAC recommendation and is used as a reference in the rapidly growing field of potassium stable isotope geochemistry. The isotopic abundance of 40K in the enriched GSC K-40 is determined with the total uncertainty of 0.075% (95% confidence level), and is suitable for precise calculation of the half-life of 40K in our ongoing decay counting study.