A TIMS-based method for the high precision measurements of the three-isotope potassium composition of small samples

Abstract

A novel thermal ionization mass spectrometry (TIMS) method for the three-isotope analysis of K has been developed, and ion chromatographic methods for the separation of K have been adapted for the processing of small samples. The precise measurement of K-isotopes is challenged by the presence of large isotope ratios in common K, and the accuracy of such measurements is compromised by isobaric interference and abundance sensitivity related issues. The combined expanded dynamic range and improved signal/noise ratio of a Triton TIMS with an adapted amplifier setup however allows measurements in the theoretically poisson-noise dominated intensity regime, while the high sensitivity of the thermal ionization-based source towards K allows this intensity regime to be reached, even with small samples. Analyses of 150 ng K samples of terrestrial basalts shows 2 s.d. 100 ppm-level or better reproducibility for mass fractionation corrected ⁴¹K/³⁹K ratios, while 10 ng K samples show 2 s.d. 200 ppm-level or better for mass fractionation corrected ⁴¹K/³⁹K ratios. The described methods are suitable for the high precision determination of internally mass fractionation corrected isotope anomalies in small samples, such as those recoverable from refractory inclusions in chondritic meteorites, or other low-level, rare or valuable materials. For meteoritic inclusions in particular, this has applications in the determination of ⁴¹K excesses, attributable to the former presence of ⁴¹Ca, and the reproducibility-levels reached are sufficient to resolve bulk radiogenic anomalies at the level suggested by prior ion microprobe studies. In addition to such radiometric applications, the K-isotope composition is potentially sensitive to cosmic ray overprint, and as such may be used to assess the level of cosmic ray irradiation in various extra-terrestrial materials.