TY - JOUR

T1 - A TIMS-based method for the high precision measurements of the three-isotope potassium composition of small samples

AU - Wielandt, Daniel Kim Peel

AU - Bizzarro, Martin

PY - 2011/2/1

Y1 - 2011/2/1

N2 - 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 K/K ratios, while 10 ng K samples show 2 s.d. 200 ppm-level or better for mass fractionation corrected K/K 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 K excesses, attributable to the former presence of Ca, 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.

AB - 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 K/K ratios, while 10 ng K samples show 2 s.d. 200 ppm-level or better for mass fractionation corrected K/K 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 K excesses, attributable to the former presence of Ca, 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.

UR - http://www.scopus.com/inward/record.url?scp=79251536225&partnerID=8YFLogxK

U2 - 10.1039/c0ja00067a

DO - 10.1039/c0ja00067a

M3 - Journal article

AN - SCOPUS:79251536225

VL - 26

SP - 366

EP - 377

JO - Journal of Analytical Atomic Spectrometry

JF - Journal of Analytical Atomic Spectrometry

SN - 0267-9477

IS - 2

ER -