Tantalum isotope ratio measurements and isotope abundances determined by MC-ICP-MS using amplifiers equipped with 10¹⁰, 10¹² and 10¹³ Ohm resistors

Author(s)

Matthias Pfeifer, N.S. Lloyd, S.T.M. Peters, F. Wombacher, Bo-Magnus Elfers, Toni Schulz, Carsten Muenker

Abstract

Due to analytical difficulties related to the low abundance of

¹⁸⁰Ta (about 0.012%), the absolute isotope composition of tantalum is not well known and possible natural variations in

¹⁸⁰Ta/

¹⁸¹Ta are so far unconstrained. Improved precision is required in order to evaluate the homogeneity of Ta isotope distributions among solar system materials and whether natural Ta stable isotope variations exist on Earth. Using a Neptune™ multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) system and different resistors in the Faraday cup amplifier feedback loops (a 10

¹⁰ Ω for

¹⁸¹Ta; 10

¹² or newly developed 10

¹³ Ω resistors for

¹⁸⁰Ta and Hf interference monitor isotopes) now allows relative analyses of

¹⁸⁰Ta/

¹⁸¹Ta with an intermediate precision of ca. ±4ϵ (ϵ refers to one part in 10 000) using 25 to 100 ng Ta and thus even for sample sizes available from meteorites (e.g., 1 g). The 10

¹³ Ω amplifier resistors proved to be of paramount importance for high-precision Ta isotope ratio measurements of low amounts of material. Tailing effects from the large

¹⁸¹Ta beam have previously been underestimated. A thorough assessment of this effect revealed a tailing contribution of ∼2.5% on the currently recommended IUPAC ratio. Potential systematic biases in the mass discrimination correction are assumed being of minor importance compared to an uncertainty of ∼0.4% achieved for the estimate of the "true"

¹⁸⁰Ta/

¹⁸¹Ta ratio. We propose a new

¹⁸⁰Ta/

¹⁸¹Ta isotope ratio of 0.00011705(41), equivalent to

¹⁸¹Ta/

¹⁸⁰Ta = 8543(30), yielding isotope abundances of 0.011704(41) % for

¹⁸⁰Ta and 99.988296(41) % for

¹⁸¹Ta, and an absolute atomic weight for tantalum of 180.9478787(38) u (all uncertainties with k = 2).

Organisation(s)

Department of Lithospheric Research

External organisation(s)

Universität zu Köln, Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Thermo Fisher Scientific Bremen, Georg-August-Universität Göttingen

Journal

Journal of Analytical Atomic Spectrometry

Volume

32

Pages

130-143

No. of pages

14

ISSN

0267-9477

DOI

https://doi.org/10.1039/C6JA00329J

Publication date

2017

Peer reviewed

Yes

Austrian Fields of Science 2012

105105 Geochemistry

Keywords

ASJC Scopus subject areas

Analytical Chemistry, Spectroscopy

Portal url

https://ucrisportal.univie.ac.at/en/publications/1ec69a87-987e-4486-88c6-5acbc6da10f5