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

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 10¹⁰ Ω for ¹⁸¹Ta; 10¹² or newly developed 10¹³ 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 10¹³ 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).