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).
Pfeifer, M., Lloyd, N. S., Peters, S. T. M., Wombacher, F., Elfers, B-M., Schulz, T., & Münker, C. (2017). Tantalum isotope ratio measurements and isotope abundances determined by MC-ICP-MS using amplifiers equipped with 1010, 1012 and 1013 Ohm resistors. Journal of Analytical Atomic Spectrometry, 32(1), 130-143. https://doi.org/10.1039/C6JA00329J