TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 53, 2019

Assessment of the secondary instrumental fractionation in TIMS: Implication for high-precision Nd isotope analysis of geological samples

Ryota FUKAI*1,2 and Tetsuya YOKOYAMA2

1Geochemical Research Center, Graduate School of Science, The university of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

(Received December 18, 2018; Accepted July 14, 2019)

Abstract: The advent of the latest-generation of mass spectrometers enables the determination of isotope ratios for several elements with very high precision. However, many obscure processes deteriorate analytical uncertainties. We investigated a likely process, the secondary instrumental fractionation of Nd isotopes during thermal ionization mass spectrometry (TIMS) analysis, which cannot be ignored in recent geochemical applications. Multiple Nd isotope measurements of the standard JNdi-1 and nine terrestrial rocks standards reveal small but resolvable isotopic fluctuations. The variations are attributed to secondary instrumental fractionation induced by the accumulation of conductive materials (e.g., H3PO4) from samples, on the ion lens assemblage surface. The secondary instrumental fractionation observed in 142Nd/144Nd ratios is corrected using the simultaneously measured 150Nd/144Nd ratios. The correction improves the 142Nd/144Nd reproducibility by a factor of ∼1.3. This second-order correction produces more precise and accurate isotopic ratios for terrestrial rocks with potentially variable 142Nd/144Nd ratios.
Key words: neodymium, 142Nd, 143Nd, TIMS, isotopic fractionation

*Corresponding author E-mail: ryota.fukai@gmail.com