Analysis of Nd isotopic composition in seawater by using newly developed solid phase extraction and MC-ICP-MS

Hirofumi Tazoe, Hirosaki University, Aomori, Japan, Hiroshi Amakawa, The University of Tokyo, Tokyo, Japan, Katsuhiko Suzuki, JAMSTEC, Yokosuka, Japan, Takuya Hara, The University of Tokyo, Kashiwa, Japan and Hajime Obata, Univ Tokyo, Kashiwa, Japan
Investigation of Nd isotopic composition in seawater is useful for tracing water masses and geochemical cycles for lithogenic elements. However, it is difficult to obtain high resolution data sets for Nd isotopic composition relative to other physical and chemical parameters because large volume of seawater and troublesome chemical separation are needed for precise analysis. Multi-collector (MC)-ICP-MS enables a high sample throughput and simplify chemical separation. We present a new analytical method for Nd isotopic composition in seawater with MC-ICP-MS by solid phase extraction using DGA Resin and Ln Resin chromatography.

Seawater samples were obtained at Stn. K2 (47oN, 160oE, 5269 m depth) and Stn. ESM (43oN, 170o 40’E, 1792 m depth) in 2016 by the training ship Oshoro Maru IV of Hokkaido University. 3 L of filtered seawater samples were acidified and transferred to land-based laboratory. Neodymium was preconcentrated by Fe-hydroxide coprecipitation with small amount of Fe (3 mg), which was collected onto membrane filter (0.45 μm pore, 90 mm in diameter). The precipitate was dissolved in 10 mL of 8 M HNO3, and Nd was separated by DGA resin. Nd fraction was directly introduced to Ln resin for removal of Sm. Nd isotopic composition was determined by MC-ICP-MS (Neptune plus). One of the advantages of this procedure is that the evaporation steps are not required. Procedural blank of Nd is 7 pg (n=3). For validation, seawater samples were also analyzed by the traditional method. 10-20 L of filtered seawater were preconcentrated by Fe-hydroxide coprecipitation (Fe 250 mg) onboard the ship. After centrifugation and removal of Fe by solvent extraction with di-isopropyl ether, Nd was purified by standard cation chromatography, TRU resin and Ln resin, and then the isotopic composition was determined by TIMS (Finnigan MAT262). The analytical results by the proposed method are consistent with those by the conventional method.