Precise determination of stable W isotopic compositions of seawater with Fe co-precipitation and double spike MC-ICP-MS

Abstract

Precise determination of the stable isotopic composition of W in seawater (δ^186/184W_sw) is crucial for better constraining the oceanic W cycle, enabling robust reconstructions of redox conditions of the ocean in the past. However, due to the extremely low concentration of W in seawater (∼0.041 to ∼0.067 nM), accurately measuring δ^186/184W_sw remains a technical challenge. Previous studies have developed methods using chelating resins for pre-concentrating W from seawater, but these approaches often suffer from lengthy procedures and incomplete W recovery and also require large sample volumes. In this technical note, we present a novel method that combines iron (Fe) co-precipitation with anion exchange chromatography to isolate W. This approach efficiently removes most of the matrix elements and generally achieves a recovery rate higher than 70%. The calibrated ¹⁸³W–¹⁸⁶W double spike was employed to correct for mass-dependent isotopic fractionation during Fe co-precipitation, chemical purification, and subsequent mass spectrometric analysis. Applying this method to seawater samples from the North Pacific open ocean, we obtained an average δ^186/184W_sw value of 0.54 ± 0.06‰ (2SD, n = 4), which is consistent with previous findings and supports the isotopic homogeneity of W in the open ocean. Therefore, the combination of Fe co-precipitation and double spike provides a more efficient way to determine the W isotopic compositions in seawater with only a trace amount of W.