Determination of the sub-picomolar concentration of dissolved palladium in open ocean seawater

Abstract

Palladium, which is used in various fields, including automobile catalysis, is a precious metal with an extremely low content (0.4 μg/kg) in the earth's crust. Previous studies have suggested its release from anthropogenic sources into a wide range of environments, including aquatic environments. However, reliable analytical methods by which it can be quantified have not yet been established owing to its extremely low concentration in aquatic environments and the vulnerability of its measurements to interfering elements. Therefore, the aim of this study was to establish a highly sensitive and accurate analytical method for its quantification in open ocean seawater using isotope dilution-inductively coupled plasma mass spectrometry (ICP-MS) and thereafter, clarify its vertical distribution. To establish the analytical method, the effects of the concentrations and compositions of the cleaning solution and eluent used during the column pre-concentration process on the analysis were examined. Further, we applied the ammonia dynamic reaction cell (NH3 DRC) during ICP-MS measurements and also optimized the NH3 gas flow rate as well as rejection parameter, q (Rpq). The open ocean seawater samples were acidified to obtain 0.5 M HCl concentration followed by the addition of 105Pd isotope enriched spike solution. Thereafter, the samples were allowed to stand for 24 h before analysis. Solid-phase extraction of Pd in seawater was performed using an anion exchange resin (AG1-X8). To remove interfering elements, a cleaning method was employed. Specifically, cadmium, with the most significant effect on Pd measurements, was removed using HNO3 (> 0.5 M), while other interfering species, including strontium and zirconium were removed using either HCl or HNO3. The flow rate of the cleaning solution and the eluate were also optimized. Thus, a pre-concentration method with a Pd recovery rate of 89–95% was established. Furthermore, the isobaric interference caused by 90Zr16O+, which increased the procedural blank value of our analytical method and could not be completely removed during pre-concentration, was eliminated by employing NH3 DRC in the ICP-MS process, while that caused by 88Sr14N1H3+ was suppressed by setting the NH3 gas flow rate above 1.8 mL/min and the Rpq at 0.81. Thus, when 470 mL of open ocean seawater samples were analyzed under these conditions, the detection limit and blank value obtained were 0.060 and 0.050 pmol/L, respectively (both of which are lower than previously reported values). Additionally, an investigation of the vertical distribution of Pd in the North Pacific using the established methods revealed that Pd concentrations showed a tendency to decreased from 0.32 pmol/L at depth of 50 m to 0.10 pmol/L at a depth of 1500 m, and at depths below 2000 m, the concentrations varied in the range 0.10–0.13 pmol/L. Therefore, our established method showed sufficient suitability for the determination of the sub-picomolar concentration of Pd in open ocean seawater.