Simultaneous quantification of Zn, Cu, Cd, and Pb in seawater via offline batch extraction using Ni-NTA chelating resin and isotope dilution ICP-MS

A streamlined, low-blank protocol has been developed for the simultaneous determination of the total dissolved concentrations of four trace metals: Zinc (Zn), Copper (Cu), Cadmium (Cd), and Lead (Pb) - in seawater. The method utilizes a small sample volume (10 mL) and offline batch extraction with Ni-NTA Superflow® chelating resin for preconcentration and matrix removal. Quantification is achieved using the isotope dilution inductively coupled plasma mass spectrometry technique (ID-ICP-MS) on a single quadrupole instrument. The sample pH was optimized to ensure maximum recoveries across all four metals for simultaneous measurement. The method demonstrates that ultrapure water (18.2 MΩ.cm) is a reliable blank matrix for Ni-NTA–based protocols, removing the need for reagent blank quantification and offering a simpler alternative to low-trace-metal or artificial seawater approaches. Trace metals were eluted using HNO 3 (Optima™; 0.5 M) , yielding effective preconcentration factors of approximately 5 for Zn, 7.5 for Cu, 2 for Cd, and 4 for Pb. The low detection limits achieved in this protocol support its application to both open-ocean and coastal seawater sample analysis. Method validation using certified reference materials NASS-7 (offshore) and CASS-6 (nearshore) showed excellent agreement with certified values. Application of the protocol to samples collected during the GEOTRACES KH-15-03 cruise produced depth profiles that are oceanographically consistent and in close agreement with previously published data. With the recent discontinuation of Nobias PA1 resin, this method offers a robust, cost-effective, and accessible alternative to measure these four key trace metals in the ocean, which serve as proxies of marine nutrient dynamics and anthropogenicity.