A new method for precise determination of iron, zinc and cadmium stable isotope ratios in seawater by double-spike mass spectrometry

‘Metal-free’ seawater doped with varying concentrations of ‘zero’ isotope standards, processed through our simultaneous method, and then analyzed by double spike MC-ICPMS for Fe, Zn and Cd isotope ratios. All values were determined within 2 σ error (error bars shown) of zero. •The first simultaneous method for isotopic analysis of Fe, Zn and Cd in seawater.•Designed for 1L samples, a 1–20 fold improvement over previous methods.•Low blanks and high precision allow measurement of low concentration samples.•Small volume and fast processing are ideal for high-resolution large-scale studies.•Will facilitate investigation of marine trace-metal isotope cycling. The study of Fe, Zn and Cd stable isotopes (δ56Fe, δ66Zn and δ114Cd) in seawater is a new field, which promises to elucidate the marine cycling of these bioactive trace metals. However, the analytical challenges posed by the low concentration of these metals in seawater has meant that previous studies have typically required large sample volumes, highly limiting data collection in the oceans. Here, we present the first simultaneous method for the determination of these three isotope systems in seawater, using Nobias PA-1 chelating resin to extract metals from seawater, purification by anion exchange chromatography, and analysis by double spike MC-ICPMS. This method is designed for use on only a single litre of seawater and has blanks of 0.3, 0.06 and