Determination of Fe, Ni, Cu, Zn, Cd and Pb in seawater by isotope dilution automatic solid-phase extraction—ICP-MS

A thorough understanding of the biogeochemical cycling of trace metals in the ocean is crucial because of the important role these elements play in regulating metabolism in marine biotas and thus, the climate. However, a precise and accurate analysis of trace metals in seawater is difficult because they are present at extremely low concentrations in a high salt matrix. In this study, we report an analytical method for the preconcentration and separation of six trace metals, Fe, Ni, Cu, Zn, Cd and Pb, in seawater using a sea FAST automatic solid-phase extraction device, analysis by a triple quadrupole collision/reaction technique with inductively coupled plasma mass spectrometry(ICP-MS), and quantification by the isotope dilution technique. A small volume(10 m L) of seawater sample was mixed with a multi-element isotope spike and subjected to sea FAST procedures. The preconcentrate solution was then analyzed using the optimized collision/reaction cell mode of ICP-MS, with NH3gas for Fe and Cd with a flow rate of 0.22 m L/min and He for Ni, Cu, Zn and Pb with a flow rate of 4.0 m L/min. The procedure blanks were 130 pmol/L, 3.0 pmol/L, 6.8 pmol/L, 37 pmol/L, 0.29 pmol/L and 0.42 pmol/L, for Fe, Ni,Cu, Zn, Cd and Pb, respectively. The method was validated using five reference materials(SLRs-6, SLEW-3,CASS-6, NASS-7 and GEOTRACE-GSC), and our results were consistent with the consensus values. The method was further validated by measuring full-water-column seawater samples from the subtropical Northwest Pacific Ocean, and our results demonstrated good oceanic consistency.