Automated on-line flow-injection ICP-MS determination of trace metals (Mn, Fe, Co, Ni, Cu and Zn) in open ocean seawater: Application to the GEOTRACES program

Abstract

Large programs in chemical oceanography currently require the determination of multiple trace elements in hundreds to thousands of open ocean seawater samples. For example, the international GEOTRACES program would benefit greatly from rapid, precise, automated, multi-element analytical methods for multiple trace metals in seawater. An automated, on-line extraction, flow-injection ICP-MS method is presented here for simultaneous determination of Mn, Fe, Co, Ni, Cu and Zn in open ocean seawater samples. A fully automated commercially available system buffers the pH of the samples on-line and extracts the metals of interest from 9mL of seawater onto a chelation resin column, which is then eluted (1.6M HNO3) directly into a magnetic sector ICP-MS. The resulting transient peaks elute in 45μL (~30s) yielding a preconcentration factor of ~200, and the total analysis cycle requires 8.75min per sample. Elemental concentrations are calculated from integrated peak areas using a combination of isotope dilution and matrix-matched standard curves. Procedural blanks determined at periodic intervals (every 10 samples) throughout each sample run are used for blank correction and for calculating detection limits (range 0.3 to 16pmolkg−1 seawater for these elements). Replicate analyses of a Ross Sea in-house consistency standard and four GEOTRACES reference seawater materials (Atlantic and Pacific, surface and deep water) are used to demonstrate precision of 1–3% (RSD) and very good agreement with reference consensus values for all elements except Co. Determined Co concentrations are ~40% lower than consensus values, consistent with the presence of strong Co ligands in seawater, and the absence of a UV-oxidation step in this method. As illustrated by oceanographically consistent profiles at the GEOTRACES Atlantic and Pacific baseline stations, this method provides highly precise measurement of trace metals over the full range of concentrations occurring in the global ocean. Moreover, the full methodology, including flow-injection hardware and control software, is readily transferable to other users.