Determination of iron in seawater by high-resolution isotope dilution inductively coupled plasma mass spectrometry after Mg(OH)₂ coprecipitation

A simple, accurate and highly sensitive method has been developed for determining the concentration of Fe in seawater. The technique applies the low-blank Mg(OH)₂ coprecipitation method of Wu and Boyle [Anal. Chem. 69 (1997) 2464-2470] to preconcentrate Fe from seawater, and uses isotope dilution high-resolution ICPMS detection to ensure accuracy and enable high sensitivity. This method has been used to determine Fe concentrations as low as 0.085nM Fe (1σ=0.019nM) in a 14ml-seawater sample. We achieved a 3σ detection limit of 0.05-0.06nM for 14ml-sample size and 0.162nM for 1.4ml-sample size. With 14ml-sample size procedure, the Fe signal at 0.085nM level is more than 10 times above ICPMS instrumental Fe blank and the procedural Fe blanks are due mainly to Fe contamination during sample preparation. The precision of the method is 3-5% at 0.18-0.31nM Fe level. The technique is demonstrated by oceanographically consistent profiles of Fe in the eastern north Atlantic and in the Sargasso Sea near Bermuda. Copyright (C) 1998 Elsevier Science B.V.