High-accuracy determination of iron in seawater by isotope dilution multiple collector inductively coupled plasma mass spectrometry (ID-MC-ICP-MS) using nitrilotriacetic acid chelating resin for pre-concentration and matrix separation

In the present paper we describe a robust and simple method to measure dissolved iron (DFe) concentrations in seawater down to <0.1 nmol L−1 level, by isotope dilution multiple collector inductively coupled plasma mass spectrometry (ID-MC-ICP-MS) using a 54Fe spike and measuring the 57Fe/54Fe ratio. The method provides for a pre-concentration step (100:1) by micro-columns filled with the resin NTA Superflow of 50 mL seawater samples acidified to pH 1.9. NTA Superflow is demonstrated to quantitatively extract Fe from acidified seawater samples at this pH. Blanks are kept low (grand mean 0.045 ± 0.020 nmol L−1, n = 21, 3× S.D. limit of detection per session 0.020–0.069 nmol L−1 range), as no buffer is required to adjust the sample pH for optimal extraction, and no other reagents are needed than ultrapure nitric acid, 12 mM H2O2, and acidified (pH 1.9) ultra-high purity (UHP) water. We measured SAFe (sampling and analysis of Fe) reference seawater samples Surface-1 (0.097 ± 0.043 nmol L−1) and Deep-2 (0.91 ± 0.17 nmol L−1) and obtained results that were in excellent agreement with their DFe consensus values: 0.118 ± 0.028 nmol L−1 (n = 7) for Surface-1 and 0.932 ± 0.059 nmol L−1 (n = 9) for Deep-2. We also present a vertical DFe profile from the western Weddell Sea collected during the Ice Station Polarstern (ISPOL) ice drift experiment (ANT XXII-2, RV Polarstern) in November 2004–January 2005. The profile shows near-surface DFe concentrations of ∼0.6 nmol L−1 and bottom water enrichment up to 23 nmol L−1 DFe.