CSIRO PUBLISHING Rapid Communication www.publish.csiro.au/journals/env A. R. Bowie et al., Environ. Chem. 2007, 4, 1–4. doi:10.1071/EN06073 Intercomparison between FI-CL and ICP-MS for the determination of dissolved iron in Atlantic seawater Andrew R. Bowie, A,B Simon J. Ussher, C William M. Landing D and Paul J. Worsfold C,E A Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC), Hobart, Tas. 7001, Australia. B Australian Centre for Research on Separation Science (ACROSS), School of Chemistry, University of Tasmania, Hobart, Tas. 7001, Australia. C School of Earth, Ocean and Environmental Sciences (SEOES), University of Plymouth, Drake Circus, Plymouth, PL4 8AA, UK. D Department of Oceanography, Florida State University,Tallahassee, FL 32306-4320, USA. E Corresponding author. Email: pworsfold@plymouth.ac.uk Environmental context. Iron is arguably the most important trace element for the growth of marine organisms and is the limiting micronutrient for primary production in many parts of the world’s oceans.The concentration of dissolved iron in seawater therefore influences the global carbon cycle and consequently Earth’s climate. Hence, it is important to understand the marine biogeochemistry of iron and quantify its spatial and temporal distribution. In order to achieve this objective, it is essential that reported open-ocean concentrations of dissolved iron are accurate. Abstract. Results from a 3-laboratory blind intercomparison exercise with two widely used analytical methods for the determination of iron in seawater are presented. The two methods used are coprecipitation followed by isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) and chemical reduction to iron(ii) followed by flow injection with chemiluminescence detection (FI-CL). The five samples used were collected from the South Atlantic Ocean as part of the IRONAGES intercomparison exercise. To avoid any inter-bottle variability, the same sample bottles were sent sequentially to three laboratories in England, Australia and the United States over a 12-month period. The results show that there is no statistical difference (P = 0.05) between the shipboard FI-CL method and the directly traceable, low blank, isotope dilution ICP-MS method for the determination of iron in surface South Atlantic seawater. There was also excellent agreement between the overall mean of the three laboratories (0.54 ± 0.03 nM) and the consensus value from an earlier community-wide separate bottle intercomparison using the same IRONAGES sample water (0.59 ± 0.21 nM). Additional keywords: biogeochemistry, environmental cycles, iron, seawater analysis. The discovery of basin-scale iron limitation in high-nutrient, low-chlorophyll (HNLC) regions, [1–3] and limitation of nitro- gen fixation in subtropical regions, [4] have demonstrated the importance of iron in the biogeochemical control and feedback mechanisms that affect ocean primary productivity. [5] In order to determine the distributions of iron in the ocean and under- stand iron biogeochemistry [6] and its impact on the functioning of marine ecosystems and carbon cycling, it is imperative that iron is measured routinely and accurately during oceanographic research cruises. Moreover, unless the oceanographic commu- nity can constrain uncertainties in the global distribution of iron, the development of accurate biogeochemical models will be restricted. Following the development of improved trace metal sam- pling and processing techniques [7] and the introduction of new highly sensitive and selective shipboard and laboratory ana- lytical methods, [8] reported dissolved iron concentrations in seawater have decreased by up to two orders of magnitude, [9] thus enabling oceanographers to obtain the first reliable open-ocean profiles. [10] Indeed, it is now possible to observe oceanographic consistency in vertical dissolved iron distributions over decadal time scales (see Fig. 1). 0 100 200 300 400 500 600 700 800 900 1000 0.0 0.2 0.4 0.6 0.8 Dissolved Fe [nM] Depth [m] A16N-Sta 31 NABE 47N NABE 59N Fig. 1. Comparison of the distribution of dissolved iron in the north-east Atlantic. A16N-Sta 31 (50 ◦ N, 20 ◦ W) data was collected in 2005 by Florida State University using 57 Fe isotope dilution ICP-MS. North Atlantic Bloom Experiment (NABE) data was collected in 1989 by Moss Landing Marine Laboratories (Martin et al. [21] ) using chelation solvent extraction–graphite furnace atomic absorption spectrometry. © CSIRO 2007 1 1448-2517/07/010001