Determination of sub-nanomolar levels of iron in seawater using ¯ow injection with chemiluminescence detection Andrew R. Bowie a,b , Eric P. Achterberg a , R. Fauzi C. Mantoura b , Paul J. Worsfold a,* a Dept. of Environmental Sciences, Plymouth Environmental Research Centre, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK b Plymouth Marine Laboratory, Centre for Coastal and Marine Sciences Prospect Place, West Hoe, Plymouth PL1 3DH, UK Received 10 October 1997; received in revised form 9 December 1997; accepted 15 December 1997 Abstract The development of a highly sensitive system for the shipboard determination of dissolved iron at the sub-nM level is presented. The technique is based on a ¯ow injection method coupled with luminol chemiluminescence detection. Dissolved Fe(IIIII) levels are determined after Fe(III) reduction using sulphite and in-line matrix elimination/preconcentration on an 8- hydroxyquinoline (8-quinolinol) chelating resin column. The detection limit (3s) is 40 pM when 1.5 ml of sample is loaded onto the column, and the relative standard deviation is 3.2% (n5) for a 1.0 nM Fe sample. One analytical cycle can be completed in 3 min. The automated method proved reliable when employed on-board the RRS James Clark Ross during Autumn 1996, mapping dissolvable Fe(IIIII) levels along the Atlantic Meridional Transect from 508N to 508S. Data from vertical pro®les through the upper water column are presented. # 1998 Elsevier Science B.V. Keywords: Flow injection; Luminol; Chemiluminescence; Iron; Preconcentration; Seawater; Atlantic meridional transect 1. Introduction Iron is a major component of the Earth's crust, but like other reactive trace elements, its dissolved con- centration in open-oceanic waters remains very low (<1.0 nM). It is an essential micronutrient for organ- isms and in certain high-nutrient, low-chlorophyll areas of the world's oceans, iron appears to limit phytoplankton growth [1,2], which may have impor- tant implications for global carbon cycles [3]. Such hypotheses have recently been tested in the under- productive waters of the equatorial Paci®c, where seeding an expanse of surface water with low con- centrations of iron sulphate triggered a massive phytoplankton bloom [4,5], and resulted in a transi- ent increase in the atmosphere±ocean CO 2 ¯ux [6]. Fe(III) is the thermodynamically stable form in oxygenated seawater, existing predominantly as inso- luble oxy-hydroxides or colloidal matter [7±9]. Fe(II) is a transient species in surface oxic waters, existing via chemical or photochemical Fe(III) reduction [10± 12], or via atmospheric deposition [13±15]. At sea- water pH, Fe(II) is oxidised rapidly by O 2 and H 2 O 2 [16]. Recently, organic complexation has been thought to occur to a signi®cant extent in marine systems [17± 19]. Laboratory studies have shown that phytoplank- ton are only able to utilise dissolved Fe 2 or Fe 3 species, and that uptake of colloidal or particulate Fe is Analytica Chimica Acta 361 (1998) 189±200 *Corresponding author. Fax: +44 1752 233009. 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00015-4