Towards a zero-blank, preconcentration-free voltammetric method for iron analysis at picomolar concentrations in unbuffered seawater Luis M. Laglera a,n , Salvatore Caprara b , Damiano Monticelli b a FI-TRACE, Departamento de Química, Universidad de las Islas Baleares, Palma, Balearic Islands 07122, Spain b Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy article info Article history: Received 20 November 2015 Received in revised form 18 December 2015 Accepted 22 December 2015 Available online 23 December 2015 Keywords: Iron Seawater Cathodic voltammetry Non-buffered analysis abstract A method with negligible blank values for the determination of total iron at the ultratrace level in seawater has been optimized and validated exploring for the first time the performance and limitations of Adsorptive Cathodic Stripping Voltammetry (AdCSV) in non-buffered solutions. The method is based on the CSV determination of the Fe-dihydroxynaphthalene (DHN) complex using atmospheric oxygen to catalytically enhance the signal via hydrogen peroxide formation at the electrode/solution interface. The accumulation of hydroxyl ions, the by-product of the hydrogen peroxide formation, increased the pH in the diffusion layer in the absence of buffer bringing it to 9, the optimum for the analytical performance of the method. Voltammograms in UV digested seawater showed no stability or reproducibility drawbacks. The negligible, lower than 5 pM, blank level, is due to the simplicity of the procedure requiring no sample manipulation and a maximum of three reagents only, necessarily the ligand DHN and a base only for those samples previously acidified to raise the pH to circumneutral values (here HCl and NH 3 according to common trace metals protocols). These reagents do not require cleaning before use, further simpli- fying the overall procedure. Analysis of seawater previously acidified at pH 1.5 with HCl and neu- tralized with ammonia showed interferences due to the buffering properties of the NH 3 /NH 4 Cl couple and the transient formation of a volatile electroactive interference that can be easily removed by simply allowing a set time before analysis. In general, the proposed method features several advantages, in- cluding high sample throughput, an excellent limit of detection at 12 pM, minimum sample handling (no preconcentration or change of matrix is required), cost effectiveness and mainly a negligible blank. The method was successfully validated using open ocean consensus samples (SAFe D2 and S). & 2015 Elsevier B.V. All rights reserved. 1. Introduction Iron is a key micronutrient for the understanding of primary productivity and the cycles of other elements in the ocean [1]. Its low solubility and strong particle reactivity are behind the low natural concentrations found in ocean waters ( o0.1 to few nanomols l 1 ) despite being one of the more abundant elements of the Earth crust [2]. Sample collection and clean analysis of dissolved iron in seawater are one of the most challenging tasks in oceanography. In the last few decades new protocols have been developed for the analysis of iron at the subnanomolar level by a battery of different analytical techniques: ICP-MS [3,4], spectro- photometry [5], chemiluminescence [6] and voltammetry [7,8]. Voltammetry is scarcely used for the determination of total iron in oceanography despite its low cost, the excellent sensitivity allowing analysis without a preconcentration step and the port- ability that permits its use in oceanographic cruises. This is caused by environmental concerns about the use of mercury electrodes, the necessity in many cases of an UV digestion step [9] and the lack of commercial flow cells that could allow its automation for flow analysis. However, despite all the effort invested in the last few decades on the development of electrodes based on new materials and surface functionalization, little to none progress has been done on providing a substitute to mercury for the analysis of iron at subnanomolar concentrations [10]. As a consequence, vol- tammetry on the mercury electrode is still the only electro- analytical technique for the study of the organic speciation of trace metals in seawater [11]. Iron analysis by adsorptive cathodic stripping voltammetry (AdCSV) at the picomolar level requires of a catalytic step to en- hance the current resulting from the cathodic reduction of the complex formed by Fe(III) and an electroactive ligand. So far, the ligand that gives the best analytical performance is 2,3 dihydrox- ynaphthalene (DHN) [7] that allows to reach a limit of detection (LOD) of 5 pM if combined with bromate [8] or naturally present Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/talanta Talanta http://dx.doi.org/10.1016/j.talanta.2015.12.060 0039-9140/& 2015 Elsevier B.V. All rights reserved. n Correspondence to: Departamento de Química, Edificio Mateu Orfila, Campus Universitario, Universidad de las Islas Baleares, Palma, Balearic Islands 07122, Spain. E-mail address: luis.laglera@uib.es (L.M. Laglera). Talanta 150 (2016) 449–454