Development of a sampling and flow injection analysis technique for iron determination in the sea ice environment
Introduction
As a potentially limiting micro-nutrient for algal growth in the oceans, iron (Fe) is strongly involved in marine biogeochemical cycling [1], [2]. Sea ice is of importance to global climate as it covers large areas of the polar oceans and this coverage shows strong seasonal changes [3], [4]. Present at the interface between atmosphere and ocean, sea ice may play an important role in the ocean–atmosphere exchange of carbon (CO2), sulphur (DMS) and Fe. The scarcity of reliable Fe data in sea ice [5], [6] mainly results from the analytical challenges encountered in this extreme environment, which makes the biogeochemical cycle of iron in sea ice virtually unknown as to date.
Recent advances in shipboard flow injection techniques for the measurement of iron in seawater have greatly facilitated the collection of reliable data [7], [8]. Not only can data now be collected in near real-time mode, but also contamination problems can be quickly identified during sampling campaigns. Most of these methods use luminol chemiluminescence for detection [9], [10], [11], [12], [13], though some rely on spectrophotometric detection [14], [15], [16]. Another characteristic of the Fe-FIA methods used today, is that they nearly all apply on-line preconcentration/matrix separation using resins of 8-hydroxyquinoline (8-HQ) immobilized on a carrier of PVC based polymer [17] or alkoxyglass [13]. Exceptions are the stopped-flow method by O'Sullivan et al. that measures total dissolved Fe directly in the sample, and those FIA applications for analysing reduced Fe(II) directly in ambient seawater in which it is required to measure the sample immediately in order not to loose any Fe(II) by rapid re-oxidation [18], [19].
The use of a column packed with 8-HQ resin aims at reaching low detection limits by on-line preconcentration followed by elution in a small volume of dilute acid. It also rejects sea-salts due to its high affinity for transition metals and low affinity for the major ions. Major ions would not only interfere with iron–luminol chemiluminescence, but would also clog the detector flow cell due to precipitation at the optimal high pH of the luminol reaction. The preconcentration technique requires tedious resin synthesis schemes, as well as column set up that can be problematic. Not every attempt to synthesize 8-HQ resin is successful and every new batch of the resin product needs careful characterization of its chromatographic properties and blank levels before it can be brought into use. Various factors control potential occurrence of backpressure problems, which may lead to limited flow-through and even severe leaking problems. They are the type of carrier resin (porosity, particle size), the length and diameter of the column, the connectors, the Teflon or polypropylene frits or nylon net to hold the resin in the column, and the flow speed. Another complication associated with 8-HQ resins is that their yields may be influenced by competition between natural organic ligands and the 8-HQ, leading to underestimations of the concentration. This is especially crucial when standard additions are performed on one seawater sample only and all the others are related to this calibration. When not using a preconcentration step, dissolved organic matter can also interfere by absorbing the luminescent signal, competing for radical intermediates or complexing Fe(II) [12]. In any case, these matrix interferences can be cancelled out by applying standard addition calibration to every sample.
Luminol (3-aminophtalhydrazide) is well known to produce strong chemiluminescence with Fe or cobalt and, to a lesser extent, with other transition metals. FIA methods for Fe are based on the Fe-mediated chemiluminescent reaction between luminol and O2 or H2O2. During the oxidation of luminol, blue light is emitted and detected by a photon counter. The peak area or peak height of the signal is proportional to the amount of dissolved Fe present in the analyte. The use of O2 or H2O2 depends on whether Fe(II) or Fe(III) is the Fe species of interest [13], [20]. The advantage of the Fe(II) driven chemiluminescence is that the reaction is instantaneous and can take place inside the detector flow cell so that analysis time can be kept short. Peak shapes are sharp and sensitivity is generally high, so that preconcentration volumes and times can be minimized. In the case of Fe(III) driven chemiluminescence, the Fe sample is delayed in a long reaction loop while being mixed with luminol, reaction buffer and H2O2 before being introduced in the detector flow cell. This leads to a longer analysis time, smearing of the signal due to wall friction in the flow circuit and lower sensitivity. The latter requires higher preconcentration factors hence higher sample volume. The Fe(II) based methods involve a lengthy reduction step, with for instance sodium sulphite to convert thermodynamically favoured Fe(III) into Fe(II), while the Fe(III) method can measure samples without this preliminary step. All FIA methods for total dissolved Fe measurement require that filtered samples are acidified at least 24 h before analysis to solubilize the iron, while unfiltered seawater samples for total dissolvable Fe should be kept for several weeks to months at low pH in order to release as much as possible the leachable particulate Fe species.
In this paper, we report a new FIA application, which can measure total dissolvable (unfiltered) and total dissolved Fe (0.2 μm filtered) directly in the sample at subnanomolar levels in natural waters with a wide range of salinities, without a preconcentration step. This application takes advantage of the high sensitivity of Fe(II) driven chemiluminescence, whilst eluding the aforementioned problems associated with the use of 8-HQ resins.
Section snippets
Instrumental
Our FIA instrument (Fig. 1) is an automated continuous flow system (FeLume, Waterville Analytical, USA) that detects chemiluminescence from the reaction of luminol and dissolved Fe(II) by directly injecting a natural water sample from a 1 ml sample loop into the detector flow cell. Sample preparation is adapted from Bowie et al. and O'Sullivan et al. To ensure that all the Fe is under the Fe(II) form, the reductant sodium sulphite (Na2SO3) in dilute ammonium acetate (NH4Ac) is added to the
Metal ions interferences
Experiments to examine metal ion interferences were conducted with unfiltered Antarctic seawater from 30 m depth to which 2 nM fresh Fe(II) was added. Individual metal ions (Co(II), Mn(II), Cu(II), Zn(II), Cr(III), Cd(II) and Ni(II)) were then spiked to study possible changes in the Fe(II) signal as a function of the type and amount of metal added. Independent tests were performed on samples at pH 1.8 and 5.3. The pH of the sample was brought from 1.8 to 5.3 by adding purified 2 M NH4Ac buffer to
Conclusions
We developed a sensitive and reliable method to measure Fe concentrations in the sea ice environment. We demonstrated the ability of this method to deal with high gradients of salinity and Fe concentrations. Accuracy and reproducibility were satisfactory. Our methodology has been successfully applied to Antarctic sea ice samples and Fe data were consistent with previously reported values. Results from the first data set reveal the potential of such measurements in shedding more light on
Acknowledgements
We would like to thank the Australian Antarctic Division, especially Ian Allison (Expedition Leader) and Rob Massom (Chief Scientist), for inviting us on the “ARISE in the East” endeavor. Captain, officers and crew of the RV Aurora Australis are thanked for all their efforts to help making our work a success. We are also grateful to the Australian Antarctic Division for arranging the loan of a clean laboratory container. The logistic support in the laboratory provided by Nathalie Roevros is
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Present address: Department of Environment North West Region, P.O. Box 836, Karratha, WA 6714, Australia.