Development of a sampling and flow injection analysis technique for iron determination in the sea ice environment

Abstract

A trace metal clean method for sampling and analysis of iron is set up and applied to sea ice and its associated snow, brine, and underlying seawater sampled during the Antarctic expedition “ARISE in the East” (Antarctic Remote Ice Sensing Experiment, AA03-V1, September–October 2003, 64–65°S/112–119°E, RV Aurora Australis). For clean sampling, a non-contaminating electropolished stainless steel ice corer is designed in conjunction with a polyethylene lathe equipped with Ti chisels to remove possibly contaminated outer layers of ice cores. A portable peristaltic pump with clean tubing is used on the ice to sample the underlying seawater (interface ice–water = 0, 1 and 30 m) and sea ice brine from access holes. Considering the extreme range of salinities (1–100) and Fe concentrations (0.1–100 nM) previously observed in similar environments, it is of paramount importance to set up a simple and sensitive Fe analyser adapted to such gradients. We use a flow injection analysis (FIA) technique and successfully demonstrate its capability to measure Fe concentrations directly in the sample without an on-line preconcentration/matrix separation step. We test the sensitivity, accuracy, precision and long-term stability of the analytical procedure. Also we explore and remediate interferences from a suite of other trace elements, such as Ni, Cd, Cr, Mn, Cu, Zn and Co. Analysis of reference materials NASS-5 and CASS-3 gives a good agreement with the certified values. Repeated measurements over a period of 5 months of an “in-house” Antarctic seawater standard yields a concentration of 1.02 ± 0.07 nM (n = 17, 1σ). The detection limit (3σ of the blank) is on average 0.12 nM. We report here results of the Fe distribution in sea ice that are in good agreement with previously published data. To our knowledge, this work provides the first complete profiles of total dissolvable and dissolved Fe in sea ice.

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.