Abstract
The trace metal iron (Fe) is an essential micronutrient for phytoplankton growth and limits, or co-limits primary production across much of the world's surface ocean. Iron is a redox sensitive element, with Fe(II) and Fe(III) co-existing in natural waters. Whilst Fe(II) is the most soluble form, it is also transient with rapid oxidation rates in oxic seawater. Measurements of Fe(II) are therefore preferably undertaken in situ. For this purpose an autonomous wet chemical analyzer based on lab-on-chip technology was developed for the in situ determination of the concentration of dissolved (<0.45 μm) Fe species (Fe(II) and labile Fe) suitable for deployments in a wide range of aquatic environments. The spectrophotometric approach utilizes a buffered ferrozine solution and a ferrozine/ascorbic acid mixture for Fe(II) and labile Fe(III) analyses, respectively. Diffusive mixing, color development and spectrophotometric detection take place in three separate flow cells with different lengths such that the analyzer can measure a broad concentration range from low nM to several μM of Fe, depending on the desired application. A detection limit of 1.9 nM Fe was found. The microfluidic analyzer was tested in situ for nine days in shallow waters in the Kiel Fjord (Germany) along with other sensors as a part of the SenseOCEAN EU-project. The analyzer's performance under natural conditions was assessed with discrete samples collected and processed according to GEOTRACES protocol [acidified to pH < 2 and analyzed via inductively coupled plasma mass spectrometry (ICP-MS)]. The mechanical performance of the analyzer over the nine day period was good (consistent high precision of Fe(II) and Fe(III) standards with a standard deviation of 2.7% (n = 214) and 1.9% (n = 217), respectively, and successful completion of every programmed data point). However, total dissolved Fe was consistently low compared to ICP-MS data. Recoveries between 16 and 75% were observed, indicating that the analyzer does not measure a significant fraction of natural dissolved Fe species in coastal seawater. It is suggested that an acidification step would be necessary in order to ensure that the analyzer derived total dissolved Fe concentration is reproducible and consistent with discrete values.
Highlights
Over the past few decades the biogeochemical cycling of the trace metal iron (Fe) in the ocean has been subject to intense research interest
Molar extinction coefficients of 27,200 ± 380 L·mol−1·cm−1 and 22,100 ± 240 L·mol−1·cm−1 were obtained with the benchtop spectrophotometer and the lab-on-chip analyzer, respectively
The microfluidic Fe lab-on-chip device is able to detect Fe concentrations with a mean limits of detection (LOD) of 1.9 nM for the long cell. This is significantly lower than other Fe in situ analyzers with reported LODs of 25 nM ( SCANNER; Coale et al, 1991; Chin et al, 1994), 70 nM (ALCHMIST; Sarradin et al, 2005), 300 nM (CHEMINI; Vuillemin et al, 2009), and 27.25 nM (IonConExplorer; Jin et al, 2013). This enables measurements of DFe concentrations in the low nM regime typically found in coastal waters
Summary
Over the past few decades the biogeochemical cycling of the trace metal iron (Fe) in the ocean has been subject to intense research interest. Widespread Fe limitation of marine primary production links the biogeochemical Fe cycle with the global carbon cycle by affecting the efficiency of the ocean’s biological carbon pump and atmospheric pCO2 (Martin, 1990). High concentrations of natural organic matter compared to the open ocean, combined with multiple Fe sources, creates a highly dynamic Fe cycle in estuarine and coastal waters. This leads to a multitude of coexisting dissolved Fe species including dissolved Fe(II), Fe(III) complexes, and less bioavailable iron oxyhydroxide colloids (Rose and Waite, 2003a)
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