Developing DGT technique for determination of iron redox species by photothermal beam spectrometry

Iron (Fe) is a redox-sensitive geogenic micronutrient with complex environmental cycle and diverse
biogenic roles. It exists in an oxidized (Fe3+) or reduced (Fe2+) state. Fe redox state reflects
environmental conditions and microbial metabolic processes in which it is involved (e.g. microbial
iron respiration). These influence its bioavailability and environmental mobility and are therefore
excellent tracers of past and present environmental change.
Diffusive Gradients in Thin-films method allows to accumulate and preconcentrate labile fraction of
trace metals in situ, and was used previously to construct 2-dimensional image of total Fe distribution
using laser ablation ICP-MS. However, there is currently no method for obtaining 2D image of
Fe redox species, although there are accurate and sensitive methods for Fe redox determination,
such as the photothermal Beam Deflection Spectrometry (BDS). BDS detects light intensity change
due to thermal field around absorbing sample and correlates this change to concentration. Thus,
light-absorbing complexes are detected after reaction of Fe2+ with 1,10-phenanthroline. Fe3+ is detected
as Fe2+ after reduction with L-ascorbic acid before reaction with phenanthroline.
In order to develop a methodology capable of 2D Fe redox imaging, we coupled DGT technique
with BDS. After exposure in different Fe concentrations, ground Chelex-100 resin gels were immersed
in phenanthroline solution (preceded by L-ascorbic acid reaction for Fe3+). Fe concentration
was measured with BDS system in dry, levelled gels. The coupled DGT-BDS method was linear in
the concentration range 0-20 μmol L-1 and had limit of detection 0.14 μmol L-1 and 0.21 μmol L-1
for Fe2+ and Fe3+, respectively.
To assess environmental applicability, we deployed DGT probe in a freshwater river sediment. We
were able to produce for the first time a 2-dimensional image of labile Fe redox species, identifying
natural variability in their distribution. Our results indicate the potential of the coupled DGT-BDS
method to advance our basic understanding of Fe redox biogeochemistry. It could significantly
improve environmental model predictions, which are currently based primarily on one-dimensional
profiles. DGT-BDS method warrants further research and development, since it is reliable, precise
and non-invasive analytical method that preserves the structural characteristics of the sample.