A novel competitive ligand equilibration-cathodic stripping voltammetry (CLE-CSV) procedure for the fast and ultrasensitive iron speciation in seawater with tenfold sample size reduction

Iron detection and speciation in seawater has a fundamental role in the understanding of oceanic biogeochemical cycles since early findings about the limitation of the primary productivity caused by subnanomolar iron concentrations in the oceanic environment. Iron speciation, i.e. its distribution between different forms (redox state II or III, inorganic, organically bound, colloidal, etc.), is of the utmost importance in understanding its behavior and connection to other oceanic cycles.

Here we present a novel competitive ligand equilibration-cathodic stripping voltammetry (CLE-CSV) procedure for iron speciation in seawater. It benefited from a recently introduced instrumental configuration which showed extreme analytical performances in terms of detection capabilities and reduced to one tenth the sample volume requirement. 2,3-dihydroxynaphthalene (DHN) and atmospheric oxygen were used as the ligand for iron and the catalytic enhancer, respectively. The chemical and electrochemical features of the system were fully investigated. The new method resulted in unparalleled detection capability, strongly reduced (to around 0.5 h) analysis time and a tenfold reduction in sample size.

The method was validated with both artificial (DTPA, TTHA) and natural (protoporphyrin IX and deferoxamine) ligands demonstrating the high accuracy and precision in determining both the concentration of the ligands and their complexation stability constants for iron. Application to the characterization of natural complex ligands as humic substances will be also shown, complemented by the investigation of iron speciation in oceanic water columns. The benefits, features and possible drawbacks of this method for the investigation of iron speciation in seawater will be finally discussed.