The organic complexation of iron and copper: an intercomparison of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) techniques

Characterization of the speciation of iron and copper is an important objective of the GEOTRACES Science Plan. To incorporate speciation measurements into such a multinational program, standard practices must be adopted that allow data from multiple labs to be synthesized. Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) is the primary technique employed for measuring metal-binding ligands and determining metal speciation in seawater. The determination of concentrations and conditional stability constants of metal-binding ligands is particularly challenging, as results can be influenced both by experimental conditions and interpretation of titration data. Here, we report an investigation between four laboratories to study the speciation of iron and copper using CLE-ACSV. Samples were collected on the GEOTRACES II intercomparison cruise in the North Pacific Ocean in May 2009 at 30 degrees N, 140 degrees W. This intercomparison was carried out shipboard and included an assessment of the viability of sample preservation by freezing. Results showed that consensus values could be obtained between different labs, but that some existing practices were problematic and require further attention in future work. A series of recommendations emerged from this study that will be useful in implementing multi-investigator programs like GEOTRACES.