Lanthanide Humic Substances Interactions Determined by Capillary Electrophoresis Inductively Coupled Plasma Mass Spectrometry

Dissolved organic matter (DOM) is well known for its strong binding capacity for trace metals. In order to better predict the role of DOM in the speciation and transport of trace metals in the environment capillary electrophoresis (CE), a molecular separation technique, was coupled to a Sector Field Inductively Coupled Plasma Mass Spectrometer (SF-ICP-MS). The combination of these two techniques allows for the study of non-labile metal speciation in aquatic samples. An extensive theoretical analysis of metal-ligand separations on the molecular scale of a CE experiment was combined with numerical simulations and experimental tests to assure accurate quantitative results. It was found that the susceptibility of metal-ligand complexes to dissociation during a CE separation can be conveniently captured with a theoretical approximation of complex half-life. Complex half-life, thus is proposed to serve as a tool for assessing the likeliness of quantitative artifacts in CE-ICP-MS. By separating lanthanide complexes with EDTA and Humic Acids (i.e. strong stable ligand competition) we have been able to determine equilibrium binding constants for all 14 stable rare earth elements (REEs), Sc and Y with Suwannee river fulvic acid (SRFA) and Leonardite humic acid (LHA) at near environmental conditions (pH 6-9, 0.01 – 0.01 mol.L-1 NaNO3, 100 nmol.L-1 Ln, 10 mg.L-1 HS). Conditional binding constants for LnHS (Kc) were found to increase gradually by 2-3 orders of magnitude from La to Lu. This increasing relative affinity reflects the lanthanide contraction, a basic chemical property of the REEs related to the gradual decrease in ionic radius from La to Lu. LogKc values were found to gradually increase with increasing pH and decrease with increasing ionic strength. Additionally, LHA logKc's were on average 1.5 log units higher than SRFA logKc's with a total range of 9.0