Capillary electrophoresis study of iron(II) and iron(III) polyaminocarboxylate complex speciation

Abstract

Environmental contextMethods for determining iron species are integral to investigations of iron cycling processes in the environment. Capillary electrophoresis is an effective tool for determining the concentrations of various iron species in solution, but the separations are highly dependent on the electrolyte composition. This study reports the use of capillary electrophoresis to separate and quantify distinct FeII and FeIII complexes with polyaminocarboxylates. AbstractThe purpose of this study was to use capillary electrophoresis to (i) separate and quantify distinct FeII and FeIII complexes with polyaminocarboxylates and (ii) develop new methods for distinguishing between FeII and FeIII in aqueous media. A 25 mM phosphate and a 50 mM 3-(N-morpholino)propanesulfonic acid (MOPS) background electrolyte (BGE), both buffered at pH 7.1, were each tested with 6 polyaminocarboxylate complexes with FeII and FeIII. Adequate separation of all FeIII-chelating agent complexes was observed with the MOPS BGE. With the phosphate BGE, sharp peaks were obtained for FeIII complexes with EDTA, HEDTA, DTPA and CDTA, however FeIII–EGTA showed excessive peak broadening, and FeIII–TMDTA showed no discernable peak. Mobilities of FeIII–EGTA, FeIII–EDTA and FeIII–HEDTA were much larger when the phosphate BGE was employed, providing evidence for ternary complex formation with phosphate during electromigration. For FeII, complexes with TMDTA and EGTA were adequately separated with either BGE, but separations of the other chelating agents resulted in their corresponding FeIII complexes due to rapid oxidation by molecular oxygen. These chelating agents may be used as preservatives and derivatising agents for the analysis of ferrous and ferric iron in environmental samples.