Effect of eluent composition and pH and chemiluminescent reagent pH on ion chromatographic selectivity and luminol-based chemiluminescence detection of Co2+, Mn2+ and Fe2+ at trace levels

The alkaline-luminol/H2O2-based chemiluminescent (CL) detection of Fe2+, Co2+, and Mn2+, separated with a Dionex CS5A ion chromatographic phase was studied by means of a multi-pump flow system allowing the variation of the post-column solution composition. A perchlorate gradient at pH 1.9 (with HCl) was used to separate cations partially complexed with 5.6 mM oxalate present in the eluent and necessary for the chosen separation phase. A 0.91 mM luminol, 3.3 mM H2O2 in 0.25 M carbonate buffer at pH 10.5 composition was chosen as CL reagent solution. The chosen pH value warrants signal repeatability and wider linearity range although absolute signal is not maximum. The CL signal was related to the pH of the two post-column mixing solutions. Calibration plots of Co2+ and Fe2+ were linear in the chosen concentration range whilst a parabolic model was the best fit for Mn2+. Detection limits were 0.24, 0.50 and 375 nM for Co2+, Fe2+ and Mn2+, respectively. The method was used to determine Co2+ at trace level in commercial copper chelates used for animal feeding. A comparison with a chromatographic method with spectrophotometric detection was made giving results comparable both in absolute values and accuracy.