Chemiluminescence of luminol in the presence of iron(II) and oxygen: Oxidation mechanism and implications for its analytical use

The validity of chemiluminescence-based methods relies upon the uniqueness of the relationship between the concentration of the analyte and the intensity of chemiluminescence produced. We have examined the chemiluminescence of luminol. (5-amino-2,3-dihydro-1,4-phthalazinedione) in the presence Of O-2, without added H2O2, to measure nanomolar concentrations of total Fe(II) (both inorganically and organically complexed) in aqueous samples. To test the validity of the method, we have developed a kinetic model that describes the two-step oxidation of luminol by superoxide and hydroxyl (or hydroxyl-like) radicals produced from the oxidation of Fe(II) by O-2 by synthesis of existing published data. This model was coupled with a model for Fe(Il) oxidation by O-2 in the presence and absence of the naturally occurring Suwannee River fulvic acid (SRFA). The production of chemiluminescence depended upon the concentrations of free radicals in both the sample and luminol reagent and the pH at which the reactions were performed. The relationship between Fe(II) concentration and chemiluminescence intensity was found to be unique at Fe(II) concentrations from 1 nM to 1 muM without organic complexation and from 1 to 32 nM in the presence of SRFA despite strong signal quenching in the latter case. This type of behavior will likely ensure a similarly unique relationship in the presence of a wide range of organic compounds; however, when other systems are being investigated, the technique should be carefully evaluated.