Metal ion quenching of fulvic acid fluorescence intensities and lifetimes: nonlinearities and a possible three-component model

The quenching of fluorescence of a fulvic acid by metal ions has been revisited in this study. Fluorescence quenching has been used in the past to establish the extent of metal ion binding to fulvic acids. Both emission and synchronous fluorescence show that the relationship between the extent of quenching and fraction of cation sites occupied is not linear. It was also found that synchronous fluorescence is a more revealing measurement than emission. Similar patterns of components can be assigned from both time-resolved fluorescent measurements and synchronous fluorescence. Correlation of these two distinct types of measurements suggests that a three-component treatment of fulvic acid luminescence may be physically meaningful. For this fulvic acid the three components may be assigned lifetimes and wavelength maxima as follows; [approximately] 50 ps (392 nm), [approximately] 430 ps (465 nm), and 4.2 ns (512 nm). The three-component model accommodates pH dependence and metal ion quenching. Stern-Volmer plots also support a (minimum) three-component model.