The acidic functional groups of humic materials are an abundant source of metal binding sites in the natural environment. Studies of metal binding to humics are of great environmental interest because the biological and physicochemical properties of metals are often changed dramatically as a result of complexation with humics. In order to understand how these heterogeneous organic macromolecules bind metals with such a large range of binding energies, lanthanide ion probe spectroscopy (LIPS) has been used to study changes in the fluorescence lifetime of the europium probe metal as it binds to these substances. A method developed by Horrocks and Sudnick for the determination of the number of water molecules bound to Eu<sup>3+</sup> was used to calculate the coordination number of humic-bound Eu<sup>3+</sup> from the fluorescence data. The peak shift of the Eu<sup>3+</sup> hypersensitive emission band (616 nm) was used to calculate the change in charge of the complex. Equations based on Horrocks and Sudnick's method were also developed to calculate the distribution of metal associated with the different types of binding sites on humic substances by computer modeling of the fluorescence life-time data.
J. W. Thomason, W. Susetyo, and L. A. Carreira, "Fluorescence Studies of Metal–Humic Complexes with the Use of Lanthanide Ion Probe Spectroscopy," Appl. Spectrosc. 50, 401-408 (1996)