FLUORESCENCE QUENCHING OF SYNTHETIC ORGANIC-COMPOUNDS BY HUMIC MATERIALS

Although fluorescence quenching has been used to determine equilibrium constants between synthetic organic fluorophores and water-soluble humic materials, limitations of the technique have not been thoroughly evaluated. Difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium methylsulfate) and 1-naphthol were used as fluorophores in quenching experiments with three humic acids. Curvature in the Stern-Volmer plots toward the Y-axis was not caused by inner filter effects or, in the case of difenzoquat, its tendency to form dimers. Mathematical methods to separate a possible combination of static and dynamic quenching components, which may have contributed to the curvature, were unsuccessful. The Stern-Volmer plots resembled those obtained by other researchers working with microheterogeneous micellar systems. This investigation shows that the quenching behavior of humic acid does not conform to simple Stern-Volmer theory, which assumes single-exponential decay. The use of fluorescence quenching to determine equilibrium constants of interactions occurring between humic materials and synthetic organic compounds should therefore be approached with caution.