Immobilization of aqueous pyrene by dissolved humic acid

Pyrene is frequently used as a fluorescent probe in the study of solution environments. Purging with nitrogen is a common practice for the elimination of oxygen from solutions, but this procedure leads to the loss of pyrene through carry-over in the nitrogen stream. The presence of humic acid in solution, even in small quantities, can have a significant retarding effect on this evaporation of pyrene. Based on the detergent model of humic acid, this is ascribed to its ability to sequester pyrene in pseudomicellar structures. This immobilization of pyrene has been found to depend on the molecular size and flexibility of the humic acid in question. The presence of metal ions in solution also exerts a strong influence by enhancing the detergent character of the humic acid. The degree of pyrene immobilization by dissolved humic acid can be used as a measure of its association with hydrophobic species.