Ion trap tandem mass spectrometry study of dexamethasone transformation products on light activated TiO2 surface.

The photocatalytic transformation of dexamethasone and the formation of its intermediate compounds have been studied using titanium dioxide as a photocatalyst. The degradation of dexamethasone occurs easily through the formation of several hydroxy derivatives whose characterization has been made by HPLC/MS/MS. Even if both oxidative and reductive processes can be operating, only oxidative products have been identified in air saturated aqueous suspensions. A pattern of reaction pathways accounting for the observed intermediates is proposed. The obtained experimental evidence may be rationalized postulating the existence of a double initial mechanism. A single oxidation step resulting from the attack by one *OH radical leading to the formation of five hydroxy-derivatives and a concomitant attack involving two *OH radicals leading to the hydroxylation of the quinoid moiety of the molecule.