Topics - Fluorescence
Fluorescence is a specific type of luminescence. If molecules are excited by electromagnetic radiation to produce luminescence, the phenomenon is termed photoluminescence. If the release of radiation is immediate, or stops on the removal of the exciting radiation, the substance is said to be fluorescent. If, however, the release of energy is delayed, or persists after the removal of the exciting radiation, then the substance is said to be phosphorescent. Due to its persistence, phosphorescence is of little use in chromatography detection, but fluorescence has been used to produce some of the most sensitive LC detectors (albeit somewhat specific in their response). When light is adsorbed by a molecule, a transition to a higher electronic state takes place and this absorption is highly specific to the substance concerned. Radiation of a particular energy (or wavelength) is only absorbed by a specific structure. The adsorption of light energy produces the characteristic absorption spectrum of a molecule. If the absorbed energy is not rapidly dissipated by collisions with other molecules, or by other means, the electron will return to its ground state, with the emission of electromagnetic energy producing fluorescence. As some energy is always lost in the process, the fluorescent light always has a longer wavelength than the excitation light.