Fluorescence: A Glowing Concern

The same principle that causes phosphorescent toys to glow in the dark enables scientists to detect petroleum and other aromatic compounds in the environment using laser fluorosensing technology. Phosphorescence and fluorescence occur when light of a shorter wavelength (i.e., ultraviolet) excites the molecules in certain compounds. To return to their normal, relaxed state, these compounds get rid of this energy by emitting visible light. Fluorescence takes place very quickly, while phosphorescence continues for quite a while after excitation has ceased.

Certain compounds can be made to fluoresce because their molecular structure is different than that of most carbon-based compounds, which have normal, carbon-hydrogen bonds. Instead, the majority of fluorescent compounds contain aromatic rings, such as benzene, which have special absorptive qualities. Few compounds found in the natural environment fluoresce, and those that do-such as the chlorophyll found in plant leaves-do so more weakly and at different wavelengths than oil. Most aromatics are petroleum- or hydrocarbon-based and, as such, are commonly found in manufactured products. Other materials that contain fluorescent substances are clothing dyes and laundry detergents.

There are considerable differences in the way various fluorescent compounds give off light. Light refined oils, for example, emit a more intense light for a longer period of time than most crude or heavy refined oils, because they contain smaller aromatics, such as naphthalene. The larger aromatics found in heavy refined oils (e.g., perylene) fluoresce more weakly and for a shorter period of time, because their excited molecules lose some of their excess energy by bumping into each other. Different classes of oil also exhibit different spectral signatures: lighter oils tend to give off light in the ultraviolet to blue region of the spectrum; crudes in the blue to green region; and heavy refined oils in the green to red region.