How does it work?

Fluorescence occurs when a fluorescent capable material (a fluorophore) is excited into a higher electronic state by absorbing an incident photon and cannot return to the ground state except by emitting a photon. The emission usually occurs from the ground vibrational level of the excited electronic state and goes to an excited vibrational state of the ground electronic state. Thus fluorescence signals occur at longer wavelengths than absorbance. The energies and relative intensities of the fluorescence signals give information about structure and environments of the fluorophores.

The component parts necessary within a typical Fluorescence Spectrometer (Spectrofluorometer) are a sample holder, incident photon source (typically a xenon lamp), monochromators used for selecting particular incident wavelengths, focussing optics, photon-collecting detector (single, or preferably multiple channel) and finally a control software unit. An emission monochromator or cut-off filters are also usually employed. The detector is usually set at 90 degrees to the light source. The intrinsic sensitivity of fluorescence is also its biggest problem and care must be taken to record a true fluorescence signal of the analyte of interest.

A fluorescence emission spectrum is recorded when the excitation wavelength of light is held constant and the emission beam is scanned as a function of wavelength. An excitation spectrum is the opposite, whereby the emission light is held at a constant wavelength, and the excitation light is scanned as a function of wavelength. The excitation spectrum usually resembles the absorbance spectrum in shape.

Most materials are not naturally fluorescent. However, useful data, particularly in fluorescence microscopy can be obtained by staining non-fluorophores with an active label.