The principle of fluorescence detection is based on the property that certain organic molecules possess of being able to emit light radiation after excitation by an ultra-violet or near-visible ray. A common example of fluorescence: when clothes react to black light that can be found in nightclubs.

In fluorescence, the wavelength of the radiation emitted by the molecule (called the emission wavelength or λem) is different and always higher than the excitation radiation’s wavelength (called the excitation wavelength or λex).

In this way, the analysis will be done with a λex/λem couple that is much more selective than the UV absorption wavelength. This method therefore allows for more specific and much more sensitive analyses than with a UV detector (when the molecules are fluorescent, which is not the case for all UV-absorbing molecules).

As opposed to a UV detector, the modules that make up the fluorescence detector must be positioned at an angle (often 90°) to the level of the sample cell. If the light source and the detector were aligned, the fluorescence detection would be disturbed by the excitation light radiation. The angle makes sure that the detected radiation only comes from the sample’s fluorescence.