Chemiluminescence is the generation of electromagnetic radiation as light by the release of energy from a chemical reaction. While the light can, in principle, be emitted in the ultraviolet, visible or infrared region, the reactions emitting visible light are the most common. They are also the most interesting and useful
In order to achieve the highest levels of sensitivity, a chemiluminescent reaction must be as efficient as possible in generating photons of light. Each chemiluminescent compound or group can produce no more than one photon of light. A perfectly efficient reaction would have a chemiluminescence quantum yield ΦCL of one, i.e. one photon/molecule reacted according to the equation:
ΦCL = ΦCE × ΦF × ΦR
The chemiexcitation quantum yield ΦCE is the probability of generating an electronic excited state in a reaction and has a value between 0 and 1, with 0 being a completely dark reaction and, when 1, all product molecules are generated in the excited state. The most useful chemiluminescent reactions will have a ΦCE of about 10-3 or greater.
One reason accounting for the growing popularity of chemiluminescent assays is their exquisite detection sensitivity. Unlike absorbance (colorimetric) or fluorescent measurements, assay samples typically contribute little or no native background chemiluminescence. Measurement of light intensity is relatively simple, requiring only a photomultiplier or photodiode and the associated electronics to convert and record signals. The lack of inherent background and the ability to easily measure very low and very high light intensities with simple instrumentation provide a large potential dynamic range of measurement.