Luminometry

Introduction:

If a substance is exposed to an energy source, for example heat or various forms of radiation, energy is usually absorbed.  the energy content of the substance is increased, this may show itself as increased temperature or as an excitation of the electrons in the molecules are excited they are often unstable and tend to lose their excess energy in order to return to the stable state.  The excess energy can be released to the environment in many forms, for example as heat, or various other forms of radiation.

One way for excited molecules to release their excess energy is by emitting visible light.  Processes in which molecules reach high energy excited states by chemical reactions and then release energy as light emission are called chemiluminescent reactions.  When the molecule reach a high energy state by absorbing light and then release energy as light emission, this process is called fluorescence or phosphorescence.

Certain chemical reactions can produce high energy states in a wide variety of different substances.  In order to give luminescence, the energy released during the reaction must be sufficient to produce light.  Consequently most of the chemical reactions that lead to chemiluminescence are oxidation reactions in which large amounts of energy are released for each oxidized molecule.

Bioluminescence:

Many chemiluminescent reactions take place in living organisms, such reactions are called bioluminescent reactions.The chemiluminescent oxidation is catalysed by enzyme called LUCIFERASES.  This enzyme is found in many species of insects, bacteria, jellyfish, fungai, crustaceans etc.

Chemiluminescence used in analysis;

In chemiluminescent analysis the light emitted during the cource of a reaction is measured.  The rate of emission of photons depends on the rate of the oxidation reaction.  The reaction rate in turn depends on the concentration of the reacting molecules present in the reacting medium.Therefore, a measurement of chemiluminescent gives a direct indication of the concentration of the reaction molecules.

Instrumentation:  

A major advantage of chemiluminescent analysis is a lower detection limit and the simple procedure. In principle all that is needed is a light detector with a spectral sensitivity that covers the emission wavelength and a device to record the detector output.  

A photoelectric detector in the form of a photomultiplier can have an extremely fast response.

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