15.4: Chemiluminscence
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The focus of this chapter has been on molecular luminescence methods in which emission from the analyte's excited state is achieved following its absorption of a photon. In Chapter 10 we considered atomic emission following excitation of the analyte by thermal energy. An exothermic reaction may also serve as a source of energy. In chemiluminescence the analyte is raised to a higher-energy state by means of a chemical reaction, emitting characteristic radiation when it returns to a lower-energy state. When the chemical reaction results from a biological or enzymatic reaction, the emission of radiation is called bioluminescence. Commercially available “light sticks” and the flash of light from a firefly are examples of chemiluminescence and bioluminescence.
The intensity of emitted light, \(I\), is proportional to the quantum yield for chemiluminescent emission, \(\Phi_{CL}\), which is, itself the product of the quantum yield for creating excited states, \(\Phi_{EX}\), and the quantum yield for emission through emission of a photon, \(\Phi_{EM}\). The intensity also depends on the rate of the chemical reaction(s) responsible for creating the excited state; thus
\[I = \Phi_{Cl} \times \frac{dC}{dt} \nonumber \]
where \(dC/dt\) is the rate of the chemical reaction.
Chemiluminescent measurements require less equipment than do other forms of molecular emission because there is no need for a source of photons and no need for a monochromator as the only source of photons are those arising from the chemiluminescent reaction. A sample cell to hold the reaction mixture and a photomultiplier tube may be sufficient for the optical bench. Because chemiluminescent emission depends on the reaction's rate, and because the rate decreases with time, the intensity of emission is time-dependent. As a result, the analytical signal is often the integrated emission intensity over a fixed interval of time.