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Non-Radioactive Tracers | Detection Methods

Non-radioactive Methods of Detection

The enormous advantage of radioactive tracers is their sensitivity, but now non-radioactive tracers rival the sensitivity of their radioactive forebears. This can be a significant advantage because radioactive substances pose a potential health hazard and must be handled very carefully.  Furthermore, radioactive tracers create radioactive waste, and disposal of such waste is increasingly difficult and expensive.  How can a non-radioactive tracer compete with the sensitivity of a radioactive one?  The answer is, by using the multiplier effect of an enzyme.  That is, if we can couple an enzyme to a probe that detects the molecule we are interested in, the enzyme will produce many molecules of product, thus amplifying the signal.  This works especially well if the product of the enzyme is chemiluminescent (light emitting) , because each molecule emits many photons, amplifying the signal again.  The light can be detected by autoradiography with x-ray film, or by a phosphorimager.
If we want to avoid the expense of a phosphorimager or x-ray film, we can use enzyme substrates that change color instead of becoming chemiluminescent.  These chromogenic substrates produce colored bands corresponding to the location of the enzyme, and therefore to the location of the molecule we are trying to detect.  The intensity of the color is directly related to the amount of our molecule of interest, so this is a quantitative method.