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Liquid Scintillation Counting

Liquid Scintillation Counting

The principle of this method is to convert the radioactive emissions from a sample to photons of visible light that a photomultiplier tube can detect.  To do this, we place the radioactive sample (a band cut out of a gel, for example),  into a vial with scintillation fluid.  This liquid contains a fluor, a compound that fluoresces when it is bombarded with radioactivity.  Thus, it converts the invisible radioactivity into visible light.  A liquid scintillation counter is an instrument that takes the vial and places it in a dark chamber with a photomultiplier tube.  There, the tube detects the light resulting from the radioactive emissions exciting the fluor.  The instrument counts these bursts of light, or scintillations, and records them as counts per minute (cpm).  This is not the same as dpm because the scintillation counter is not 100% efficient.  One common radioisotope used by molecular biologists is 32P. The B-rays emitted by this isotope are so energetic that they create photons even without a fluor, so a liquid scintillation counter can count them directly, though at a lower efficiency than with scintillation fluid.