Cerenkov counting is the preferred method of measurement for 32P and other energetic β-emitters. Cerenkov radiation is seen as blue light, produced when a high energy β-particle (>263 kev in water) traverses a medium of high refractive index at a velocity greater than the speed of light in that medium. For 32P in water, about 35% of all decay events can be detected in a coincidence counter. Various enhancements, e.g., the addition of trace additives which absorb the initial light and re-emit in a manner more suitable for the photomultipliers, are capable of improving the performance.
The cell coil is sandwiched between two thin disks of plastic scintillator that produce light from decay events which are not captured within the coil and are sufficiently energetic to pass through the tubing wall and to the scintillator. The combined counting of Cerenkov events as well as light produced in the plastic scintillator, more than doubles the efficiency: a LabLogic "Cerenkov cell" is capable of delivering 80% counting efficiency for 32P.
In Cerenkov counting, HPLC eluate merely passes through a Teflon coil and then either to waste or to collection. There is no need for either liquid or solid scintillator. Backgrounds are low. Contamination is almost never a problem.
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