The TopCount Microplate Scintillation and Luminescence Counter has been widely used in biopharmaceutical assays involving both radionuclide and luminescence measurement. A new feature allows the use of TopCount for measuring a high energy beta emitter such as 32P by Cerenkov counting, a well-known measurement technique that has the main advantage of not requiring the addition of scintillation cocktail. Since cocktail is not required for measurement, there is little or no sample preparation, no chemical quenching, and the cost and time requirement to add cocktail is avoided. These advantages make Cerenkov counting an attractive alternative to liquid scintillation counting of 32P in microplate based assays. Cerenkov counting performance in the TopCount is documented for a variety of sample matrices and assays. Typical TopCount efficiency, background, and crosstalk data are presented for both liquid and filter based assays. Quench correction for colored samples is also demonstrated.
Cerenkov radiation occurs when charged particles traveling at or greater than the speed of light pass through a sample medium (usually liquid) where there is an exchange of energy from the charged particle to the molecules of the medium.1 β energies in excess of 1 MeV are more desirable since the counting efficiencies are higher and closer to those observed by liquid scintillation counting. The main advantage to Cerenkov counting is the fact that scintillation cocktail is not required for measurement. The popularity of this technique is well documented for a variety of applications where higher energy beta emitters are used. For example, 32P (Emax= 1710 keV) is used in molecular biology and enzyme activity assays because of the ease by which nucleotides and proteins can be labeled by phosphorylation reactions. Nucleoti