Instrumentation for Radioisotope Assays

Conventional scintillation counters such as the Microbeta (Wallac/Perkin Elmer, Turku, Finland) or the TopCount (Packard, Meriden, USA) use photomultiplier detection systems that count 8 or 12 wells at a time, resulting in a readout time of 40 minutes per 384-well microplate. Bialkali photocathodes (Sb-Rb-Cs or Sb-K-Cs) used in standard photomultipliers have a maximum spectral response at about 420 nm, with a quantum efficiency for detection of up to 30%. Thus, the aforementioned instruments are ideally suited for filtration assays and SPA assays with the blue-emitting YSi and PVT beads. 

SPA technology requires only pipetting steps and there is no need to use scintillation cocktails or to perform a separation, and thus it is ideally suited for automation by robotic liquid handling systems. However, as a consequence of the nonseparation SPA method and due to the screening of colored synthetic or natural compounds, it is essential that the detection instrument accurately assesses the level of color quenching and corrects the observed count rate (cpm) to the true activity (dpm) [49]. 

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