Radiometric Detection

One of the oldest methods used for the quantitation of drug molecules is radiometric analysis. This generally involves quantitation of radiation from beta-emitting radioactive isotopes such as 14C, 3H or 32P. Radiometric analysis is one of the most precise, sensitive, and efficient detection methods; however, there are many technical and social challenges with using this technology. Radiometric analysis begins with a radioactive isotope being incorporated into the drug molecule via a chemical synthesis procedure. Generally a specific carbon atom is replaced with 14C, or a specific hydrogen atom is replaced with 3H. The presence (or absence) of the radioactive isotope can be detected which directly correlates with the presence or absence of the drug molecule.Radiometric analysis is normally performed using liquid scintillation counting . Scintillation detection is primarily “light” detection, meaning that it detects the number of light particles. Light is produced when the radioactive isotopes emit beta particles that excite fluor molecules that emit light. The light is detected using a photomultiplier tube and converted into “counts” or “disintegrations”. Thus the results are presented as counts per minute (cpm) or disintegrations per minute (dpm).