Abstract: Prior art methods for quantitation of radioisotopically labeled molecules employ isotopes emitting a single electron, positron or gamma-photon. Their sensitivity is limited by background events which cannot be distinguished from the decays of the isotopic label. There are isotopes decaying with concurrent production of a positron and a gamma photon, with a subsequent positron-electron annihilation producing paired 511 keV gammas with opposite momenta. The Coincident Gamma-photon Detector (CGD) registers a count when coincident gamma-photons of known energies are detected. When set to the triple gamma signature of a particular isotope, the CGD achieves exceptional background rejection with resultant improved capacities to quantitate minute traces of the isotope. With the increased sensitivity thus achieved, there are advantageous novel uses of the multiple gamma producing isotopes, for the quantitation of molecules in which they can be incorporating or adducted to.