biophysical scintillation proximity assay RIA Radio Immuno Assay PSI-MI spa SPA MI:0099 SPA relies upon the fact that a beta particle emitted from a radioisotope decay can excite a fluorophore only when it is at a very short distance in water solution (few micrometers). The ligand is labelled with a radioactive atom and its potential partner is fixed to fluorophore containing beads, the emitted fluorescence proving their interaction can be measured in a scintillation counter. The scintillator measures only the amount of bound radiolabelled ligand. Competition experiment with cold competitor can be done to estimate the binding affinities (50% inhibitory concentration [IC50], cold ligand versus labelled ligand). Loss of signal can also be used to measure substrate cleavage by an enzyme, and labelled antibodies used to titrate the degree of modified residue present.