Chemistry. Unstable, radioactive isotopes undergo spontaneous nuclear disintegration emitting alpha or beta particles and sometimes gamma rays. Major use in biology is to trace the fate of atoms or molecules that follow the same metabolic pathway or enzymic fate as the normal stable isotope, but that can be detected with high sensitivity by their emission of radiation. Also used to locate the position of the radioactive metabolite, as in autoradiography and to measure relative rates of synthesis of compounds from radioactive precursors.
Commonly used radioisotopes are tritium (3H), carbon-14 (14C), calcium-45 (45Ca2+), iodine-131 (131I), phosphorus-32 (32P) und sulfur-35 (35S).