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tracer,an identifiable substance used to follow the course of a physical, chemical, or biological process. In chemistry the ideal tracer has the same chemical properties as the molecule it replaces and undergoes the same reactions but can at all times be detectible and quantitatively assessed. In biochemistry tracers have been in use since the beginning of the 20th cent. Using synthetic methods, Franz Knoop in 1904 made various derivatives of fatty acids, the degradation of which he studied by feeding the derivatives to dogs and by monitoring the appearance of unusual products in the dogs' urine. From these studies were obtained the first descriptions of the metabolic pathway for fatty acid catabolism. About these sorts of experiments, however, the argument could always be made that the derivatives were "unphysiological," that is, did not occur naturally and might be handled by the enzymes of the body differently than "physiological" compounds. This difficulty was overcome in 1935 when Rudolf Schoenheimer and David Rittenberg described the use of the isotope deuterium (identical to the hydrogen atom except that it contains an extra neutron) in following biochemical reactions. They argued persuasively that deuterium-labeled compounds (those having a deuterium atom substituted for a hydrogen) were essentially indistinguishable from nonlabeled compounds as far as metabolic processes were concerned but that the amount of deuterium in any given sample could be quantitatively determined by the properties of the water produced upon combustion of the sample. Although this was the first declaration of the general usefulness of the approach, George Hevesy in 1923 was the first investigator to use an isotope in metabolic studies; he explored lead transport in the bean plant using radioactive thorium. Radioactive isotopes are more easily detected than nonradioactive ones, such as deuterium; therefore, when the radioactive isotopes of various atoms commonly occurring in organic molecules became widely available after World War II, metabolic studies proliferated. Isotopes in common use today include carbon-14, iodine-131, nitrogen-15, oxygen-17, phosphorus-32, sulfur-35, tritium (hydrogen-3), iron-59, and sodium-24.
A foreign substance, usually radioactive, that is mixed with or attached to a given substance so the distribution or location of the latter can later be determined; used to trace chemical behavior of a natural element in an organism. Also known as tracer element.
A thread of contrasting color woven into the insulation of a wire for identification purposes.
Med any radioactive isotope introduced into the body to study metabolic processes, absorption, etc., by following its progress through the body with a gamma camera or other detector