Isotopic tracers

Radioactive atoms were first used as tracers in biochemical work, but inorganic elements such as lead interfered with biological processes. The use of stable isotopes like carbon-13, nitrogen-15, oxygen-18, and hydrogen-2 became significant for biochemical analysis due to their distinguishable mass differences.
 Rudolf Schoenheimer Germany 1935 Isotopic tracers

Radioactive atoms were first used as tracers in biochemical work, but inorganic elements such as lead interfered with biological processes. The use of stable isotopes like carbon-13, nitrogen-15, oxygen-18, and hydrogen-2 became significant for biochemical analysis due to their distinguishable mass differences.