Isotopes are atoms with the same atomic number but different mass numbers. Hevesy introduced trace amounts of radioactive isotopes into chemical or biological systems to create the tracer method. This allowed researchers to quantify the movement of tiny amounts of material and to measure reaction rates that had been impossible to observe. Because the isotope’s radiation can be detected externally, samples remain intact during study. The technique now underpins metabolism research, groundwater analysis and industrial process control, bringing wide societal benefits. Modern PET scans and environmental tracer studies are direct descendants of his work.

Hevesy introduced β- and γ-emitting radionuclides as labels and quantified their decay with ionization chambers and scintillation counters. Using isotope dilution he performed precise mass balances in reaction systems, identifying rate-determining steps and equilibrium constants. In the 1930s he visualized iron metabolism in vivo, elucidating ferritin synthesis and hematopoiesis, a milestone in medical science. By later combining stable-isotope ratio mass spectrometry, he opened a route to high-sensitivity tracing without radioactivity. These advances spread to isotope geochemistry, nuclear medicine and radiation safety, creating a cross-disciplinary platform. Current pharmacokinetic assays and environmental isotope studies rest on the theoretical and experimental foundations Hevesy established.

Hevesy introduced trace amounts of short-lived radionuclides such as ^32P and ^24Na and derived diffusion coefficients and reaction orders from time-resolved decay measurements. His isotope dilution protocol, in which a known specific activity is diluted and total radioactivity measured, revolutionized quantitative analytical chemistry. He proposed statistical models for self-absorption and background correction in radiation detectors, systematically reducing measurement errors. In vivo, he quantified iron turnover times, establishing diagnostics for hematologic disorders. Using stable ^18O labeling he dissected solvent isotope effects on enzyme kinetics, partitioning the contribution of proton transfer. These achievements underpin modern tracer kinetics, mechanistic chemistry and even performance assessments of high-level radioactive waste migration, cementing isotope tracers as a cornerstone of quantitative science.