Die Methode der Isotopenverdünnung

Abstract

The isotope dilution principle, invented by Hevesy in 1932 and applied since 1939 to mixtures of similar often organic substances, makes the determination of substances possible that cannot be isolated in quantitative yield. It is based on the near-equality of the chemical properties of isotopes. Stable or radioactive isotopes, more often the latter, are applied. Inactive substances are separated after addition of active isotopes, and the yield in separation found by radiation measurement. Radioactive substances (natural radionuclides or pre-tagged substances), on the other hand, may be assayed after addition of inactive isotopic carrier and (incomplete) separation. The latter variant (“inverse isotope dilution method”) is more suitable for trace analysis. However, except for natural radionuclides, it can in most cases be applied only to systems that are from the beginning “under the control of the analyst,” i.e., to systems where the substance to be determined may be labelled in known specific activity. Nevertheless, the substance in question can in some cases be labelled at a later stage by a (usually stoichiometric) reaction with a radioactive reagent (combination of isotope dilution and radioreagent method). By analogy, the term “ isotope dilution” is also used for numerous methods in technology, biology and medicine, where unknown volumes or amounts of substances in inaccessible volumes are estimated by addition of isotopes in known quantities and subsequent determination of the dilution, which results from their distribution over the system.