The principle of the recoil-less nuclear resonance absorption, or Mossbauer spec­troscopy, was discussed in detail in Section 5.4.7. Here, the emphasis is on its chemical applications: Mossbauer spectroscopy allows the analysis of chemical compounds that contain elements which have a Mossbauer nuclide. From a practi­cal point of view, the most important is iron, the Mossbauer nuclide of which is 57Fe. Its abundance in natural iron is about 2.2%. The gamma radiation source is 57Co, and its gamma radiation of 0.0144 MeV can excite the nucleus of the 57Fe isotope. The chemical state (valency) and the chemical environment modify the energy of the nuclear levels, which can be measured, thus allowing the study of the chemical species.