Radioactivity is the emission of particles and/or energy by the nucleï of an atom. Radioactive labelling introduces only small changes into a compound and has little influence on reactions. Most common are \({\alpha}\) - ( \(\mathsf {He}\) -nucleï), \({\beta}\) - (electrons) and \({\gamma}\) - (photons) radiation. Some elements have more than one stable isotope; the rarer one can be used for labelling too and is detected by mass spectrometry.