Different isotopes of one chemical element have different radioactive properties. Those isotopes that are not radioactive are called stable. Whilst stable isotopes of the same element maintain the same chemical characteristics and therefore react in the same way, the mass difference as a result of an extra few neutrons results in partial separation of the light from heavy isotopes during chemical reactions (isotopic fractionation). For example, the difference in mass between the two main isotopes of hydrogen H1 (1 proton, no neutron) and H2 (also known as deuterium; 1 proton, 1 neutron) is almost 100%. Therefore, a significant fractionation will occur.
Commonly analysed stable isotopes include oxygen, carbon, hydrogen and sulphur. These isotope systems have been under investigation for many years as they are relatively simple to measure. Recent advances in mass spectrometrey (ie: muliple-collector inductively coupled plasma mass spectrometrey) now enable the measurement of heavier stable isotopes, such as Iron, copper, zinc, molybdenum, etc.