Isotope fractionation Isotope fractionation Isotope fractionation describes processes that affect the relative abundance of isotopes, often used in isotope geochemistry. Normally, the focus is on stable isotopes of the same element. Isotopic fractionation in the natural environment can be measured by isotope analysis, using isotope-ratio mass spectrometry, to separate different element isotopes on the basis of their mass-to-charge ratio, an important tool to understand natural systems. For example, in biochemistry processes cause a fluctuation in the amount of isotopes of carbon ratios incorporated into a biological being. The difference between the true amount of carbon and the amount in the plant is
Standard atomic weight is thus a more special value of the relative atomic mass. It is defined as the "recommended values" of relative atomic masses of sources "in the local environment of the Earth's crust and atmosphere as determined by the IUPAC Commission on Atomic Weights and Isotopic Abundances." (CIAAW) In general, values from different sources are subject to natural variation due to a different radioactive history of sources. Thus, standard atomic weights are an expectation range of atomic weights from a range of samples or sources. By limiting the sources to terrestrial origin only, the CIAAW-determined values have less variance, and are