Isotopes are atoms of the same element whose nucleus contains the same number of protons (Z), but a different number of neutrons (N) and, consequently, different masses. Hydrogen, the first and most abundant element in the universe, from which the others were formed, has three isotopes, Hydrogen, also called protium, whose nucleus consists of a single proton, Deuterium, which has a nucleus formed by a proton and a neutron and tritium, which has two neutrons and a proton in its nucleus.
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The atomic mass given in the periodic table is the weighted average of the mass of that element, taking into account the natural abundance of its isotopes. Most elements have one or more, up to 10 for tin (Sn), stable isotopes with known natural proportions (abundances). The Table below presents examples of stable isotopes of elements, their respective masses and natural abundances. Small variations in these proportions allow the characterization of different materials.
The natural isotopic variations are given with respect to defined patterns for each element and their notation is done in δ‰ to more conveniently and legibly express the variations of isotopic ratios that are normally in the third to fifth decimal places.
The internationally used standards for the determination of isotopic variations for the most commonly used light elements as well as their respective ratios (R) can be seen in the table below.
These variations occur due to the phenomenon called Isotopic Fractionation, which occurs in physical processes such as change of state, chemical processes such as precipitation and biological processes such as photosynthesis.
The variations in the concentrations of isotopic species, under different conditions or phases in a physical system, subject to a certain process, are what determine different isotopic signatures of the materials or substances subject to analysis. The isotopic signatures with respect to δ13C‰ of different materials are presented below.
Isotopic signatures with respect to δ13C‰ of different materials (Wagner T., Magill CR, Herrle JO,2018)
