CONCEPT OF STABLE ISOTOPES AND THEIR APPLICATIONS

Concept of Stable Isotopes and their Applications

Concept of Stable Isotopes and their Applications

Isotopes are atoms of the same element that differ in atomic mass, due to differences in the number of neutrons contained in the atoms' nuclei. Isotopes are forms of a given chemical element that have different atomic masses. For a particular element, the isotopes have the same numbers of protons, and so have the same atomic number. However, each isotope has a different number of neutrons and therefore has a different atomic mass. Stable isotopes are those isotopes that do not undergo radioactive decay; so their nuclei are stable and their masses remain the same. However, they may themselves be the product of the decay of radioactive isotopes. (Fontes 1989)

Isotope fractionation

Isotope fractionation is the physical phenomenon which causes changes in the relative abundance of isotopes due to their differences in mass. There are two categories of isotope effects: equilibrium and kinetic. (Epstein 1953 213)

An equilibrium isotope effect will cause one isotope to concentrate in one component of a reversible system that is in equilibrium. If it is theta heavier isotope that concentrates in the component of interest, then that component is commonly referred to as enriched or heavy. If it is the light isotope that concentrates then the component is referred to as depleted or light. In most circumstances the heavy isotope concentrates in the component in which the element is bound more strongly and thus equilibrium isotope effects usually reflect relative differences in the bond strengths of the isotopes in the various components of the system. (Kendell Coplen 1985 1437-1440)

A kinetic isotope effect occurs when one isotope reacts more rapidly than the other in an irreversible system or a system in which the products are swept away from the reactants before they have an opportunity to come to equilibrium. Normally, the lighter isotope will react more rapidly than the heavy isotope and thus the product will be lighter than the reactant.

It should be noted that isotope fractionation will only occur in systems in which there is both an isotope effect and a reaction that does not proceed to completion. Thus, even in the presence of an isotope effect, there will be no isotope fractionation if all the reactant goes to a single product because all the atoms have reacted and thus the ratio of the heavy to light isotope must be the same in the product as it was in the reactant.

Global Mean Water Line (GMWL) and local variations

The Global Meteoric Water Line is an equation defined by the geochemist Harmon Craig [1] [2] that states the average relationship between hydrogen and oxygen isotope ratios in natural terrestrial waters, expressed as a worldwide average. (Craig 1961 1702-1703)

> dD = 8.0 * d18O + 10permille

A meteoric water line can also be calculated for a given area, and used as a baseline within that area. Kinetic fractionation will cause the isotope ratios to vary between localities within that ...