instrumental techniques applied to mineralogy and geochemistry

Alike as two water drops: distinguishing one source of the same
substance from another
Clemente Recio Hernández
Departamento de Geología y Laboratorio de Isótopos Estables, Fac. de Ciencias, Univ.
de Salamanca. Plaza de la Merced, S/N. 37008 – Salamanca. http://www.usal.es/isotopos
Introduction
It is usually possible to identify a substance by using chemical or physical methods.
However, when two substances are alike, physicochemical methods are unable to
distinguish between different sources of that same substance. When the question is not
“how much”, but rather “where from”, stable isotopes may be a helping tool: isotope
ratio measurement is a tool useful in a large range of fields, frequently completely
unrelated; not a science in itself.
In fact, every process of biogeochemical interest will involve substances that
contain one or more of the light elements H, C, N, O and S, in different proportions: they
are in the air we breath, the food and drink that we take, the environment that we live in
… And the processes that determine the direction and magnitude of isotopic
fractionation are fundamental ones, responding to basic thermodynamic principles that
cannot be tampered with.
Definitions: Elements and Isotopes
Isotopes are atoms of the same element that have the same numbers of protons and
electrons, but different numbers of neutrons. The difference in the number of neutrons
between the various isotopes of an element means that these have similar charges, but
different masses. For example, among the hydrogen isotopes, deuterium (denoted as D
or 2 H) has one neutron and one proton. This is approximately twice the mass of protium
( 1 H), whereas tritium ( 3 H) has two neutrons and is approximately three times the mass of
protium.