Isotopic ratio refers to the ratio of the atomic abundances of two isotopes of the same element, e.g., 18O/16O or 143Nd/144Nd. An advantage of using ratios rather than absolute abundances of a particular nuclide is a better precision. Comparing the two signals 143Nd and 144Nd can be done at the ppm (one part in one million) precision level, i.e., two to three orders of magnitude more precisely than the counting of either nuclide individually.
Isotopic ratios change as a result of (1) thermodynamic fractionation, (2) radioactive ingrowth, (3) the presence of nucleosynthetic compounds, and (4) spallation reactions. Ratios of a radiogenic isotope (e.g., 143Nd) to a stable isotope (e.g., 144Nd) are used in geochronology and provide information on the geochemistry of sample source environment; ratios of stable isotopes provide information about temperatures and sources.