Quantum number for particle families.

(or isotopic labelling) a technique used to track the passage of an isotope (an atom with a detectable variation in neutron count) through chemical reaction, metabolic pathway, or a biological cell.

Adding stable isotopes to trace.

(physics, chemistry) A set of ratios of the abundances of different isotopes of specified elements in a sample, used as a diagnostic test of the sample's origin, or of its age

Mass of a specific isotope.