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All possible labeling patterns for a 3 carbon molecule

In 13C tracer experiments, molecules that are labelled with carbon atoms of mass 13u (u: atomic mass unit) - often called Carbon-13 - are used to determine intracellular reaction rates. Carbon-13 is a stable isotope and hence not radio active. Depending on the positon and number of labelled carbons in a molecule, different labelling patterns are possible (see picture on the right). Each isomer shown in the picture differs by the topology (position) of the carbon atom, thus they are calles isotopomers.

For a $n$ atom molecule there exist $2^n$ isotopomers.