Understanding Isotopic Enrichment

Introduction
Advances in mass spectrometry have led to increased interest in materials labeled with stable isotopes. More researchers have become interested in the relative abundances of molecules enriched in stable isotopes. Questions arise about nomenclature and conventions used in stable isotope chemistry. Here we will describe our specifications for enrichment and how they are related to calculations of theoretical distributions for isotopologues.^a  

Isotopic Distributions 
Let’s say, we want to know how many molecules will have a certain number of labeled atoms. When we compare the relative amounts of molecules with different numbers of labeled atoms we have the distribution of isotopologues. We need to know the maximum number of atoms that may be labeled. This is the number of labeled atom “sites” in the molecule.  

Enrichment 
We need to know the probability of finding a labeled atom at any single site. This is the “enrichment.” Enrichment is specified as a percentage. The enrichment is not the same as probability that a molecule will be fully labeled.  

Examples 
Glycerol (D₅, 99%) has five labeled sites. In this case, the subscript 5 stands for the number of sites. Up to five of the hydrogen atoms in glycerol may be replaced by deuterium atoms. The chance of finding a deuterium atom at any of these sites is 99%.

^a Isotopologues are molecular species that differ only in isotopic substitution (e.g., H₂O and D₂O. Isotopologues may have different numbers of substituted atoms (D for H, ¹³C for ¹²C, etc.). They can easily be confused with isotopomers, which have the same number of isotopic atoms, but differ in their placement of isotopic atoms.​​​​