Definitions

Some important definitions are provided in this section.

Isotopologues

Molecular entities that differ only in their isotopic composition (number of isotopic substitutions), according to its IUPAC definition. It consists of isotopic forms that either:

1. do not have the same isotopic composition but have the same nominal mass (e.g., ¹³CH₃NH₂and CH₂DNH₂)

2. or have neither the same isotopic composition nor the same nominal mass (e.g., CH₄, CH₃D, CH₂D₂). The ability of the MS analyzer to distinguish different isotopologues is determined only by the elemental formula of the molecule and the resolution of the MS analyzer.

Tracer isotopologues

Isotopologues that differ only in their isotopic composition (number of isotopic substitutions) of the tracer element (e.g. CH₃NH₂, CH₂DNH₂ and CHD₂NH₂ are considered as tracer isotopologues in a ²H-labeling experiments). A molecular entity containing \(n\) tracer atoms has \(n+1\) tracer isotopologues. Tracer isotopologues have different nominal masses, hence they can be distinguished both at low and high MS resolution.

Isotopic cluster

Group of MS peaks that originate from a unique molecular entity, i.e. with the same elemental composition but different isotopic compositions, according to its IUPAC definition. A given peak of the isotopic cluster typically contains several isotopologues. The number and the nature of isotopologues overlapped in each peak is determined only by the elemental formula of the molecule and the resolution of the MS analyzer. The intensity of each peak depends on the abundance of each isotopologue, and thus on the incorporation of tracer.