Most organic and biochemical compounds contain hydrogen, carbon, oxygen, and nitrogen. Naturally occurring stable isotopes include:

• Carbon-13 (¹³C) – 1.1% natural abundance

• Deuterium (²H or D) – 0.01%

• Nitrogen-15 (¹⁵N) – 0.36%

• Oxygen-18 (¹⁸O) – 0.2%

In stable isotope chemistry, isotopic enrichment refers to the mole fraction of a specific isotope at a defined molecular position, expressed as a percentage.

Examples include:

• D₂O (99.8%)

• Methyl iodide (D₃, 99.5%)

• Acetic acid (¹³C₂, 99%)

Isotopic enrichment applies to specific labelled sites within a molecule. For multi-labelled compounds, enrichment may also be reported as an aggregate value across labelled positions.

It is important to distinguish between isotopic enrichment and species abundance:

• Isotopic enrichment describes the percentage of a particular isotope at a specific molecular site.

• Species abundance refers to the percentage of molecules with the same overall isotopic composition (isotopologues).

For example:

Methyl iodide (D₃, 99.5%) does not mean that 99.5% of molecules are CD₃.
At 99.5% isotopic enrichment:

• ~98.5% of molecules are CD₃

• ~1.5% are CD₂H

Understanding this distinction is critical for accurate mass spectrometry (MS) and nuclear magnetic resonance (NMR) analysis.

For deuterated compounds, species abundance for each isotopologue can be calculated using a binomial expansion, assuming equal isotopic enrichment at each labelled site.

Using Pascal’s Triangle, one can determine the statistical distribution of isotopologues for molecules containing multiple deuterium atoms.

Let:

• D = fractional isotopic enrichment (percentage ÷ 100)

• H = fractional hydrogen abundance (H = 1 − D)

Example: D₃ Molecule at 99% Enrichment

Possible isotopologues:

• D₃

• D₂H

• DH₂

• H₃

The fractional species abundance of the D₂H isotopologue is calculated as:

3(0.99)²(0.01) = 0.0294

Converted to percentage: 2.94% species abundance

Tables illustrating species abundance for molecules containing one to nine deuterium atoms at varying enrichment levels are available in the downloadable brochure.

These statistical relationships apply not only to deuterium (²H), but also to other stable isotopes such as:

• Carbon-13 (¹³C)

• Nitrogen-15 (¹⁵N)

• Oxygen-18 (¹⁸O)

Species abundance values are calculated using polynomial expansion under the assumption of random isotope distribution.

Note: In rare cases, authentic multi-labelled compounds may show slight deviations due to manufacturing processes where isotopic enrichment differs across labelled sites.