Rapid Determination of Isotopic Purity of Stable Isotope (D, 15N, or 13C)-Labeled Organic Compounds by Electrospray Ionization-High-Resolution Mass Spectrometry

Abstract

Rationale

Stable isotope-labeled organic compounds, containing D, ¹⁵N, or ¹³C, have widespread applications in chemistry, biology, environmental science, and agriculture. However, the isotopic purity calculations for these labeled organic compounds are usually complicated, especially in mixed isotopes-labeled scenarios. Herein, the electrospray ionization-high-resolution mass spectrometry (ESI-HRMS) was applied to determine the isotopic purity for stable isotope-labeled organic compounds, containing D, ¹⁵N, or ¹³C.

Methods

The representative isotopolog ion with its specific molecular formula was proposed to represent various labeled states. The isotopic purity was calculated using the corrected intensities of representative isotopolog ions by removing the natural isotopic contributions from preceding peaks. A unified equation has been proposed for the calculation of isotopic purity for various labeled situations including D, ¹⁵N, or ¹³C.

Results

Several case studies were presented and our calculated isotopic purities were all consistent with the isotopic purities provided in the certificate. In-source CID method was applied for the labeled compound (molecular weight > 400 u), when the maximum resolution setting was insufficient to differentiate isobaric isotopolog ions.

Conclusion

Finally, a workflow with a Python calculation program was summarized for determinations of the isotopic purity for mono isotope-labeled or mixed isotopes-labeled organic compounds, involving D, ¹⁵N, or ¹³C, by using ESI-HRMS to assign the representative isotopolog ions with accurate mass and excluding the isobar interference.