Ionization Mechanism of Positive-Ion Nitrogen Direct Analysis in Real Time

  • Liguo Song
  • Wei Chean Chuah
  • Xinyi Lu
  • Edward Remsen
  • John E. Bartmess
Research Article


Nitrogen can be an inexpensive alternative to helium used by direct analysis in real time (DART), especially in consideration of the looming helium shortage. Therefore, the ionization mechanism of positive-ion N2 DART has been systematically investigated. Our experiments suggest that a range of metastable nitrogen species with a variety of internal energies existed and all of them were less energetic than metastable helium atoms. However, compounds with ionization energies (IE) equal to or lower than 10.2 eV (all organic compounds except the extremely small ones) can be efficiently ionized. Because N2 DART was unable to efficiently ionize ambient moisture and common organic solvents such as methanol and acetonitrile, the most important ionization mechanism was direct Penning ionization followed by self-protonation of polar compounds generating [M+H]+ ions. On the other hand, N2 DART was able to efficiently ionize ammonia, which was beneficial in the ionization of hydrogen-bonding compounds with proton affinities (PA) weaker than ammonia generating [M+NH4]+ ions and large PAHs generating [M+H]+ ions through proton transfer. N2 DART was also able to efficiently ionize NO, which led to the ionization of nonpolar compounds such as alkanes and small aromatics generating [M–(2m+1)H]+ (m=0,1…) ions. Lastly, metastable nitrogen species was also able to produce oxygen atoms, which resulted in increased oxygen adducts as the polarity of organic compounds decreased. In comparison with He DART, N2 DART was approximately one order of magnitude less sensitive in generating [M+H]+ ions, but could be more sensitive in generating [M+NH4]+ ions.

Graphical Abstract


Ionization mechanism, N2 DART He DART 



L.S. greatly appreciates the financial support of this research by the Lucas Research Grant Program from Forensic Sciences Foundation (FSF) and the University Research Council (URC) Grant from Western Illinois University (WIU).

Supplementary material

13361_2017_1885_MOESM1_ESM.docx (486 kb)
ESM 1 (DOCX 485 kb)


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Copyright information

© American Society for Mass Spectrometry 2018

Authors and Affiliations

  1. 1.Department of ChemistryWestern Illinois UniversityMacombUSA
  2. 2.Department of ChemistryUniversity of TennesseeKnoxvilleUSA
  3. 3.Mund-Lagowski Department of Chemistry and BiochemistryBradley UniversityPeoriaUSA

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