Ionization Mechanism of Positive-Ion Nitrogen Direct Analysis in Real Time
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.
KeywordsIonization 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).
- 2.Chernetsova, E.S., Morlock, G.E.: Determination of drugs and drug-like compounds in different samples with direct analysis in rreal time mass spectrometry. Mass Spectrom. Rev. 30, 875–883 (2011)Google Scholar
- 4.Zhang, J.L., Huo, F.F., Zhou, Z.G., Bai, Y., Liu, H.W.: The principles and applications of an ambient ionization method-direct analysis in real time (DART). Prog. Chem. 24, 101–109 (2012)Google Scholar
- 10.Cody, R.B., Dane, A.J.: Direct analysis in real time (DART). In: Domin, M., Cody, R. (Eds.) Ambient ionization mass spectrometry. Royal Soc. Chem. (2014)Google Scholar
- 11.Musselman, B., Tice, J., Crawford, E.: Enabling automated sample analysis by direct analysis in real time (DART) mass spectrometry. In: Domin, M., Cody, R. (Eds.) Ambient ionization mass spectrometry. Royal Soc. Chem. (2014)Google Scholar
- 17.Song, L.G., Bartmess, J.E.: Ionization mechanism of direct analysis in real time (DART). In: Domin, M., Cody, R. (Eds.) Ambient ionization mass spectrometry. Royal Soc. Chem. (2014)Google Scholar
- 25.Shi, X.Y., Su, R., Yang, H.M., Lian, W.H., Wan, X.L., Liu, S.Y.: Detection of pharmaceuticals by nitrogen direct analysis in real time mass spectrometry. Chem. J. Chin. Univ.-Chin. 38, 362–368 (2017)Google Scholar
- 29.Slanger, T.G.: Reactions of electronically excited diatomic molecules. In: Fontijn, A., Clyne, M.A.A. (eds.) Reactions of small transient species, kinetics, and energetics. Academic Press, New York (1983)Google Scholar
- 32.NIST Standard Reference Database Number 69. http://webbook.nist.gov/chemistry/ (accessed August 11, 2017).
- 35.Harrison, A.G.: Chemical ionization mass spectrometry. CRC Press, Boca Raton, FL (1983)Google Scholar