Investigation on the binary ionization choices for large conjugated amines during electrospray ionization

Abstract

Rationale: Generally, amines form protonated cations ([M + H]⁺ ) in positive polarity during electrospray ionization (ESI). However, it was found that large conjugated amines (LCAs) had binary ionization choices of generating either radical cations (M^•+ ) or [M + H]⁺ during ESI. Investigation on the mechanism would further our understanding of ESI.

Methods: In this work, the binary ionization behavior of LCAs was reported and studied. Internal factors (functional groups and sizes of conjugated systems) and external factors (solvent type, flow rate, and electrode position) were systematically investigated and discussed.

Results: For the internal factors, electron-donating groups and large conjugated structures of LCAs were conducive to the generation of M^•+ . For the external factors, aprotic solvent, higher flow rate, and shorter distance from the electrode to the spray cone facilitated the formation of M^•+ but hampered the generation of [M + H]⁺ .

Conclusion: The present study illustrated that the formations of M^•+ and [M + H]⁺ for LCAs were two independent processes. The M^•+ cations of LCAs were formed on the surface of the electrode through electrochemical oxidation, whereas the [M + H]⁺ cations were generated following the typical ESI evolution process. By regulating the external factors, the ionization results of LCAs could be well modulated.