Two competing ionization processes in electrospray mass spectrometry of indolyl benzo[ b ]carbazoles: formation of M +• versus [M + H] +

RATIONALE Ionization in electrospray ionization mass spectrometry (ESI‐MS) mainly occurs as a result of acid‐base reactions or coordination with metal cations. Formation of the radical cation M +• in the ESI process has attracted our interest to perform further investigation. METHODS A series of indolyl benzo[ b ]carbazoles were investigated using a quadrupole ion trap mass spectrometer equipped with an ESI source or an atmospheric pressure chemical ionization (APCI) source in the positive‐ion mode. Theoretical calculations were performed using the density functional theory (DFT) method at the B3LYP/6‐31G(d) level. RESULTS Both the radical ion M +• and the protonated molecule [M + H] + were obtained by ESI‐MS analysis of indolyl benzo[ b ]carbazoles, while only [M + H] + was observed in the APCI‐MS analysis. The relative intensities of M +• and [M + H] + were significantly affected by several ESI operating parameters and the nature of the substituents. CONCLUSIONS Formation of M +• and [M + H] + was rationalized as two competing ionization processes in the ESI‐MS analysis of indolyl benzo[ b ]carbazoles. Copyright © 2014 John Wiley & Sons, Ltd.