title: Two competing ionization processes in electrospray mass spectrometry of indolyl benzo[b]carbazoles: formation of M⁺• versus [M + H]⁺.
abstract: BACKGROUND
  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.
authors.affiliation: College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310014, China.|Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou Normal University, Hangzhou, 311121, China.
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou Normal University, Hangzhou, 311121, China.
authors.forename: Kezhi
authors.initials: K
authors.lastname: Jiang
authors.affiliation: Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou Normal University, Hangzhou, 311121, China.
authors.forename: Jingfeng
authors.initials: J
authors.lastname: Zou
authors.affiliation: College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310014, China.
authors.forename: Zuguang
authors.initials: Z
authors.lastname: Li
mesh_terms: D002227:Carbazoles / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D008958:Models, Molecular; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods
chemical_list: D002227:Carbazoles; D002412:Cations; D011522:Protons