title: Paper spray mass spectrometry applied in the monitoring of a chemical system in dynamic chemical equilibrium: the redox process of methylene blue.
abstract: BACKGROUND
  The monitoring of chemical systems in dynamic equilibrium is not an easy task. This is due to the high rate at which the system returns to equilibrium after being perturbed, which hampers the possibility of following the aftereffects of the disturbance. In this context, it is necessary to use a fast analytical technique that requires no (or minimal) sample preparation, and which is capable of monitoring the species constituting the system in equilibrium. Paper spray ionization mass spectrometry (PS-MS), a recently introduced ambient ionization technique, has such characteristics and hence was chosen for monitoring a model system: the redox process of methylene blue.
  
  
  METHODS
  The model system evaluated herein was composed of three cationic species of methylene blue (MB), which coexist in a dynamic redox system: (1) [MB](+) of m/z 284 (cationic MB); (2) [MB + H + e](+•) of m/z 285 (the protonated form of a transient species resulting from the reduction of [MB](+) ); (3) [MB + 2H + 2e](+) or [leuco-MB + H](+) of m/z 286 (the protonated leuco form of MB). Aliquots of a MB solution were collected before and after the addition of a reducing agent (metallic zinc) and directly analyzed by PS-MS for identification of the predominant cationic species at different conditions.
  
  
  RESULTS
  The mass spectra revealed that before the addition of the reducing agent the ion of m/z 284 (cationic MB) is the unique species. Upon the addition of the reducing agent and acid, however, the solution continuously undergo discoloration while reduced species derived directly from cationic MB (m/z 285 and 286) are detected in the mass spectra with increasing intensities. Fragmentation patterns obtained for each ionic species, i.e. [MB](+) , [MB + H + e](+•) and [leuco-MB + H](+) , shown to be consistent with the proposed structures.
  
  
  CONCLUSIONS
  The PS-MS technique proved to be suitable for an in situ and 'near' real-time analysis of the dynamic equilibrium involving the redox of MB in aqueous medium. The data clearly demonstrated how the redox equilibrium shifts depending on the disturbance caused to the system.
authors.affiliation: Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 31270-901, Brazil.
authors.forename: Camila Cristina Almeida
authors.initials: CC
authors.lastname: de Paula
authors.affiliation: Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 31270-901, Brazil.
authors.forename: Alberto
authors.initials: A
authors.lastname: Valadares
authors.affiliation: Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 31270-901, Brazil.
authors.forename: Marina
authors.initials: M
authors.lastname: Jurisch
authors.affiliation: Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 31270-901, Brazil.
authors.forename: Evandro
authors.initials: E
authors.lastname: Piccin
authors.affiliation: Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte, MG, 31270-901, Brazil.
authors.forename: Rodinei
authors.initials: R
authors.lastname: Augusti