Surface-assisted reduction of aniline oligomers, N-phenyl-1,4-phenylenediimine and thionin in atmospheric pressure chemical ionization and atmospheric pressure photoionization.

Reduction of the oligomers formed from on-line electropolymerization of aniline, the compound N-phenyl-1,4-phenylenediimine, and the thiazine dye thionin was observed in both an atmospheric pressure chemical ionization and an atmospheric pressure photoionization source. The reduction, which alters the mass of these analytes by 2 Da, was shown to occur by means of a surface-assisted process which involves reactive species, possibly hydrogen radicals, generated from protic solvents in the ionization plasma. Reduction was minimized by limiting protic solvents, by using a high heated nebulizer temperature, and by using a clean, heated nebulizer probe liner. The expected generality of this reduction process, and the possibility of similar reduction processes in other plasma ionization sources are discussed in relation to the use of these ion sources for on-line electrochemistry/mass spectrometry experiments.