The invention provides improvements in reagents for use in electron transfer dissociation ionization techniques for use in mass spectrometry, particularly for sequencing peptides and proteins using mass spectrometric techniques involving electro-spray ionization and MS/MS characterization of fragment ions. The novel reagents used in the inventive methods allow for more effective determination of protein sequences, especially of long peptides or post-translationally modified protein fragments. Use of the polycyclic aromatic hydrocarbons azulene, homoazulene, and acenaphthylene, and homodimers and heterodimers thereof, are described.