Where Pep-NH + and Pep-N stands for protonated and unprotonated amine, imidazole and guanidine groups in peptides. X -stands for anions such as halides, phosphate, hydrogen phosphates, sulfate, hydrogen sulfate, nitrate, hydroxide, and carboxylates. In solid phase with the usually acidic matrix as the media, Pep-N is mostly protonated, and the isolated ions predominates. Upon laser irradiation, in the gas phase the equilibria represented by equations (5-6) will have a strong tendency toward ion pairs and neutral molecules.