protonated molecule

A protonated molecule is (usually) an ion formed by  addition of a proton to the neutral molecule M ,  namely (M+H)⁺. The  process of chemical ionization using a reagent gas  such as methane forms such a protonated molecule. The  transfer of a proton from one molecule to the other in the gas phase is an acid/base reaction in which the relative proton affinities of the reacting species describe the energetics of the reaction. Protonated molecules formed in other  ionization sources (such as fast atom bombardment, electro-spray ionization, or MALDI) may not be the end result of such well-defined acid/base reactions. The term protonated molecular ion has been used to describe (M+H)⁺ but is usually discouraged.