In solution phase, the ion pair partner of an ion can be easily displaced by another ion with the same charge in the solution, and the exchanged ions can be the same or different types of ions. For example, the ammonium ion that is paired with a phosphate ion in nucleic acid can be easily displaced by sodium and vice versa. The chloride ion that is paired with a guanidium ion in peptide can be easily displaced by a hydrogen sulfate ion and vice versa. However, in gas phase, such exchange of ions between ion pairs is expected to have a high kinetic barrier because one of the steps for the exchange is the separation of ion pairs, and the energy needed for the separation is as high as 138 kJ/mol as analyzed earlier. Therefore, the exchange of ions between ion pairs in gas phase is difficult. This phenomenon is beneficial for ion detection as the change of metal cations of an ion pair to an ammonium cation that can transfer its proton to the counter anion has a high energy barrier.