Ion-Ion Chemistry for the Analysis of Biomolecular Ions via Tandem Mass Spectrometry: A Tutorial Review

Gas-phase ion-ion reactions lead to well-defined changes in mass and charge that are readily detected via mass spectrometry. They have unusually large cross sections, which allow for rates on the order of 1-1,000 s-1 and, as a result, enable a variety of analytically useful measurements. Such applications rely on one or more of a variety of reaction mechanisms, such as proton transfer, electron transfer, metal ion transfer, and selective covalent bond formation. Electrodynamic ion traps make excellent reaction vessels for ion-ion reactions due to their ability to trap one or both polarities of ions, thereby allowing reactions to proceed with high reactant to product conversion. Understanding the underlying phenomena of ion-ion reactions, as well as the conditions under which they proceed, is essential to designing future experiments. This tutorial review summarizes the underlying phenomena of gas-phase ion-ion reactions and related practical considerations needed to optimize these reactions in an electrodynamic ion trap.