Ionization method refers to the mechanism of ionization while the ionization source is the mechanical device that allows ionization to occur. Ionization methods work by either ionizing a neutral molecule through electron ejection, electron capture, protonation, cationization, or deprotonation.

Protonation - basic residues

A proton is added to a molecule, producing a net positive charge of 1+ for every proton added. Positive charges tend to reside on the more basic residues of the molecule, such as amines, to form stable cations. Protonation can be achieved by MALDI and ESI.

Deprotonation - acidic residues

The negative charge of 1- is achieved through the removal of a proton from a molecule. Deprotonation is more useful with acidic residues. Deprotonation can be achieved by MALDI or ESI.

Cationization - carbohydrates

Cationization involves adding positively charged ions by addition of a cation adduct such as alkali or ammonium. It is very suitable carbohydrates. Cationization can be achieved by MALDI or ESI.

Transfer of a charged molecule to gas phase

This is generally achieved through the desorption or ejection of charged species from the condensed phase into the gas phase. This transfer is commonly achieved via MALDI or ESI.