In the proton transfer mechanism, an ionization reagent containing at least one hydrogen atom in the molecule (MH n(𝑛≥1) ) is bombarded by the electron beam and generates a molecular ion MH n + (Equation ). When an analyte molecule A is introduced into the ionization chamber, MH n + collides with it and donates one proton to A during the collision producing a protonated analyte ion [A + H] + (Equation ), which is then recorded by the detector.