In the adduct formation mechanism, molecular ions (M + ) are formed through a similar procedure as in the proton transfer mechanism, however, no hydrogen atom is required in the reagent molecule M in this case (Equation ). When M + collides with an analyte molecule, A, a stable adduct complex [M + A] + can be formed (Equation ). "+" in the fragmentation formula indicates the addition of an extra proton or fragment from chemical ionization.