The tendency for ion neutralization in gas phase via the formation of the ion pairs in equations (2-4) should be similar as discussed for matrix ionization, and the equilibrium constant should be in the order of 10 -7 disfavoring the free ions. The tendency for ion neutralization in the gas phase via proton transfer as described in equations (3-4) can be estimated by the corresponding equilibrium constants, and they are 400 kJ/mol or 10 21 and 584 kJ/mol or 10 30 , respectively when pKa of 903 kJ/mol of guanidinium is used for R3NH + , pKa of 1303 kJ/mol is used for Pep-COOH, pKa of 719 kJ/mol is used for [M+H] + , and the plume temperature is assumed to be 1,000 K. Under the same assumptions but with the temperature being 2,200 K, the numbers become 462 kJ/mol or 10 11 for equation (3), and 593 kJ/mol or 10 . In all cases, the equilibria strongly favor the neutral species.