The tendency for ion neutralization via ion pair formation in the gas phase can be estimated using the equilibrium constant between the ion pair [M-H] -⁞[M+H] + and the ions [M-H] -and [M+H] + in equation (1). Assuming the distance between the ions in an ion pair is 1 nm and the plume temperature is 1,000 K, the energy needed to separate the ion pair into free ions is 138 kJ/mol, which corresponds to an equilibrium constant of 10 7 favoring the neutral ion pair. If a distance of 0.5 nm is assumed, the numbers become 276 kJ/mol and 10 14 , respectively (calculations are given in supporting information).