The tendency for ion neutralization via proton transfer in the gas phase can be estimated using the equilibrium constant between the neutral matrix molecule M, and the ions [M-H] -and [M+H] + in equation (1). In gas phase, pKa values are given in Gibbs energy, and those for carboxylic acids are around 1350 kJ/mol (see supporting information). Proton affinity of double bonded oxygen is around 719 kJ/mol. Thus, the equilibrium constant is 631 kJ/mol or 10 33 assuming that the plume temperature is 1,000 K. At 2,200 K, the numbers are is 640 kJ/mol and 10 16 , respectively. In both cases, the equilibrium strongly favors the neutral matrix molecules.