The discrepancy may be accountable by several hypotheses. One is that the plume temperature may be higher than 1,000 K. For example, at 2,200 K, the ratio of ion pair over the matrix becomes 10 -6 , and the estimated ion yield became 1.1 × 10 -8 (1.1 × 10 -2 × 10 -6 ), which is close to observed yields. The second hypothesis is that the neutralization via proton transfer process may not be able to reach thermal equilibrium, and the ion pair to neutral ratio in the plum is between 3.5 × 10 -2 , the value in solution or solid phase, and 10 -13 , the value in gas phase at 1,000 K. The third hypothesis is that the trace impurities such as alkaline salts in the matrix affect the ratio of ion to neutral molecules. When the counter cation of the carboxylate is a metal, once in the gas phase, because exchange of ions of ion pairs in gas phase is difficult, the carboxylate and metal ion pair will behave like the permanent salts. Similarly, if the counter anion of the protonated matrix is a weaker base such as hydrogen sulfate rather than the carboxylate, the ion to neutral ratio in the gas phase will also be higher. Finally, it is also possible that other ionization pathways proposed in the literature played a more important role in this case.