The Ion pair thermal model: Key elements of the ion pair thermal model include: (1) Desorption and ionization are caused by high temperature. (2) The plume is at thermal equilibrium at certain stage of the desorption and ionization process. (3) Ions in the plume may come from the solid sample or be formed in gas phase due to thermal equilibrium, and the plume is overall charge neutral. (4) Ions in the gas phase have a strong tendency toward neutralization via either proton transfer or ion pair formation. (5) Ion to neutral ratio of any species is controlled by gas phase thermal equilibrium. (6) High energy ion pairs are separated by ion extraction potential, and the separated ions are detected by the spectrometer. The following provides more details about the model in the context of information in the literature.