The ion pair thermal model has a different view about ions in the plume from the previous models. It assumes that individual free ions do not exist in the plume, all ions exist as ion pairs, and the plume is overall charge neutral. Ions preformed in the sample can be ejected into the gas phase, but they may not contribute to the pool of ions that are extracted and detected by the spectrometer, or they may only contribute a small portion of it, the latter of which is beyond the ion pair thermal model. Once in the gas phase, the preformed ions have a strong tendency to neutralize either through proton transfer or through ion pair formation. Neutral molecules ejected into the gas phase may also be ionized in the plume, and these ions may also neutralize though proton transfer and ion pair formation. Gas phase thermal equilibrium controls ion formation and ion neutralization, and determines the relative amounts of the forms of each species including their ion to neutral ratio. The ion pairs that are not neutralized through proton transfer have different energies. A small portion of the ion pairs that have significantly higher energy than average is separated to free ions by the ion extraction potential, and these free ions are detected by the spectrometer.