In Thermal Ionisation, also referred to as Surface Ionisation, chemically-purified material loaded onto a filament which is then heated may cause some of the loaded element to be ionized as it boils off the hot filament.

The likelihood of ionisation is a function of the filament temperature, the work-function of the filament substrate and the ionization energy of the element.

This is summarised in the Saha-Langmuir equation:

(Y1/Y0) = (g1/g0) exp [(WF-IP)/kT]

Y1/Y0 = ion to neutral ratio
g1,g0 = statistical weights of ion and neutral states
WF    = Surface Work Function
IP    = element Ionisation Potential
k     = Boltzmann's constant
T     = surface temperature

One application of thermal ionisation is Thermal Ionisation Mass Spectrometry (TIMS). The ions coming of the filament are directed into a mass spectrometer to analyse the elements or isotopes present in the sample.