Thermal ionization

In thermal ionization, also referred to as surface ionization, chemically-purified material loaded onto a filament which is then heated to cause some of the material to be ionized as it boils off the hot filament. Filaments are generally flat pieces of metal around 1-2mm wide, 0.1mm thick, bent into an upside-down U shape and welded to steel posts that supply a current.

Saha-Langmuir equation
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:


 * $$\frac{Y_1}{Y_0} =  \frac{g_1}{g_0} \exp \Bigg(\frac{\phi-IP}{kT}\Bigg)$$


 * $$\frac{Y_1}{Y_0}$$ = ion to neutral ratio


 * $$\frac{g_1}{g_0}$$ = statistical weights of ion and neutral states


 * $$\phi$$ = surface work function


 * IP = element ionization potential


 * k = Boltzmann's constant


 * T = surface temperature

Thermal ionization mass spectrometry
One application of thermal ionization is thermal ionization mass spectrometry (TIMS). The ions being produced at the filament are directed into a mass spectrometer to analyze the elements or isotopes present in the sample.