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# 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:[1]

$\frac{Y_1}{Y_0} = \frac{g_1}{g_0} \exp \Bigg(\frac{\phi-IP}{kT}\Bigg)$
$\frac{Y_1}{Y_0}$ = ion to neutral ratio
$\frac{g_1}{g_0}$ = statistical weights of ion and neutral states
φ = 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.[2]