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

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). This method is widely used in radiometric dating, where the sample is ionized under vacuum. The ions being produced at the filament are focussed into an ion beam and then passed through a magnetic field to separate them by mass. The relative abundances of different isotopes can then be measured, yielding isotope ratios.

When these isotope ratios are measured by TIMS, mass-dependent fractionation occurs as species are emitted by the hot filament. Fractionation occurs due to the excitation of the sample and therefore must be corrected for accurate measurement of the isotope ratio.

There are several advantages of the TIMS method. It has a simple design, is less expensive than other mass spectrometers, and produces stable ion emissions. It requires a stable power supply, and is suitable for species with a low ionization energy, such as Strontium (Sr), and Lead (Pb).

The disadvantages of this method stem from the maximum temperature achieved in thermal ionization. The hot filament reaches a temperature of less than 2500 degrees Celsius, leading to the inability to create atomic ions of species with a high ionization energy, such as Osmium (Os), and Tungsten (Hf-W). Although the TIMS method can create molecular ions instead in this case, species with high ionization energy can be analyzed more effectively with MC-ICP-MS.

See also

• Irving Langmuir
• Meghnad Saha