This can be a highly efficient method and has the experimental advantage of producing ions with a small energy spread characteristic of the filament temperature, typically a few tenths of an electron volt, as compared with beam energies of thousands of electron volts.
The filaments, generally made of platinum, rhenium, tungsten, or tantalum, are heated by current.
Surface ionization requires a nearby source of atoms, often another filament operating at lower temperatures.
Samples can also be loaded directly on the filament, a widely used and successful technique and one that has resulted in many interesting chemical treatments of the sample when it is deposited on the filament.
One such application changed lead from a difficult to an easy element to analyze, enabling important geochronological and environmental measurements.
A disadvantage of thermal ionization is the possible change in isotopic composition during the measurement. This effect is caused by Rayleigh distillation, wherein light isotopes evaporate faster than heavy ones.
Studies done on isotopes that come from radioactive decay; such as, those used in determining the ages of rocks, encounter this problem, but it is correctable using the measured values of the isotopes that are not radiogenic.
With few exceptions the use of a thermal source requires the chemical separation of the sample. Useful data are commonly obtained on extremely small (nanogram) samples.