This ionization method is especially useful for large biological molecules. One or more laser beams are tuned in resonance to transistions of a gas-phase atom or molecule to promote it in a stepwise fashion above its ionization potential to create an ion. Solid samples must be vaporized by heating, sputtering, or laser ablation. A spark source ionizes analytes in solid samples by pulsing an electric current across two electrodes. If the sample is a metal it can serve as one of the electrodes, otherwise it can be mixed with graphite and placed in a cup-shaped electrode.
Thermal ionization is used for elemental or refractory materials.
A sample is deposited on a metal ribbon, such as Pt or Re, and an electric current heats the metal to a high temperature. The ribbon is often coated with graphite to provide a reducing effect.