Secondary Ion Mass Spectrometry (SIMS)

The ionisation method used in secondary ion mass spectrometry (SIMS) is closely related to FAB. A beam of positively or negatively charged ions is generated, but no collision cell is used to convert the ion beam into neutral particles. This ion beam is used directly to bombard the sample surface. The most commonly used ions are Cs⁺ and O₂⁺ for positive ion beams, and O⁻ for negative ion beams. Cs⁺ and O ions are generated by thermal ionisation and plasma sources, as described previously.

Please note that both Cs and O are reactive species, rather than inert ones. In SIMS, this is intentional, as both will be implanted into the sample and will affect its chemical and physical properties. However, the way in which they influence these properties is such that the use of Cs+, or the use of O₂⁺ or O⁻ as positive ions, leads to more efficient generation of negative ions.

Cs and O beams are most commonly observed in DC sources; the high acceleration voltages employed result in severe fragmentation of molecules within the sample, meaning that no molecular information is retained during the analysis. Their use is regarded as a hard ionisation method.

To circumvent this problem, pulsed sources of small and large cluster ions (Au³⁺, Bi³⁺, C₆₀⁺, Ar₂₀₀₀⁺) have also been developed; these are considered gentler ionisation methods and provide greater molecular detail in the resulting mass spectra. These sources typically operate in pulsed mode, further reducing damage to the sample surface.

ToF, sector magnet and quadrupole mass spectrometers are all commonly used instruments in SIMS.