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How does secondary ion mass spectrometry produce ions from a sample surface?

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Moon rock crystals — surface analysis A scanning-electron-microscope photograph of pyroxene and plagioclase crystals (the long and the short crystals, respectively) that grew in a cavity in a fragment of Moon rock gathered during the Apollo 14 mission.

In secondary ion mass spectrometry (SIMS), ions are produced through a process known as sputtering.

A primary ion beam, typically composed of inert gas ions with kinetic energies between 0.3 and 10 keV, is directed at the sample surface. When these energetic ions penetrate the atomic or molecular lattice of the solid, they transfer significant momentum to the lattice atoms or molecules. This collision causes substantial disruption, resulting in the expulsion of atoms, molecular fragments, or atom clusters from the surface layers.12

While the majority of these expelled particles are neutral, a small fraction—typically 0.1 percent or less—are emitted as ions. These "secondary ions" are then collected and focused by an ion lens into a mass spectrometer for analysis. This technique allows for both surface analysis and, through the continuous erosion of the material, depth profiling of the sample.12