How does dynamic secondary ion mass spectrometry create a depth profile?

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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), depth profiling is achieved by utilizing the "dynamic" mode of operation.

In this mode, a primary ion beam with a high current density—typically around $10^{- 6}$ amperes per square cm—is directed at the sample surface. This intense bombardment causes rapid erosion of the material through a process known as sputtering, where the primary ions penetrate the lattice and expel atoms, molecules, or molecular fragments from the surface. As the beam continuously "bores" into the solid over time, it effectively eats away the surface layers, allowing the mass spectrometer to analyze the composition of the material at increasing depths.12