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Why is secondary ion mass spectrometry highly sensitive to tiny amounts of surface material?

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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.

Secondary ion mass spectrometry (SIMS) is highly sensitive to trace amounts of surface material because it utilizes a primary ion beam to induce a process known as sputtering. When energetic primary ions strike a surface, they transfer momentum to the lattice, causing the expulsion of atoms, molecules, and molecular fragments.1

Although the yield of secondary ions is typically very low—often 0.1 percent or less—the technique remains exceptionally sensitive because even a very low primary ion current density produces a sufficient number of secondary ions to generate a detectable mass spectrum. This high sensitivity allows for the detection of materials present at concentrations as low as 1 part per million (10⁻⁴ percent) of a monolayer.12

While SIMS is the least quantitative of the surface analytical techniques due to significant matrix effects that cause sensitivity variations between elements, its ability to detect minute quantities of both organic and inorganic species makes it a powerful tool for surface characterization.2