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How does surface contamination from air affect X-ray photoelectron spectroscopy measurements?

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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 the context of surface analysis, all real-world samples exposed to the atmosphere inevitably accumulate surface contamination. When a sample is left in the air, it typically picks up a layer of carbon from organic contaminants, and reactive materials may develop a film of surface oxides or be modified by atmospheric moisture.1

In X-ray photoelectron spectroscopy (XPS), this contamination manifests as a significant carbon signal in the spectrum. While these layers are generally thin enough that the underlying surface can still be sampled, it is crucial to recognize their presence because they can alter the intensity of the signals obtained from the material of interest. Being aware of this contamination is essential for accurately interpreting the elemental and chemical state data provided by XPS.12