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How accurate is X-ray photoelectron spectroscopy for measuring surface composition?

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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.

X-ray photoelectron spectroscopy (XPS) is a highly effective technique for the qualitative and quantitative analysis of surface composition, capable of detecting all elements except hydrogen and helium.1

Quantitative Accuracy

XPS provides reliable quantitative data. Without extensive calibration, surface composition can be estimated using published sensitivity factors to within ±30 percent of the actual value. In well-calibrated systems, the relative standard deviation of measurements can reach ±5 percent or better.1

Factors Affecting Accuracy

While XPS is powerful, several factors can influence the accuracy of the results:
* Chemical State: XPS is sensitive to the chemical environment of elements, allowing for the identification of different oxidation states through "chemical shifts".1
* Surface Contamination: Real-world samples often accumulate atmospheric contamination, such as carbon or oxides, which can affect signal intensity.2
* Sample Damage: Although photons are generally less destructive than electron or ion beams, X-rays can occasionally damage photosensitive materials or alter their oxidation states, potentially leading to inaccurate measurements.3
* Detection Limits: Sensitivity varies by element, ranging from 1 percent for some light elements to less than 0.1 percent for heavy elements.1