XPS spectra are obtained by illuminating the sample surface with monochromatic X-rays and measuring the energy of the photo-emitted electrons with an information depth of up to 10 nm for standard soft X-ray excitation sources. Thus, it provides qualitative and quantitative information about the elemental composition and the chemical state of the surface.
The classical XPS analysis technique is used under UHV conditions and this strongly restricts the type of samples that can be investigated mainly to solid samples or liquids with a very low vapor pressure. Therefore, model systems rather than real samples in their generic environments can be investigated by using standard XPS techniques in UHV.
Thus, in Near-Ambient-Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) the sample is surrounded by a gas atmosphere and no ultra-high vacuum (UHV) conditions are required in the analysis area. Therefore, investigations of a large variety of different samples, including insulating samples, biological samples, gases, liquids and their interfaces are easily accessible. When measuring XPS in a gas atmosphere, the emitted photo-electrons from samples are scattered by collisions with surrounding gas molecules before entering the hemispherical electron analyzer.