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NAP-XPS: Surface Chemistry Analysis

NAP-XPS is a less traditional form of XPS (x-ray photoelectron spectroscopy), it allows samples to be analysed at relatively high pressures, for example, at greater than 2500 Pa. By using the NAP-XPS, the XPS can effectively probe moderately volatile liquids, biological samples, porous materials or even polymeric materials that outgas significantly.

 

XPS and UPS (ultraviolet photoelectron spectroscopy) are used to analyse the surface chemistry of a material. XPS spectra are obtained by illuminating the sample surface with monochromatic x-rays and eventually measuring the photo emitted electrons. With XPS, the elemental composition and the chemical state of the detected elements can be determined quantitatively in the surface region with an information depth of up to 10nm. 

 

Many NAP-XPS system designs allow for investigations of a large variety of different samples, including insulating samples, gases, liquids and their interfaces that are not accessible through the standard XPS systems. The sample under investigation is the central part of every XPS system - in a NAP-XPS system, the sample needs to be positioned underneath the analyser nozzle, which is the entrance to the analysis section of the machine. Samples can range from flat surfaces to really rough three-dimensional structures; they can be tiny with a diameter of just a few hundred microns, or as large as 10mm in NAP-XPS.

 

Aside from the sample itself and the analyser, an x-ray source is the third component required to perform an XPS analysis of a surface to gain insights into its chemical composition on the molecular level. The energy from the x-ray source is low when compared to medical x-ray sources, that the stainless steel or µ-metal chamber walls of the machine will absorb them completely, to ensure that no x-rays will leave the system. The beam of x-rays is focused into a spot on the sample that needs to be analysed - about 300 µm in diameter defining the analysis region.

 

X-ray photoelectron spectroscopy is based on the physical principle of photoionization. Whenever a photon of high enough energy interacts with an atom in the uppermost surface layers (roughly 10nm) an electron of a specific energy is being released. The energy is characteristic for the element and the nearest neighbours of the atom in the surface layer. Therefore, it is not only possible with XPS to determine the surface concentration of an element down to 0.1 atomic percent, but also its binding conditions to other atoms and molecules. 

 

In conventional XPS systems where the analysis region needs to be kept under strict vacuum conditions, an effect occurs that is named surface charging. For example, the sample is nonconductive and the impinging x-ray photons continuously removing electrons from the analysis region than the sample will be slowly charged positively. The escaping electrons will be influenced in their path and energy by the positive charges at the surface. To prevent the surface from charging up, conventional XPS systems are fitted with electron and ion guns that can be turned in a way that additional charges will be brought to the surface region compensating the lost charges due to the photoionization process. 

 

NAP-XPS systems are not conventional XPS systems, but Ambient Pressure XPS systems. Which means that it does not require the sample to be kept under vacuum. A low pressure of less than 50 mbar is sufficient. This extremely high working pressure, when compared to conventional systems, opens up the field of biology, biochemistry, astrobiology, medicine and others. Due to the non-vacuum conditions to operate in, means that the sample being analysed can be outgassing or even be a liquid with a vapour pressure in this range.

 

NAP-XPS also works at synchrotron beamlines with variable photon energies. A special beam entrance stage either with differential pumping or a Si3N4-window has to be used in this case.


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