Surface analysis, in analytical chemistry, is the study of that part of a solid that is in contact with gas or a vacuum. When two phases of matter are in contact, they form an interface. The term surface is usually reserved fro the interface between a solid and a gas or between a solid and a vacuum; the surface is considered to be that part of the solid that interacts with its environment. A surface analysis method is a technique for discovering the chemical structure of an extremely shallow and thin area called the surface number atomic layer of the solid matter. By using current surface analyzers, the elemental composition and chemical state of materials that exist only in the surface number atomic layer can be analyzed, which is not possible with other analysis methods.