Ordinarily, chemical analysis is concerned with the mass composition of a specimen and the sample is taken to obtain a representative, uniform product for the analytical operations. However, numerous occasions arise in which knowledge of both the composition and the nonuniformities in the material is required. Surfaces often receive special treatments, as in the case of coatings or the surface effects of processing which change them from the body as a whole. It is a fundamental requirement in the examination of such surfaces that a very thin film must suffice for the sample. Moreover, identification alone is not always enough. Often a diagnosis is possible only when the exact distribution and form of the surface components can be charted. Thus, the second requirement of surface analytical methods is the localization of identified products to permit recognition of significant distributive patterns.