Special analysis of thinnest layers on the surface requires sophisticated analysis methods. The surface chemistry is determined by the top nanometer and our analysis methods look right there: the top 5-10nm.
Local distributions of substances may be intentional (structure) or unwanted (stains). We can find that out with our methods by means of an imaging analysis in the micrometer range.
If you need to go deeper, the underlying micrometers below the surface are also accessible by sputter removal.