The backscattered electron signal in scanning electron microscopy is sensitive to changes in the atomic number of the specimen. This atomic number information can be measured by calibrating the output of a backscattered electron detector from two known materials. The atomic number of an unknown sample can then be determined by measuring the backscattered electron signal from the unknown sample.
Knowing the atomic number factor of an unknown sample and the heavy elements (Z > 10 as determined from X-ray analysis), the existence of light elements can be detected. From a knowledge of the sample and valence information, the composition of a sample can often be characterised in terms of its chemical formula.
Robinson, V. N. E.
"Materials Characterization using the Backscattered Electron Signal in Scanning Electron Microscopy,"
Scanning Microscopy: Vol. 1
, Article 11.
Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss1/11