Castaing, in his remarkable PhD thesis on the electron microprobe, not only detailed the construction of the prototype of what was to be the first commercial electron microprobe and demonstrated its application as a working tool in metallurgy, geochemistry and other materials sciences. In, addition, he also outlined the physical factors involved in the x-ray emission process and developed the general scheme of data correction for quantitative analysis that is still in use today.

In his thesis, Castaing outlined the nature of the processes involved in electron inelastic scattering leading to inner shell ionization and x-ray production (the so-called “stopping power” correction) and the loss of generated x-ray intensity due to elastic scattering of energetic electrons resulting in their emission from the sample (the so-called “backscatter” correction). He showed that the combination of these factors resulted in a correction primarily dependent upon the sample’s average atomic number (the so-called “atomic number” or “Z” correction), which for a thin specimen or one with minimal x-ray absorption and fluorescence would suffice in relating relative intensity to relative concentration. He noted that, in what is known as “Castaing’s first approximation”, the ratio of emitted x-ray intensity of a given line from element “A” in a specimen to that in a standard, KA = IA,spec/IA,std, can be related functionally to the mass concentration of the analyzed element: