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The absolute x-ray intensity depends on many factors but it is simply a function of specimen thickness and composition by assuming the other factors are constant. Fortunately, this assumption is correction in most cases. However, in EDS measurements, especially for elemental quantifications, because of the dependence of thicknesses the absolute intensities are not used to convert to the elemental concentrations. Instead, the ratio of two or more x-ray intensities is evaluated and then converted into weight fractions of the elements. The reason for that is the ratio of measured intensities from two different elements does not depend on thickness, but is simply related to the corresponding weight-fraction ratio, C_A/C_B, given by [1],

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where,
           k_AB – The k-factor
           C_A – The concentration of element A
           C_B – The concentration of element B
           I_A – The intensity from element A
           I_B – The intensity from element B

To convert the concentrations to percentages (%), it is necessary to normalize all the concentrations of the elements based on the relationship of ΣC_n = 1, including all elements. Here, C_n is the concentration of element n. Note that at a constant accelerating voltage of a given incident beam the k-factors are independent of specimen thickness and the complexity of compositions in a thin specimen.

[1] Cliff, G. and Lorimer, G. W. (1975) J. Microsc. 103, 203.