EDS Quantification of Elements & Quantification Modes
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Quantification of the intensities of the measured X-ray peaks and thus the elemental concentrations is more problematic than qualification because the interaction of the X-rays that are collected from the specimen depends on the specimen composition itself. Therefore, the measured X-ray intensities of an element also depend on the concentration of the other elements in the specimen. These matrix effects can normally be corrected by EDS software. The procedure of quantification depends on whether the specimen is a thin film (e.g. in STEM and TEM measurements) or a bulk material (typically in SEM measurement).
The relative intensity of EDS peaks depends on the properties of the EDS detector significantly so that the elemental quantification based on EDS needs specimen standards. In accurate EDS quantifications, the appropriate corrections such as stopping power, back-scattering, X-ray absorption and secondary X-ray fluorescence within the specimen, should be evaluated and applied to the raw EDS data. In addition to the specimen itself, the X-ray generation process is also affected by the probe size, current, and convergence angle. Therefore, it is necessary to accurately measure the absolute x-ray intensity, monitor the beam current, and determine the specimen thickness at each analysis point, however, which is very difficult and time-consuming. Fortunately, for thin specimens, the problem of the uncertainty can be mostly addressed with the k- (Cliff-Lorime) and ζ-factors, and thus elemental concentration quantification can be done with reasonable accuracy by comparing the peak intensities with k-factors in EDS spectra. It is still important to know that, without calibration by using standards, the quantified atomic concentrations are false and useless in most cases.
For SEM-EDS method, in order to quantify the elemental concentrations accurately, clean, flat, smooth samples, and calibration with proper standards are needed. Without the proper standards to "re-produce" the interaction phenomena that occur within the sample, it is impossible to accurately convert the measured X-rays into exact elemental concentrations.These interactions are determined by the atomic number of the elements, the effects of X-ray absorption, and the effects of fluorescence within the sample. The two most commonly used modes for SEM-EDS are ZAF and Phi-Rho-Z even though many mathematical correction programs are generated and provided by different EDS manufacturers.
When there are no significant differences in their X-ray absorption coefficients for all the elements in the specimen, or when the specimen is thin enough, X-ray absorption corrections may be neglected to obtain an accuracy of 10-20%. However, to obtain an accuracy in the range of 5-10%, an absorption correction should be applied with an estimate of the specimen thickness.
However, overall, the basic procedure for quantitative analysis using EDS is:
Furthermore, for more accurate EDS qualifications, we also need to minimize the effects of channelling enhanced emission and thus, measurements should be performed with the crystalline orientation far away from the exact Bragg conditions and with a highly convergent beam rather than a parallel beam.
There are different algorithms which can be used for calculating quantitative data: