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There are some differences in XRF and SEM‐EDS such as sample handling, experimental conditions, sample stress, and excitation sources [1, 8]. Sample handling processes for XRF are quite easy when compared to those used for SEM‐EDS. Unlike for SEM‐EDS, the electrical conductivity requirement of the sample is not needed for μ‐XRF [3, 8]. In SEM analysis, the excitation is carried out by electrons which transport electrical charges to the sample. These charges have to be removed, and therefore the sample must be electrically conductive. Otherwise, image quality and quantification are affected. In cases of a non‐electrical conducting samples, coating by carbon (C) or gold (Au) of the samples is quite important. In SEM analysis, this is the only additional effort needed for analyzing samples such as glass, minerals, and plastics. On the other hand, these kinds of samples are analyzed directly using μ‐XRF, which reduces the sample preparation time significantly.

During SEM experimentation, sample surface must be of a very high quality due to the low penetration depth of electrons inside the materials. Thus, polishing of the sample needs more attention for better results. On the contrary, this kind of sample treatment is not required for μ‐XRF analysis.

μ‐XRF analysis can also be performed in air or pre‐vacuum. Due to this advantage of μ‐XRF, vacuum‐sensitive materials (organic samples) are easily analyzed [3, 8]. Also, simple liquids or wet samples such as pastes, slurry, etc. can also be analyzed quickly. This makes μ‐XRF applicable for a wide range of materials. Sometimes a vacuum condition is needed for μ‐XRF just to avoid the absorption of the fluorescence radiation in air, and thus 10–50 mbar pressures are sufficient. In SEM analysis, the absorption of electrons must be avoided and thus pressures required are in the range down to 0.01 mbar.

Some differences also exist between the excitation by X‐rays in XRF and by electrons in SEM with respect to the sample stress. The absorption of electrons by the target material is accompanied by a higher impact of energy into the material that heats up the material and stresses it. Sometimes, it damages the materials. Contrarily, the absorption of X‐rays did not produce high energy impact into the sample, and thus the heating effect is negligible, which reduces the sample stress. Therefore, higher excitation intensities can be used for μ‐XRF analysis.

A comparison of the analytical performances of XRF and SEM‐EDS indicates the differences for sensitivity particularly for analysis of traces. To analyzed trace elements, the sensitivity mainly depends on the peak/background ratio [23]. For electron excitation, the background intensity is higher because of the bremsstrahlung of the electron beam. The spectral background for X‐ray excitation is mainly due to the scattering of the bremsstrahlung of the tube on the sample. The other factor that influences peak/background ratio is the peak‐intensity which is determined by the quantity of the element and also by its excitation efficiency i.e. the excitation conditions and the cross‐sections for the excitation. Figure 1.4 clearly shows the excitation efficiency for electrons and X‐rays that indicates that the cross section of electrons and X‐rays for exciting the K‐shell depends on the atomic numbers of the atoms [23]. This diagram reveals the high efficiency (high cross section) of electron excitation for lighter elements. In electron microscopes, lighter elements can easily be detected; even boron (B) or beryllium (Be) can be measured. However, heavy elements can be detected easily by excitation with X‐rays with better efficiency. Therefore, all the elements with atomic number greater than 20 (such as Ca) exhibit higher peak intensities and better sensitivities with X‐ray excitation [23]. This also results in better LODs in the case of heavy elements, as demonstrated in Figure 1.5 [23].

Figure 1.4 K‐shell cross sections for electron and X‐ray irradiation. Here X‐denotes the emission due to X‐rays.

Source: Reproduced from Haschke and Boehm [23] with permission from Elsevier.

Figure 1.5 Limit of detection for electron and X‐ray excitation (for similar measurement conditions for electron and X‐ray excitation).

Source: Reproduced from Haschke and Boehm [23] with permission from Elsevier.

Use of X‐ray excitation makes it possible to obtain a deeper view of the sample that provides a more illustrative analysis for bulk materials [8, 23]. Using poly‐cap optics, μ‐XRF provides similar information as offered by SEM‐EDS the spot diameter and it can also be used to examine the coating systems with thicker layers and multiple layer systems [8, 23]. Overall, as compared to SEM‐EDS, μ‐XRF offers easier sample handling, experimental conditions, and other factors which provide fast analytical results and increases its applicability for a broad variety of materials.

SEM‐EDS and μ‐XRF have a high potential for analyzing non‐homogeneous materials and thus μ‐XRF is considered a complementary analytical technique to SEM‐EDS. The other analytical capabilities of the two methods are shown in Figure 1.1 and tabulated in Table 1.1. Combined EDS and μ‐XRF provide complete sample characterization by combining the better light element sensitivity of electron excitation with the better trace element sensitivity of XRF. Simultaneous mapping with μ‐XRF and electron beam excitation combines the advantages of both the techniques, exciting the light elements (C to Na) using the electron beam and heavier elements by μ‐XRF [3, 8, 23].