[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The energy absorbed by the sample is much smaller in

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The energy absorbed by the sample is much smaller in the case of X-ray excitation compared to electron excitation and the absorption occurs within a larger volume of the sample (not just in a very thin surface layer as for elec- trons). Very sensitive samples can be analysed with high excitation intensity risking minimal or no damage at all during the measurement period. The pos- sibility to measure in air together with the negligible heating of the sample is valuable not only for organic samples or thin foils but also for minerals if they contain, for example, bonded water. For such samples, the heating induced by electron excitation may reduce the sample volume by evaporation of the water during the measurement. The same sample can be measured for a much longer time without any damaging effects when X-ray beams are used for excitation. Remember that in micro-XRF systems the X-ray source is provided by a low power X-ray tube. There are also differences in the analytical performance between EPMA and micro-XRF with advantages and disadvantages for both methods. EPMA has advantages regarding spatial resolution because the electron beam can be focussed to a much smaller spot size than the X-ray beam. The smallest spot size for SEMs is in the nm-range whereas for micro-XRF bench top units the limit is in the range of around 10 µ m. In addition, with EPMA the analysis of light elements down to Boron is possible. This is due to the different excitation modes. With an electron beam instrument it is quite straightforward to reduce the excitation voltage to be close to the absorption edge of the element of interest enhancing the excitation probability. Furthermore, the cross sections for electron and X-ray excitation differ by more than two orders of magnitude for light elements rendering the excitation by electrons more efficient at these energies. Additionally, electrons are absorbed in the upper surface of the sample which is at the same time the part of the sample that contributes mainly to the fluorescence radiation of light elements. Fluorescence radiation that is excited in deeper layers will be absorbed in the sample itself. In the case of X-ray excitation the absorption of the primary beam also occurs in deeper layers but the fluorescence radiation of light elements does not reach the sample surface.
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444 B. Kanngießer and M. Haschke Table 7.5. Typical limits of detection for micro-XRF and EPMA for a selection of elements in ppm Element Micro-XRF EPMA Ti 100 1000 Cr 80 800 Mn 50 800 Fe 40 800 Ni 30 900 Cu 20 1000 Mo 200 2000 Sn 300 4000 Pb 200 5000 On the other hand, the fact that X-rays penetrate the sample deeper may also be advantageous for certain applications. For the analysis of bulk materials a deeper penetration of the exciting beam implies that a larger representative sample volume will contribute to the detected signal. In the case of coated materials, it is possible to measure thicker coatings or multiple coatings. Another important advantage is the significantly higher sensitivity for trace element analysis. This is a result of the better peak-to-background
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