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

In effect the thickness of the sample should fit the

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the penetration depth of X-rays in case of heterogeneous samples. In effect, the thickness of the sample should fit the lateral resolution fixed by the microbeam size. This means that if 1- µ m resolution is required we will quite often be in the thin target approximation. Referring to Fig. 7.26 we see that the thin target approximation ( z < 0 . 3 D 1 / 2 ) is valid for practically all elements in biological samples where precisely one can make such thin slices. It would be the case for most elements in geological samples if 1- µ m thick slices could be obtained which is quite uncertain for soil samples for example. For metallurgical samples (Fe matrix) we shall nearly always need samples of less than 1- µ m thick to be in this approximation. Quantitative analysis seems easier in the thin target approximation. Nev- ertheless the area mass of the sample is needed which may seem trivial but which is not so in many cases. The uncertainty on thin target thicknesses is Si 0.1 1 100 Thickness ( m m) K Ca Ti Cr Mn Fe Element Ni Cu Zn Pb Thick target Thin target Infinite target 10 Fig. 7.26. Thin, thick and infinite target thicknesses as a function of elements
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484 A. Simionovici and P. Chevallier often quite large and significant variations from point to point can exist. In other samples the heterogeneity is so large that even a good knowledge of the thickness is insufficient to have a rough idea of the area mass. This is the case of soil samples where important void fractions makes the problem even more difficult. In laboratory XRF, the intensity of the Compton and Rayleigh scattered radiation from the sample is often used for quantitative purposes. Unfortunately, it is not as simple as that for polarized synchrotron radiation and this technique has not been validated yet. As the beam polarization can change from one experimental run to another or even during the same run if the orbital plane changes only very slightly, it cannot yet be considered as a reliable parameter. Anyhow, for such thin samples we will have to take into account the contribution of the scattered radiation from the sample’s supporting substrate, which is often a thick silicon slice. In conclusion, contrary to what is often praised, absolute quantitative microscopic fluorescence analysis is still very difficult to achieve in most cases. 7.2.4 Elemental Mapping Acquisition Procedure The software used during the acquisition should allow to automatically make point analysis where the coordinates of each points of interest are stored in a file, line scans (not restricted to horizontal or vertical lines) simply by setting the coordinates of the end points and of course polygonal maps of various areas of the sample. The program places the sample in the starting position, already referenced on the indexed external microscope, then starts the acquisition for a preset time, stores the data, and repeats this cycle of operations until all points have been analyzed.
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