## 4 Results and Discussion
To predict the results of \(\upmu\)-XRF experiments, simulations of the X-ray fluorescence spectra from two different samples were performed with XMI-MSIM. Simulations were performed for the beam focused with AEXML, as a worst-case scenario in terms of expected beam intensity and the best scenario in terms of spatial resolution. All the simulations were carried out for a silicon drift detector equipped with ML3.3 Extreme window, helium atmosphere and backscatter geometry (90\({}^{\circ}\) angle between the beam and the sample plane and 45\({}^{\circ}\) angle between the sample plane and the detector axis) with a sample-detector distance of 3 cm and an acquisition time of 1 s. Two excitation modes were chosen: monochromatic (5 keV, 10 keV and 15 keV) with DMM and polychromatic with the white beam filtered with various aluminium filters. The sample of biological thin tissue section (4 \(\upmu\)m), one of the most demanding materials, was chosen. The elemental composition of the sample plane and the detector axis is shown in Fig. 11. The sample of biological thin tissue section is shown in Fig. the tissue was selected based on the literature data [15] and is given in Table 4, together with the simulated elemental intensities. The simulated spectra for both excitation modes are shown in Figures 12 and 13. As can be seen for 5 keV, 10 keV, and white beam filtered with 1 mm AI, the expected count rates of the elements up to zinc are high enough to make fast XRF imaging feasible even with the acquisition times being much lower than 1 s. Heavier and low-abundant elements like Se, Br, Rb and Sr can be efficiently excited with only the polychromatic beam; however, the detection of those elements will be very difficult due to the poor signal-to-noise ratio. In order to acquire information about the capabilities of analysis of higher \(Z\) elements with the high-energy tail of the polychromatic excitation spectrum, a 1 mm-thick soda-line glass (72% SiO\({}_{2}\), 14% Na\({}_{2}\)O, 12% CaO, 2% Al\({}_{2}\)O\({}_{3}\)) sample doped with some elements (Se, Sr, Zr, Mo, W, Pb, Th, U) at 100 \(\upmu\)g/g level was used. The results of the simulations for 1 mm, 2 mm and 4 mm aluminium filters are shown in Table 5 and Figure 14. Since the high energy limit of the DMM monochromator is at 15 keV, high Z elements cannot be excited with the monochromatic beam (Sr, Zr on K shell and Th, U on L shell). However, Figure 14 indicates that they can still be efficiently excited by the hardened white beam allowing analysis of such elements in samples of, i.e., geological origin. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & & \multicolumn{2}{c}{**Weight Fraction**} & \multicolumn{2}{c}{**DMM**} & \multicolumn{3}{c}{**White Beam Filtered with AI**} \\ \hline
**Z** & **Elem. ** & **Value** & **Unit** & **5 keV** & **10 keV** & **15 keV** & **0.5 mm** & **1 mm** & **2 mm** \\ \hline
1 & H & 8.1 & \% & & & & & & \\ \hline
6 & C & 64.8 & \% & & & & & & \\ \hline
7 & N & 9.8 & \% & & & & & &  
Figure 12: Simulated XRF spectra of thin tissue slice sample excited with monochromatic beam focused by AEXML. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Elem.** & **Line** &
\begin{tabular}{c} **Weight Fraction** \\ **[\(\mu\)g/g]** \\ \end{tabular} & **1 mm Al** & **2 mm Al** & **4 mm Al** \\ \hline
34 & Se & K\(\alpha\) & & 120k & 17k & 990 \\ \hline
38 & Sr & K\(\alpha\) & & 92k & 24k & 2300 \\ \hline
40 & Zr & K\(\alpha\) & & 50k & 17k & 2500 \\ \hline
42 & Mo & K\(\alpha\) & & 19k & 8400 & 1700 \\ \hline
74 & W & L\(\alpha\) & & 24k & 2400 & 130 \\ \hline
82 & Pb & L\(\alpha\) & & 44k & 6800 & 440 \\ \hline
90 & Th & L\(\alpha\) & & 30k & 8000 & 840 \\ \hline
92 & U & L\(\alpha\) & & 23k & 7300 & 890 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Elemental XRF intensities (in counts per second) for simulated glass sample. Figure 13: Simulated XRF spectra of thin tissue sample excited with polychromatic beam focused by AEXML.