X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg Flock
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Название: X-Ray Fluorescence Spectroscopy for Laboratory Applications
Автор: Jörg Flock
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9783527816620
isbn:
Figure 3.1 Information depth of fluorescence radiation.
Because the energy of the primary radiation must be higher than that of the fluorescence radiation, the penetration depth is always greater than the information depth. These geometric relations are shown in Figure 3.1.
The information depth dinformation of the radiation can be estimated from Lambert–Beer's law (2.5), as well as from the measurement geometry shown in Figure 3.1. If it is assumed that about 95% of the fluorescence radiation comes from this layer, the information depth is calculated as
This relation for a few different matrices and ψ = 90° is shown in Figure 3.2.
For the fluorescence energy of a specific element, the information depth is shown here for different matrices. Since the information depth is only an approximate measure of the layer thicknesses contributing to the measurement signal, this representation is sufficient for an estimation. For other matrix compositions, it is possible to interpolate between these relations. It should also be taken into account that the information depth in Figure 3.2 is given for a perpendicular incidence. Usually however, there is an incident angle close to 45°, which reduces the information depth according to (3.1) by a factor of about 1.4.
Figure 3.2 Information depth for different matrices.
Table 3.2 Information depth for different fluorescence lines in various materials.
| Fluorescence line | Energy (keV) | Graphite (μm) | Silicon oxide (μm) | Steel (μm) | Lead (μm) |
|---|---|---|---|---|---|
| B-Kα1 | 0.18 | 4 | 0.13 | 0.01 | 0.01 |
| F-Kα1,2 | 0.68 | 3.7 | 1.7 | 0.4 | 0.3 |
| Mg-Kα1 | 1.25 | 20 | 7 | 2 | 1 |
| S-Kα1 | 2.31 | 116 | 15 | 10 | 5 |
| Cr-Kα1 | 5.41 | 1 600 | 100 | 104 | 7 |
| Ni-Kα1 | 7.48 | 4 000 | 300 | 30 | 17 |
| Cd-Kα1 | 23.17 | 14 500 | 8 000 | 700 | 77 |
Table 3.3 Information depth and accumulated intensity in the case of 10 wt% Ni in different matrices.
| Matrix | Mass absorption coefficient (g/cm2) | Information depth (μm) | Count rate in case of 10 wt% (cps) |
|---|---|---|---|
| Steel | 333 | 10.3 | 6 000 |
| Aluminum | 60 | 170 | 35 000 |
| Polyethylene | 15 | 1 300 | 170 000 |
Table 3.2 gives an overview of the information depth of different fluorescence lines in various materials. It shows, the information depth covers 6 orders of magnitude for these materials. This has to be considered for the sample preparation. The surface layer of the material to be analyzed needs to be homogeneous and representative of the corresponding fluorescence line in this material.
The influence of a different absorption of fluorescence radiation in the matrix on the information depth and the measured intensity is demonstrated in Table 3.3 for the element Ni. It shows very strong changes not only in information depth but also in accumulated intensity depending on the matrix. A factor of almost 30 is observed for the same content of Ni in different matrices. These changes must be considered in case of quantification.
Considering these information depths, the volume contributing to the measurement signal as well as the corresponding sample mass can be estimated depending on the excited sample surface, the sample matrix, and the element under consideration. Table 3.4 summarizes the volume and the analyzed mass in metallic and mineral matrices for different spot diameters. These volumes and masses are very small in metallic or mineral matrices, i.e. only a few mm3; or mg contribute to the measurement signal, but in the case of light matrices such as aqueous solutions or polymers they can be relatively large.