Information Depth and Analyzed Volume
The information depth is an important parameter for the assessment of sample preparation. It depends on the energy of the fluorescence radiation of the elements of interest as well as on the matrix of the sample. In the case of XRF, the incident radiation penetrates into the material, is absorbed on its way through the sample, and thereby generates fluorescent radiation. The radiation must reach the sample surface in order to be detected by the spectrometer. On its way, the fluorescence radiation is attenuated, i.e. only the radiation from a well-defined sample layer reaches the sample surface. This is referred to as the information depth.
Table 3.1 Preparation technologies for solid samples.
| Sample type | Compact and homogeneous | Compact and inhomogeneous | Powder-like | Liquid |
|---|---|---|---|---|
| Example | Metals, glasses | Minerals (ores, rocks, etc.), glasses, metal alloys, metal swarfs | Minerals (ores, slags, soil, sludge, etc.) powder, dust | Solutions, melts |
| Homogenization | Usually not necessary, where appropriate remelting | Metals, glasses: remelting ⟹ transfer into compact homogeneous material (from here see compact and homogeneous) Minerals: crushing, grinding ⟹ transfer into powder-like material or direct analysis ⟹ position-sensitive analysis (see Chapter 13) | Grinding, digestion, melting, solution | Stirring, shaking, where appropriate, filtering of solid components (as powder-like material) |
| Shaping | Sawing, cutting, turning, drilling, milling | Pouring of the powder in sample cups, pressing of disks, preparation of melted disks | Pouring in sample cups | |
| Surface preparation | Polishing or milling with a roughness adapted to the elements to be analyzed | Realized with shaping | Given by covering of the sample cup |
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
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