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
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| Excited area (mm) | Volume/analyzed mass for | ||||
|---|---|---|---|---|---|
| Si in SiO2 | Si in steel | Fe in SiO2 | Fe in steel | Cd in aqueous solution | |
| ∅ 20 | 6 mm3 | 0.6 mm3 | 72 mm3 | 23 mm3 | 7500 mm3 |
| 16 mg | 5 mg | 190 mg | 175 mg | 7.5 g | |
| ∅ 0.3 | 0.001 mm3 | 0.0001 mm3 | 0.005 mm3 | 0.0015 mm3 | 1.7 mm3 |
| 0.003 mg | 0.0016 mg | 0.015 mg | 0.04 mg | 1.7 mg |
The sample preparation must therefore guarantee that the material of this surface layer within its information depth sufficiently characterizes the material to be analyzed. This means
that in the case of surface contamination it is important to ensure that their thicknesses are small compared to the information depth and thus cannot influence the analytical result too much;
that the surface roughness should be small against the information depth so that the absorption lengths of the fluorescence radiation are not influenced by topological effects – this influence can be reduced by rotating the sample during the measurement; or
that for light matrices and high fluorescence energies, the information depth can exceed the sample thickness. Then the measured fluorescence intensity depends not only on the concentration of the analyte but also on the sample thickness. This is shown in Figure 3.3 for the intensities of Cd in polymer samples of different thicknesses. This problem can be solved by using the same amount of sample material for all analyzed samples, i.e. both measured and reference samples.
However, this problem is somewhat mitigated because both the incident and the fluorescence radiation hit the sample at an angle less than 90° and, therefore, the analyzed volume is limited (see Figure 3.4). This means that the sample thickness does not have to be greater than the saturation thickness and in the case of the detection of heavy elements in light matrices, sample thicknesses that are less than the information depth can also generate sample-independent fluorescence intensities in case of an appropriate excitation geometry. Nevertheless, this analyzed sample volume defined by the excitation geometry must also be considered during quantification.
3.2.3 Infinite Thickness
The infinite thickness of a sample is another important parameter to consider, in particular for the analysis of layered samples (see Chapter 14). Like the information depth it depends on the element in question, other elements in the matrix, and the respective measuring geometry but it is largely independent of the spectrometer type. Typical dinfinite is assumed as three times dinformation, since the usable thickness range cannot be expanded to infinity, because the slope of the calibration curve for thick layers approaches asymptotically the infinite value. Infinite thickness does not mean that no other radiation can penetrate this layer – radiation of higher energy will not completely be absorbed in layers of this thickness. The infinite thickness, for example, of Si is about 15 μm, but only approximately 21% of the fluorescence radiation of Cu will be absorbed in this layer. In the matrix, backscattered Cu radiation can even enhance the fluorescence intensity of Si.
Figure 3.3 Cd intensities measured on polymer samples of different thicknesses.
Source: Courtesy of S. Hanning, FH Münster.
Figure 3.4 Analyzed volume limited by the measurement geometry.
3.2.4 Contaminations
For all preparation steps, the sample material comes into contact with other materials as well as with the laboratory environment. This can cause contaminations. These should, however, be avoided as far as possible in order not to influence the analytical result. Contaminations by the laboratory environment are usually in the range of trace amounts, in case of observing good laboratory practice. Another source of impurities is the contaminations by the sample preparation tools (e.g. mills, crushing tools, melting crucibles, etc.). Furthermore, if the preparation tools are not sufficiently cleaned, cross-contamination can occur as a result of transfers between the individual samples.
Table 3.5 Contaminations in the range of traces by preparation tools (main contaminations in bold).
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Because contamination cannot be completely avoided, such as by abrasion of the preparation tools, it is important that these contaminations do not affect the analyte. This is especially the case when the analytes are present in trace amounts. Changes in the contents of the main components due to traces can be neglected as well as any noticeable influence on the matrix.
Table 3.5 summarizes some typical impurity elements that can be introduced during the different steps of sample preparation.
3.2.5 Homogeneity
The homogeneity of samples for bulk specimens as well as glasses or metals and also for powder pellets can be determined by measurements at different specimen positions with a position-sensitive instrument; some examples are shown in Section 16.6. It is also possible to determine inhomogeneities by repeated measurements after the removal of sample layers. In this case, inhomogeneities perpendicular to the surface of the sample can be identified, in other words, layer structures. Furthermore, СКАЧАТЬ