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.
Table 3.4 Estimations of the sample volume and mass that contribute to the measured signal.
| 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