X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg Flock
Чтение книги онлайн.
Читать онлайн книгу X-Ray Fluorescence Spectroscopy for Laboratory Applications - Jörg Flock страница 25
Название: X-Ray Fluorescence Spectroscopy for Laboratory Applications
Автор: Jörg Flock
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9783527816620
isbn:
Minor sample inhomogeneities as well as surface roughness can be partially compensated by a rotation of the sample during the measurement.
One possibility for the determination of the reproducibility of the preparation procedure is to repeat several times the entire analysis process including sample preparation, measurement, and evaluation. One can then compare the results obtained from the repeated measurements.
Sufficient homogeneity or reproducibility is achieved if the average value and standard deviation of the results of repeated measurements at the same sample position or on the same sample differ only insignificantly from the results obtained from different sample positions or at different samples. The calculation of the mean value and the standard deviation is shown in Section 6.1.1. Examples for homogeneity tests are discussed in Section 16.6.
3.3 Preparation of Compact and Homogeneous Materials
3.3.1 Metals
Metals here mean both pure metals and metal alloys. This may be a wide variety of iron alloys, nonferrous metal alloys, or hard metals.
The preparation of metals for a measurement depends on the starting material as well as on the analytical task. Casting from the melt is possible. In the case of inhomogeneous alloys, remelting is also possible. The resulting specimens first have to be adapted to the geometry of the respective measuring instrument by mechanical processing, such as cutting, drilling, or sawing and then a clean and plane surface must be prepared by milling, grinding, or polishing. For the sampling from liquid melts, probe samplers are available with which, under reproducible conditions, homogeneous samples can be produced whose external form is directly fitted for the measuring instruments (see Figure 3.5). For castings, depending on the casting mold, removal of a surface layer is required to eliminate the influence of surface segregation and corrosion. Castings should always be produced under the same conditions, i.e. temperature and mixing of the melt, cooling rate, and sample geometry to ensure high reproducibility.
Conversely, the material to be analyzed can also present a compact metal body that already has been subjected to mechanical deformation, e.g. plates, bars, or tubes; they must be prepared by modifying the shape and by processing the surface to a measurement sample. The aim is always to produce a homogeneous sample with low surface roughness.
Figure 3.5 Specimen generated by sampling from a melt.
Figure 3.6 Influence of surface roughness on the measured fluorescence intensity.
Source: According to Willis et al. (2014).
Usually, the surface roughness obtainable by grinding or milling is sufficient for the analytical task. If a surface treatment is required by polishing, care must be taken that the polishing agent does not penetrate into the sample and cause contamination. This is especially likely in the case of relatively soft sample matrices, such as aluminum samples, in conjunction with hard polishing agents. This effect can be avoided by using other surface processing methods, such as milling. For hard sample matrices, polishing is often the only way to process the sample surface. Here, the risk of contamination is relatively low and can be largely avoided by subsequent cleaning procedures, for example, washing in an ultrasonic bath.
The measured intensities can be influenced by the surface roughness of the samples. It should be taken into consideration that this influence is greater for small fluorescence energies, in particular for heavy matrices, but smaller for high fluorescence energies in light matrices. The dependence of the measured fluorescence intensities of pure metals on the surface roughness of the sample is shown in Figure 3.6.
3.3.2 Glasses
Glasses are amorphous substances that can contain very different elements, usually in the form of oxides; they are dissolved in glass formers and have typically a good homogeneity and a high surface quality. Glasses are available in very different forms for the analysis: commercial flat glasses, molded glass, and glass in art objects. Glassy beads are also frequently produced with the goal to obtain homogeneous samples from minerals or other small-particle materials. In this way, not only surface effects but also mineralogical influences can be neglected.
To keep contaminations small during the melting process, Pt crucibles are used. During the melting of the glasses, low melting temperatures are required in order to avoid or reduce the loss of easily volatile elements in the melting process. LiBO2 (lithium metaborate), Li2B4O7 (lithium tetraborate), or Na2B4O7 (sodium tetraborate or borax) are used as melting agents and glass formers (see Section 3.4).
Flat glasses usually only require shaping to fit the sample to the geometry of the measuring instrument; they already have a flat surface and are also homogeneous. The shaping can be carried out by cutting, breaking, or sawing. Remelting should be avoided, because a possible loss of easily volatile elements can occur.
For the preparation of glasses, the procedures described for metals can also be required to remove surface layers or for improving the surface quality, in particular to avoid the effects of laminations in the glass composition. As a result of the generally light matrices of glasses, the requirements for the surface quality are typically comparatively low, in particular when determining elements with high fluorescence energies due to their large information depths. The specific preparation techniques are again strongly dependent on the sample type and the analytical task. More detailed information on the preparation techniques for glasses can be found in the descriptions of the appropriate applications in Section 9.7.
Shaped glasses or art objects are often irregularly shaped, which means that sufficiently large areas are not available for a conventional large area analysis. In the case of valuable objects, sample preparation usually is not possible in order to avoid any damage to the object. Here, spatially resolved methods are recommended.
3.4 Small Parts Materials
Powder-like materials can be of various nature – minerals such as ores, rocks, or sands, slags or metal swarfs, but also polymer granules or secondary raw materials.
The sample preparation starts usually with crushing of the samples, which is usually combined with a mixture and division of the crushed material, in order to achieve sufficient representativity. For geological samples it is recommended to remove metallic components; Fe-containing particles for example can be removed by a magnetic separator, and other metals for example by flotation. After further grinding or cutting steps, the final making of the test sample can be carried out by filling the test particles into a sample cup or vessel, by pressing a tablet, or by producing a fusion bead. Depending on the starting material, the processing methods and sample quantities given in Table 3.6 are usually used.
СКАЧАТЬ