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 3.18 Transmission of low-energy radiation of sample cup films.
For highly volatile solutions, sample cups with a cover are offered to reduce the evaporation process (see right cup in Figure 3.17).
The preparation steps for liquids are generally determined by their state. In the case of a homogeneous liquid, the preparation is simple – the liquid can readily be filled into the sample cup and measured.
On the other hand, a multiphase liquid, having different densities, for example, can be homogenized by stirring or shaking. The measurement should then be carried out shortly after the homogenization in order to avoid a new segregation. Another possibility is the separation of the individual phases and their separate analysis.
If particulate material is suspended in liquid, homogenization by shaking and stirring is also required, in an ultrasonic bath, for example. The measurement should also be done quickly, in order to avoid new sedimentation. Another possibility is to filter the suspended particles in order to analyze the filtered particles and the liquid separately.
3.5.2 Special Processing Procedures for Liquid Samples
Various other preparation techniques are also being used for liquids. An increase in the detection sensitivity is obtained by drying up the solvent of a liquid dropped onto a sample carrier (see Sections 11.3.2 and 12.4.1). This method enriches the analytes, and at the same time reduces the spectral background, thereby increasing the analytical sensitivity.
Other possibilities are the transfer of a liquid directly into a quasi-solid body, for instance, by gel formation or by the absorption of the liquid in a porous material such as alumina or coal. The precipitation of solutes by a targeted chemical reaction and its recovery by filtering or the absorption of solutes and the analysis of the adsorbent is also possible. In all cases, depending on the composition of the primary material, the most favorable option and chemical reaction must be selected. To ensure sufficient analytical accuracy, test measurements on reference samples of known composition should be made, in order to determine parameters such as recovery rate and reproducibility before the actual analysis.
Some of the preparation procedures are described in connection with the corresponding applications in Chapter 11.
3.6 Biological Materials
Biological materials are generally inhomogeneous. For an analysis, it is therefore first necessary to decide whether these inhomogeneities are to be determined by the analysis or whether the mean composition of the sample is of interest and it therefore must be homogenized before the analysis.
If the inhomogeneities of the sample are to be recorded, the use of spatially resolved analyses techniques is required (see Section 4.3.3 for instrumentation, Chapter 13 for the measurement algorithms, and Chapter 16 for examples). The sample preparation for position-sensitive measurements depends on the texture of the primary materials. Solid materials, such as plant parts or bones, can directly be analyzed in certain circumstances; alternatively, the samples are to be cut or ground in order to obtain flat surfaces for the analysis. In the case of tissues, it is possible to convert these into a solid sample by freeze drying, for example, or by preparing microsections for the analysis.
If, on the other hand, the primary material should be homogenized for the analysis, various preparation procedures are available, again depending on the sample state.
For solid materials, such as wood or bone, direct analysis is possible if the samples are sufficiently large and not strongly structured. They can be polished and then directly analyzed.
In the case of inhomogeneous solid samples, it is also possible to prepare pressed pellets. For this purpose, the material must be dried for several hours at 65–85 °C. Higher temperatures should be avoided to reduce the loss of easily volatile elements. After the material is crushed or cut (see Section 3.4.4), it can be pressed without binder, since it has low hardness and therefore produces stable pellets, where applicable also at elevated temperatures.
Soft materials, such as human or animal tissues, should be first dried by heating or by freeze-drying. In this way, the prepared material can then be further processed by cutting or grinding and eventually pressed into pellets. If only a small amount of material is available, it is possible to press the sample into a binder tray (see Figure 3.10).
Another possibility is the ashing of biological materials. For this purpose, the material is heated to temperatures above 900 °C under oxygen supply. Thus, all hydrocarbons escape from the sample, but also highly volatile elements such as P, S, and even important analytes such as As or Hg. The ashes can then be further processed like small-particle material, i.e. as pressed pellet or as a fusion bead.
Another possibility to prepare tissue materials is to freeze them and then produce microsections. These can then be put on a sample carrier and analyzed. Glass slides as well as thin plastic films are suitable as sample carriers. Especially in the case of glass, care must be taken to ensure that the carrier material does not contain any analyte elements.
Finally, biological fluids can be prepared for the analysis like other liquids. Examples of this are described in Section 12.4.3.
From this compilation, it becomes clear that a wide variety of preparation techniques for biological materials are available. Depending on the sample type as well as on the analytical targets, the most favorable option of sample preparation has to be selected. Like always, it is useful to carry out test preparations on comparable reference samples prior to the actual analysis in order to determine sensitivities and recovery rates.
3.7 Small Particles, Dust, and Aerosols
Dusts and aerosols are solid and liquid particles in the air, which occur in very different quantities depending on the location and the respective environmental conditions. Usually they are only present in very small amounts in the range of 50–500 ng/m3. In order to achieve enough signal intensity an enrichment of the air components must be made by accumulation. This can be achieved by depositing the material from an air stream on filters or impactors. The filters can be analyzed directly. This means the preparation of dusts and aerosols already starts with the collection of the sample material and depends on the available amount, i.e. the collected material as well as the filters used. The amount of the collected material is determined by the amount of air throughput, i.e. the airflow and collection time.
Plastic films made of Teflon or polycarbonate (Nucleopore or Millipore) are often used as filters. The dust particles are deposited on these films. The filters do not contribute to the fluorescence signal but can increase the scattering background of the spectra if they are too thick. It is also important to ensure that the coverage is not too high in order to reduce the matrix interaction СКАЧАТЬ