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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As melting agent, usually borates are used because they cannot be detected analytically by XRF and they reduce the melting point of the samples. Most commonly, lithium metaborate and lithium tetraborate are used depending on the sample composition. Samples with high contents of Al, Si, S, or Fe react better with metaborate, and samples with high contents of Na, Mg, K, and Ca better with tetraborate. Mixtures of these two melting agents are also frequently used. Further melting agents such as sodium tetraborate are available for special applications, in order to further reduce the melting point. It must be ensured that analyte elements are not also contained in these materials. The usual fluxes as well as their melting temperature can be found in Table 3.9.
Figure 3.13 Arrangement of layers of melting agent and sample in the fusion mold.
Table 3.9 Flux agents for different applications.
| Flux | Chemical notation | Melting temperature (°C) |
|---|---|---|
| Lithium metaborate | LiBO2 | 850 |
| Lithium tetraborate | Li2B4O7 | 925 |
| Mixtures | LiBO2 + Li2B4O7 | 825 for a 50 : 50 mix |
| Sodium tetraborate | Na2B4O7 | 741 |
Table 3.10 Sample–flux ratios for typical materials in grams (valid for diameter of the fused bead of 32 mm).
| Material | Ratio of sample to Li2B4O7 | Ratio of sample to Li2B4O7 + LiBO2 (mixture of 1 : 1) | Ratio of oxidizer (LiNO3) to the sample flux mix |
|---|---|---|---|
| Alumina | 0.6 : 6 | ||
| Bauxite | 0.5–1 : 6 | ||
| Cement | 2–3 : 6 | ||
| Chromium oxide | 0.1 : 10 | ||
| Coal ash | 0.6 : 6 | 1 : 1 | |
| Ferric oxide | 0.4 : 6 | ||
| Ferrous oxide | 0.4 : 6 | 1 : 1 | |
| Magnesia | 0.6 : 6 | ||
| Rocks | 0.5–1 : 6 | ||
| Silica | 1 : 6 | ||
| Slags | 0.5–1 : 6 | ||
| Sulfate | 1 : 6 | ||
| Sulfide concentrates | 0.3–0.6 : 6 | 2–3 : 1 | |
| Sulfide ores | 0.4–0.8 : 6 | 2 : 1 | |
| Titania | 0.4–0.6 : 6 | ||
| Zirconia | 0.4–0.6 : 6 |
The fluxes are used in varying ratios to the sample and sometimes also with an additional oxidizer depending on the different sample qualities. Table 3.10 gives an overview of the different types of powder materials, the ratio of the amount of the flux to the amount of the sample to be used, and if an additional oxidizer is recommended. The weighing should be carried out with an uncertainty of approximately 1 mg. In any case, it is recommended to make tests in the preparation of the different sample qualities to ensure the manufacturing of high-quality fusion beads.
It has to be taken into account that due to the flux the sample is diluted, which means both the slope of the calibration curve and the sensitivity of the analysis will be reduced with increasing flux content.
Fluxes are hygroscopic, in particular lithium tetraborate. Therefore, the melting agents should be dried before use, and preheating at 105 °C over a longer period (overnight) is recommended.
Other additives are sometimes also used in the melting process. These may be oxidizing agents and glass-forming agents, but also substances that aim to improve the quality of the fusion bead or have an influence on the analytical process.By strong absorbers, such as BaO or La2O3, the matrix interaction is increased and fusion beads can then be treated as infinitely thick samples, in which the intensity of the fluorescence signal does not depend on the sample thickness, i.e. their information depth is smaller.
Sometimes, also lithium bromide (LiBr), lithium iodide (LiI), or ammonium iodide (NH4I) is added to the molten sample in small amounts (one or two drops) СКАЧАТЬ