X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg FlockЧитать онлайн книгу.
material should be ground. Here the grain sizes are not required to be as small as for pressed pellets. Grain sizes <63 μm are usually sufficient.
As pre-treatment the material is often dried by annealing. This takes place usually at temperatures of approximately 950 °C, in the case of refractory materials in accordance with ISO 12677 at 1025 °C, and should be carried out until the material mass is constant. For this purpose, the samples are placed in an Al2O3 dish and heated in a muffle furnace. In this case, the loss of ignition (LOI) must be taken into account from the evaporation of moisture or volatile materials; see Section 3.4.4.3.
Pre-oxidation of the sample material may be necessary if the sample contains pure metals that can attack or alloy with the crucible material, which can lead to a reduction in its melting point and thus damage the crucible material. The following oxidizing agents are used:NH4NO3 for low concentrations of reducing materials (oxidation at temperatures of approximately 300 °C for 1–2 minutes)LiNO3 for slags from steel production (oxidation at temperatures of approximately 815 °C for 5 minutes)NaNO3 for metal sulfides (oxidation at temperatures of 700 °C for 15 minutes). In this case, the nonvolatile Na has to be taken into account in the subsequent analyses orSrNO3, which not only oxidizes the material but also increases the mass absorption of the fusion bead.Further protection of the crucible material is possible by a layering of the oxidizing agent and the sample material, so that the crucible material only comes into contact with the melting agent, as shown in Figure 3.13.
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