Thermally stable oxide

Beryllium compounds have a pronounced covalent character, and the structural unit is commonly tetrahedral. The small size of the magnesium cation results in a thermally stable oxide with low solubility in water. 

We have already seen in Chapter 2 that choice of atomizer system to be used may have a dramatic effect upon sensitivity, and thus upon signal-to-noise ratio. It is necessary to choose not only between flames, electrothermal atomization (ETA), and cold vapour and hydride generation techniques (which are discussed in Chapter 6), but sometimes also between different flames. Those elements which tend to form thermally stable oxides, such as Al, Ti, Si, Zr, may only be determined in a hotter, reducing nitrous oxide-acetylene flame. They cannot be determined with useful sensitivity in the air-acetylene flame. Some elements, Ba and Cr for example, may be determined in air-acetylene, but are more efficiently atomized in nitrous oxide-acetylene. 

For some elements, especially those which tend to form thermally stable oxides, fuel-to-oxidant ratio may have a dramatic effect upon atomic absorbance signal. Figure 4, for example, illustrates the effect of increasing fuel flow upon aluminium determination. 

Thorium. Th02 is one of the most thermally stable oxides known, but it forms a slightly oxygen-deflcient, congruently-vaporizing solid ThOi.998 (3) at temperatures of about 2500°C. Th02 is a fluorite-type dioxide with a = 5.999A. 

The detection limits of flame AAS are particularly low for fairly volatile elements, which do not form thermally stable oxides or carbides and have high excitation energies, such as Cd and Zn. Apart from these and some other elements such as Na and Li the detection limits in flame AAS are higher than in ICP-AES (see Table 20 in Section 10). 

Incomplete atomization of the analyte causes so-called chemical interferences. They are due to the fact that atomic absorption can only occur with free atoms. Thus reactions in the flame which lead to the formation of thermally stable species decrease the signals. This fact is responsible for the depression of calcium signals in serum analysis by the proteins present, as well as for the low sensitivities of metals that form thermally stable oxides or carbides (Al, B, V, etc.) in flame AAS. A further example of a chemical interference is the suppression of the absorbance of earth alkali metals as a result of the presence of oxyanions (X) such as aluminates or phosphates. This well-known calcium-phosphate interference is caused by the... 

In flame AFS, elements which form thermally stable oxides such as Al, Mg, Nb, Ta, Zr and the rare earths are hampered by insufficient atomization. This is not the case when an ICP is used as the fluorescence volume. Here the detection limits for laser excitation and non-resonant fluorescence are lower than in ICP-AES (Table 18) [663]. ICP-AFS can be performed for both atomic and ionic states [664]. 

In addition, thermally stable oxides, like perovskites or hexa-aluminates, which are used as active material or washcoat in catalytic combustion, can be used to manufacture high temperature supports. Consequently, promising materials that are reported here could be used as support and active phase in one and the same material or as a washcoat on a support made of another material. 

Stable compound formation will always cause a depressive effect. Typical examples are the lowering of alkaline earth metal absorbances in the presence of phosphate, aluminate, silicate and some other oxo anions, the low sensitivity of metals which form thermally stable oxides (refractory oxide elements), and the depression of the calcium signal in the presence of proteins. In addition, some refractory oxide elements may also form stable carbides, especially in rich hydrocarbon flames. 

The observation point in the flame may also affect the absorption signal. If the analyte forms a thermally stable oxide, the degree of the interference may vary in different positions of the flame. 

Recommendation 2b. Coating approaches that promise to provide damage tolerant oxide composites should be evaluated to prove or disprove their viability. Based on the preliminary results discussed in Chapter 6, the committee has concluded that research should be focused on the following areas weakly bonded, thermally stable oxide coatings (e.g., rare-earth phosphates of the general formula Me P04) and the development of oxide composites that do not require fiber... 

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