Refractory oxide forming element

Conventional AA instruments (Figure 1) use a flame atomization system for liquid sample vaporization. An air-acetylene flame (2300°C) is used for most elements. A higher temperature nitrous oxide-acetylene flame (2900°C) is used for more refractory oxide forming elements. Electrothermal atomization techniques such as a graphite furnace can be used for the direct analysis of solid samples. 

In the earliest days of flame AAS, air-propane and air-butane flames were often used to atomize samples, largely because they had a reputation for being simple and safe in operation. However it was soon found that such flames were not satisfactory for breaking up many thermally stable chemical compounds into the free atoms required to obtain atomic absorption. If samples and standards are not atomized to the same extent, erroneous results are obtained. Nowadays the most commonly used flame is the air-acetylene flame. This flame is safe and relatively inexpensive to use, and sufficiently hot at ca. 2200 °C to atomize molecules of many common elements. However it is not sufficiently hot to break the element-oxygen bonds of some elements, the so-called refractory oxide-forming elements. These include, for example, aluminium and silicon. Such determinants require a hotter flame. Also atomization efficiency of some elements may be influenced by matrix elements and ions. For example, phosphate or aluminium depress the atomic absorption signals of calcium in an air-acetylene flame. Thus there is a need for a safe, inexpensive and reliable higher temperature flame in AAS. 

Catalysts for the chemical industry have to be characterized with respect to their trace impurities and major components. Not only is their composition when they are used initially in chemical reactors important, but also their alteration in the course of time. As carbide forming elements such as V and Ti are often used, atomic absorption spectrometry could be problematic. This also applies to catalysts for exhaust gas detoxification in cars. Noble metals such as Pt, Pd and Rh are fixed on alumina supports often also containing cerium compounds. Both for the determination of the stoichiometry but also for the monitoring of the noble metal contents in used catalysts, AAS suffers from problems because of the need for sample dissolution as well as for the requirement to determine refractory oxide forming elements. 

The most extreme example of isotopic anomalies is provided by the laboratory analyses of individual preserved presolar dust grains extracted from primitive meteorites (Anders and Zinner 1993). These micron-size or smaller grains of SiC, graphite, and (less commonly analyzed) refractory oxides formed in the outflows of evolved stars and their isotopic compositions of C, N, O and other major and even trace elements quantitatively reflect the unique nucleosynthetic environment of that particular star, which may differ from average solar system compositions by one or more orders of magnitude. That such... 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

In both atomization modes, as thin unstable ligaments, and/ or sheets disintegrate into round droplets, atomization gas may plausibly be trapped into the droplets under certain conditions. For alloys with alloying elements which readily react with atomization gas, for example, oxidize to form refractory oxides, solidification may be delayed and spheroidization is prevented so that rough flakes may form. For such alloys, the atmosphere in the spray chamber must be inert and protective to avoid the formation of any refractory and to foster spheroidal shape of droplets. 

Rapid and complete solvent evaporation is required for optimum performance. Atomisation occurs in the flame reaction zone, i.e. the conversion of sample molecules into atoms. Three factors affect the number of atoms formed. Firstly, the anion with which the metal atom is combined. Calcium chloride for instance is more easily dissociated than calcium phosphate. The second factor is flame temperature. Higher temperatures cause more rapid decomposition and, indeed, are often specifically required for elements which form refractory oxides. Finally, gas composition may affect the rate of atomisation if the constituents in the gas react with the sample or its derivatives. In the outer zone of the flame the atoms are burned to oxides. In this form they no longer absorb radiation at the wavelength of the uncombined ground state atoms. 

Choice of a dissolution method also depends on the range of substances to be analyzed, but in some specific instances the chemical form of the element is of even more importance. For example, particles formed at high temperature usually contain refractory oxides and silicates which may require fusion with either an acidic or basic flux, or hydrofluoric acid treatment to remove silica, or a combination of these techniques. 

Several years ago, Taylor et al. (2,3) showed that the oxygen plasma-etch resistance of carbon-based polymers could be markedly enhanced by incorporating certain atoms into the polymer chain. Particularly effective were those elements such as silicon or titanium that form a refractory oxide during oxygen RIE. The oxide is formed at the surface of the resist and greatly retards the subsequent etching rate of the remaining resist. 

All future alternatives will require new resists and processes, and for the first time, manufacturing lines will be using at least two different resists. These new materials must have satisfactory sensitivity, resolution, and process latitude. In addition, the deep-UV tools will have limited depth of focus (1-2 (xm) and will be useful only with relatively planar surfaces. Multilayer-resist schemes have been proposed to overcome these limitations, and the simplest is the bilevel scheme that requires a resist that can be converted, after development, to a mask resistant to O2 reactive ion etching (RIE). Resistance to O2 RIE can be achieved by incorporating an element into the resist structure that easily forms a refractory oxide. Silicon performs this function very well and is relatively easy to include in a wide variety of polymer structures. 

Magnesium, like calcium, is subject to the effect of anions, although to a lesser degree. The most serious interference derives from refractory acidic oxides formed in the flame from a number of elements, particularly aluminum and silicon. The effect of phosphate and sulfate is much less marked than with calcium (Fig. 17), hut if aluminum or silicon is also present in the solutions, magnesium depression is much more severe than with either of these interfering agents alone. These interferences can be overcome by the addition of strontium, lanthanum, or calcium. Leithe and Hofer (L4, L5) showed that magnesium could be determined... 

Research must be undertaken to demonstrate that LEI is adaptable to a wider variety of samples and analytically-useful flames. This will require further consideration of methods to discriminate against or remove low ionization potential interferents. Preliminary results have indicated that the use of an acetylene-nitrous oxide flame for the determination of metals which form refractory oxides exacerbates electrical interferences when samples contain IA elements 39). The much higher flame temperature produces higher concentrations of ions whose effects cannot be entirely mitigated by using an immersed electrode. 

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