Thermal pretreatment-atomization

Double curves (thermal pretreatment/atomization curves) are used to determine the limits for both thermal pretreatment and atomization temperatures for the elements and matrices involved. These curves can also be applied to drawing conclusions about the atomization mechanism. In the first curve the absorbance signal at the optimum atomization temperature is plotted versus the pretreatment temperature as the variable. In the second curve the absorbance at the optimum pretreatment temperature is plotted versus the atomization temperature as the variable (Figure 65). The pretreatment curve (ash curve) shows the temperature to which the sample can be heated without loss of the analyte. From this curve it is also possible to derive the lowest temperature at which the element is quantitatively volatilized. From the atomization curve can be derived the temperature at which the atomization is first evident, the appearance temperature, and the optimum atomization temperature at which the maximum atom cloud density is attained. 

It is possible to draw conclusions about the atomization mechanism from the thermal pretreatment/atomization curves, when melting point, boiling point, and decomposition point for the analyte and its compounds are entered on these plots. Figure 66 shows the thermal pretreatment/atomization curves for cadmium and aluminium. The initial pre-atomization losses and the appearance temperature are below the melting point of cadmium. This... 

Figure 65 Thermal pretreatment/atomization curve. A The absorbance measured at the optimum atomization temperature B The absorbance plotted versus the atomization temperature 1 The maximum thermal pretreatment temperature 2 The lowest temperature at which the analyte is quantitatively volatilized 3 The appearance temperature of the analyte 4 The optimum atomization temperature...

Figure 66 Thermal pretreatment/atomization curve for cadmium and aluminium...

The analyte may be lost during the thermal pretreatment phase for two reasons (i) The analyte may be present in the sample as a compound which is appreciably volatile at the thermal pretreatment temperature used (ii) The analyte may be converted into a volatile form by a matrix component or solvent. From the thermal pretreatment/atomization curves (described in section 4.5) it can immediately be seen, whether or not the thermal pretreatment temperature is too high. These plots also show the best thermal pretreatment and atomization temperatures for a given matrix. 

Perkin Elmer introduced, in 1984, a Stabilized Temperature Platform Furnace (STPF) (Figure 49). There are two inert gas flows (internal and external). The heating rate of the furnace is high (about 2000 Ks ), which allows the use of lower atomization temperatures. After the thermal pretreatment a small temperature step (<1600 K) is used in order to prevent the atomization of the sample before the equilibrium has been reached. 

The process taking place during the thermal pretreatment (ashing) and atomization stages must be known in order to solve the analytical problem with high precision and accuracy. The chemical and physical characteristics of the analyte will determine its behaviour in the furnace. Chemical environment (matrix) of the analyte is also very important. It is possible to predict whether a given element can be determined in a particular matrix and to define the best operating conditions. 

Thermal Pretreatment. In this stage the analyte is separated from the interfering matrix components. Biological samples decompose to carbon and produce lots of soot and smoke. Inorganic compounds distill, sublime, or decompose to mist. If these processes take place at the same time as the atomization of the analyte, the measurement of the absorption signal would be impossible. 

The use of too high a thermal pretreatment temperature or too long a time results in the loss of significant quantities of the analyte before the atomization stage. This is particularly important in the determination of easy volatile elements, such as Hg, As, Se, Cd, Zn, and Pb. The complete removal of the matrix before the atomization stage is possible if the analyte exists in a thermally stable compound. 

In 1977 Dittrich and in 1978 Fuwa proposed, independently, that non-metals could be determined by molecular spectrometry at high temperatures with electrothermal vaporization (ETV-MAS). The method is relatively simple and allows determinations at sub p.p.m.-levels. Sample and reagent solutions are introduced into the graphite tube as a mixture or one after another. The reproducibility is usually better when the sample and reagent solutions are mixed together before introduction. However, if precipitation occurs by mixing the solutions, then they must be introduced separately into the atomizer. The sample is dried and ashed like in GF-AAS. After the thermal pretreatment steps, the conditions of the furnace are chosen so that... 

Use of modifiers (see below) to stabilize the analyte during thermal pretreatment, to delay volatilization until the temperature in the atomizer is sufficiently high and stable for efficient atomization, and to buffer the composition of the gaseous phase. 

For interferences other than spectral, the analyte itself is directly affected. The nonspectral interferences are best classified according to the stage at which the particular interference occurs, i.e. solute-volatilization and vapour-phase interferences. A nonspectral interference is found when the analyte exhibits a different sensitivity in the presence of sample concomitants as compared to the analyte in a reference solution. The difference in the signal may be due to analyte loss during the thermal pretreatment stage in the electrothermal atomizer analyte reaction with concomitants in the condensed phase to form compounds that are atomized to a lesser extent, analyte ionization or change the degree of ionization caused by concomitants. 

Electrothermal atomizers for AES share many of the advantages associated with their use for AAS. The argon inert gas that prevents oxidation of the graphite tube ensures that minimal quenching occurs, although a number of other interferences may occur. By and large, the interferences are very similar to those experienced in traditional electrothermal AAS, such as carbide formation (for some analytes), scatter by particulate matter, and losses during thermal pretreatment. A more comprehensive overview of interferences may be found in the sections on AAS. The use of matrix modifiers to assist in the separation of the analyte from the matrix is still often necessary. 

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