Thermal Desorption and Criteria for its Identification

Matrix Modification Technique Up to this point, consideration of the mechanisms and kinetics of sublimation and evaporation have been limited to crystalline substances (representing an individual phase). However, the thermodynamic approach used above is not restricted to these processes only. There are some other heterogeneous processes where it can be of use. Among these are, in particular, the processes of heterogeneous catalysis and of the interactions of solid substances with gases and metal vapours. The application of this approach to a mechanism of sample atomization in electrothermal AAS (ET AAS) will be considered further. 

The ET AAS technique (see Fig. 5.2) is based on fast evaporation of samples to be analysed in a miniature tube furnace (6-8 mm in diameter and 20-30 mm in length) made of graphite [5]. A light beam from the source of a line spectrum (usually a hollow cathode lamp) passes through this tube and the value of the light absorption by free atoms of analyte is measured. A grating monochromator is used to separate the most sensitive resonance line from the atomic spectrum of the element emitted by the light source. 

Samples in nano- or microgram amounts are introduced into the furnace in a form of solutions. These aliquots (10-30 xL) are dried and, before the atomization, are subjected to a preliminary p3Tolysis, which is aimed to remove (evaporate) more volatile matrix components from the sample. To achieve more efficient separation, 1-10 pg of one of the platinum group metals (most often, palladium) is added to the sample in the form of a nitrate solution. This additive serves as a chemical matrix modifier preventing evaporation of the analyte during the pyrolysis stage. 

An interpretation of the mechanism of analyte retention on the furnace surface has been one of the most controversial problems in ET A AS for years. Since the first studies in this field in the 1970s, two different mechanisms have been suggested. According to the first one, the adsorption/desorption mechanism, it is supposed that the anal de is distributed on the furnace surface in the form of a monolayer of free atoms or molecules, retained on the surface by means of physical or chemical adsorption forces. According to the second one, the condensation/evaporation mechanism, the sample is distributed in a form of solid microparticles retaining all the thermochemical characteristics of the original (bulk) substance. 

Numerous studies have been performed in this field during the last few decades. A non-isothermal method, traditionally used in thermal analysis was applied to measure the rate constants, which were then used in an Arrhenius plot to determine the E parameter. The magnitude of E has been compared with the enthalpies (A Ef ) of various possible reactions and this was used to elucidate the actual atomization mechanism. In spite of this, the mechanism of matrix modification, as well as some other evaporation/atomization problems, still remains unrevealed. 

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