Kinetic Model of Entry of Free Atoms into an Electrothermal Atomizer

Abstract

A kinetic phenomenological model of the processes of atomization for atomic-absorptional spectrometry with electrothermal atomization that is based on solution of a one-dimensional equation of diffusion with two independent sources is suggested. The model takes into account the processes occurring both on the surface of a graphite furnace and inside its wall. The proposed model is used for describing the electrothermal atomization of copper and zinc.Introduction. It is known that in the method of atomic-absorption spectrometry with electrothermal atomization (ETAAS), a metered amount of the sample analyzed is delivered onto the graphite surface of a furnace (or platform) on which its thermal treatment and atomization occur. A liquid sample not only spreads over the surface but also penetrates into the pores of the furnace material under the action of capillary forces, as a result of which the conditions of sample atomization are altered [1,2]. It is evident that here the state and properties of the graphite surface play a significant role.It has been shown earlier [2-6] that the form of the analytical signal depends substantially on the state of the furnace surface and the degree of its wear, and the necessity of creating a model that would take into account penetration of a sample into the surface pores has been substantiated [2]. Such a model may also be of use in studying the influence of permanent modifiers on the mechanism of atomization [7].To construct a correct phenomenological model of sample atomization in a graphite furnace it is necessary to consider two sources of free atoms: one on the furnace surface and the other inside its walls. The present work is devoted to the solution of this problem. For experimental checking of the model, we selected copper and zinc, the mechanisms of atomization of which are different.Theory. In constructing the model, we use some facts and assumptions.1) The atomic absorption A(t) is proportional to the mass M(t) of the free atoms that are present in the analytical volume of the furnace in a gas phase.2) In delivering metered amounts of a sample into the furnace (or onto the platform), a portion of the liquid sample penetrates into graphite under the action of capillary forces [1, 2]. As the total initial mass of the atoms M 0 placed inside an atomizer we take the mass of the atoms participating in formation of an analytical signal. It can be represented as the sum of the masses of atoms on the surface of the furnace and inside its pores: