Theoretical study of the formation mechanism of molecules сaf for fluoride determination using electrothermal molecular absorption spectrometry

Abstract

In order to select the temperature-time heating program, the calibration method, the prediction of matrix interference and to increase the sensitivity and accuracy of the halogens determination using the electrothermal molecular absorption spectrometry it is essential to know the formation mechanism of the measured molecules. Thermochemical processes of gaseous molecules CaF formation, which are used in the electrothermal molecular absorption determination of fluorine, were studied using the method of the thermodynamic simulation. All calculations were performed using the HSC 6.1 software package with its own database of thermodynamic data. For calculations, the non-equilibrium thermodynamic system, which was realized in the graphite furnace, was divided into the thermodynamic subsystems that correspond to the main stages of time-temperature program: drying of the sample, pyrolysis, vaporization and formation of gaseous diatomic molecules. The composition of the condensed sample residues after drying and pyrolysis stages, the composition of the gas phase in the analytical zone of the graphite furnace during vaporization and the formation of molecules stages were determined by the calculations. This study theoretically describes thermochemical processes of the formation of gaseous CaF molecules using thermodynamic simulation for electrothermal molecular absorption determination of fluorine. The calculations make it possible to establish that gaseous molecules CaF^g are formed during the course of the thermal decomposition of the condensed fluoride (CaF₂^c). The correct execution of thermodynamic simulation was confirmed by the comparison of the theoretical temperature curves of pyrolysis and the formation of molecules CaF^g with the experimental curves. The developed procedure of the thermodynamic simulation can be applied to study the formation of other dimeric molecules used for molecular absorption analysis with electrothermal atomization.