Electrothermal atomization of silver in graphite furnaces. Part 1. A two-precursor mechanism

Abstract

A two-precursor kinetic mechanism is proposed to describe the electrothermal atomization of silver. The effects of the superposition of two simultaneous atomization processes on the absorbance profiles and the Arrhenius plots are presented, both by numerical simulation and according to experimental measurements. Also, a new method is proposed for the determination of the rate constant of dissipation of the atomic vapor from a single absorbance pulse, under conditions of extended time-varying temperature. By numerical simulations, two temperature regions of atomization, with well defined atomization energies, are predicted by the Arrhenius plots for systems in which the vaporization processes are at least partially time resolved. In agreement with the two-precursor model, a double peak structure is detected in the experimental absorbance profiles of silver, and the Arrhenius plots also show two temperature regions of atomization. Employing a heating rate of 300 K s −1, average atomization energies of 120 and 250 kJ mol −1 are obtained in the low and the high temperature region, respectively. However, at a higher heating rate of 700 K s −1, an atomization energy that increases from 125 to 240 kJ mol −1, as the initial mass of the analyte increases, is obtained in the low temperature region, whereas a mass average atomization energy of 117 kJ mol −1, with a first kinetic order of release, is obtained in the high temperature region. The results seem to indicate simultaneous first order atomization from dispersed particles, and also from small clusters, whose size increases as the initial mass of Ag increases. Employing this two-precursor mechanism, with the experimental atomization energies and pre-exponential factors, an excellent description of the entire absorbance profiles and their corresponding Arrhenius plots is obtained.