VAPORIZATION AND ATOMIZATION OF LEAD AND TIN FROM A PYROLYTIC-GRAPHITE PROBE IN GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRY

A novel technique has been used to investigate the processes of vaporization and atomization of lead and tin in graphite probe furnace atomic absorption spectrometry. Using this technique it has been found that, for lead and tin, vaporization and atomization are separate processes and lead and tin are vaporized as molecular species. Furthermore, rate constants and activation energies for the vaporization of the molecular species of lead and tin have been determined. The activation energies for vaporization have been found to be 86 +/- 4 kJ mol-1 for the lead molecular species and 120 +/- 6 kJ mol-1 for the tin molecular species. It is suggested that these activation energies represent the energy for desorption of PbO and SnO from the graphite surface. Experimental results have been compared with those predicted by an earlier model based on homogeneous gas-phase thermodynamic equilibrium. The effects of the gas-phase chemical modifiers, H-2 and O2, on the atomic absorption signal profiles of lead and tin are presented and discussed. Atomization mechanisms have been proposed for lead and tin and compared with those proposed by other workers. PbO and SnO have been proposed as the most likely gaseous molecules of lead and tin, respectively, formed by vaporization.