MECHANISMS OF ATOMIZATION OF LEAD FROM NITRIC-ACID, HYDROCHLORIC-ACID AND SODIUM-CHLORIDE MATRICES IN ATOMIC-ABSORPTION SPECTROMETRY USING A GRAPHITE PROBE FURNACE

A novel technique has been used to investigate the processes of vaporization and atomization of lead in graphite probe furnace atomic absorption spectrometry. Using this technique it has been found that vaporization and atomization of lead are separate processes and lead is vaporized as molecular species. The following mechanisms are proposed. Prior to atomization of Pb, PbO is released from an HNO3 matrix, whereas PbCl2 is released from an HCl or an NaCl matrix, into the-vapour phase. Atomization of Pb from the above matrices occurs by dissociation of the gas-phase PbO or PbCl2 molecules. Rate constants and activation energies for the release processes of the lead molecular species have been determined. The activation energy for the release of both PbO and PbCl2 has been found to be about 26 kJ mol-1. It is suggested that this activation energy represents the energy for desorption of PbO or PbCl2 from the graphite surface. The effects of the sample matrix compositon and of the gas-phase chemical modifier, H-2, on the atomic absorption signal profiles of lead are presented and discussed. Experimental results have shown that H-2 added to the Ar purge gas increases dissociation of the lead molecular species mentioned above in the gas phase and reduces chloride matrix interferences. These experimental results are consistent with those predicted by an earlier model based on homogeneous gas-phase thermodynamic equilibrium, proposed by some of the present workers.