GAS-PHASE THERMODYNAMIC-EQUILIBRIUM MODEL AND CHEMICAL MODIFICATION IN GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRY

In conformity with the gas-phase thermodynamic equilibrium model proposed earlier, addition of H2 or CO gas to Ar purge gas or of ascorbic acid to aqueous solutions of the analytes was found to shift the absorbance pulses of Zn, Pb, Se, Sn, As and Cr to lower temperatures and addition of CO2 gas to Ar purge gas was found to shift the absorbance pulses of Pb, Se and Cr to higher temperatures. The results are consistent with the hypothesis that the absorbance pulse shifts caused by addition of ascorbic acid in solution are due to the production of H2 and CO by pyrolysis of the ascorbic acid. The gas-phase dissociation equilibrium of the analyte oxides is disturbed by the addition of H2, CO or CO2 to the gas phase, resulting in H2 or CO shifting the absorbance pulses to lower appearance temperatures and CO2 shifting them to higher appearance temperatures. Also, H2 and CO were found to eliminate the matrix interference caused by hydrochloric acid in the determination of Pb, Se and As, which were the only elements studied for matrix interferences. When 100% CO was used as the purge gas, large enhancements of Se and As signals were observed. Ascorbic acid was found to react with Se and As in acidic aqueous solutions to form colloidal suspensions, which gave no absorbance signals.