Effect of ascorbic acid on the appearance temperature of lead in graphite furnace atomic absorption spectrometry

Abstract

A thermodynamic (gas phase) equilibrium model of atomisation has been used to explain the shift of the appearance temperature of the Pb atomic absorption signal to a lower value when ascorbic acid is used as a matrix modifier. Experimental results of the effect of gas-phase composition on the appearance temperature of the Pb atomic absorption signal are presented. The partial pressure of H2 was determined at various temperatures during the ashing and the atomisation cycle. The partial pressure of CO, CO2 and CH4 were found to be very low for the tube wall heating rate of 1.7 K ms–1; CO was found to be present in appreciable amounts for the tube wall heating rate of 1.5 K ms–1. The results are consistent with the hypothesis that H2 and CO released by the pryolysis of ascorbic acid decrease the partial pressure of O2 in the furnace and thereby cause the equilibrium position of the reaction PbO(g)⇌ Pb(g)+½ O2 to shift to the right, favouring production of more Pb(g), which results in a lowering of the appearance temperature of the Pb atomic absorption signal in agreement with the equilibrium model.