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.