CHEMICAL MODIFICATION BY ASCORBIC-ACID AND OXALIC-ACID IN GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRY

The effects of addition of ascorbic and oxalic acids on the atomic absorption signal for lead are explained by a gas-phase thermodynamic equilibrium model. The gas-phase composition of the graphite furnace was determined as samples containing ascorbic or oxalic acid were pyrolyzed. Hydrogen and carbon monoxide were identified as two of the major pyrolysis products. The amounts of these reducing gases, formed in the atomization cycle, varied with the modifier used, the charring temperature employed, and the condition of the surface of the pyrolytically coated graphite tube. The extent of the shift in the appearance temperature showed correlation with variations in concentrations of hydrogen and carbon monoxide produced by pyrolysis. Results for oxalic acid clearly showed that the appearance temperature shifts for lead in pyrolytically coated tubes cannot be explained by a surface reaction mechanism.