EFFECTS OF CONDITIONS FOR PYROLYSIS OF ASCORBIC-ACID AS A CHEMICAL MODIFIER ON THE VAPORIZATION MECHANISM OF GOLD IN ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY

The effect of the ramp and hold times and the temperature of the pyrol ysis step of ascorbic acid as a chemical modifier on AA signals for go ld have been investigated, Three methods of pyrolysis for ascorbic aci d were used: (1) pyrolysis of ascorbic acid before deposition of a gol d solution on the platform surface; (2) pyrolysis of ascorbic acid aft er ashing a gold solution; and (3) charring a solution containing both ascorbic acid and gold, Although pyrolysis methods (2) and (3) gave a delayed single absorption peak for gold compared with that in the abs ence of ascorbic acid, in pyrolysis method (1) a double peak appeared even at a high pyrolysis temperature of 1500 degrees C, However, as th e ramp time increased from 5 to 80 s from a drying temperature of 120 up to 1500 degrees C, the first peak increased and concomitantly the s econd peak decreased with an isosbestic point; the integrated absorban ce remained constant, The concentration dependence of the gold signals indicates that a fractional-order of release is shown for the first p eak and a first-order process is obtained for the second, indicative o f gold atoms adsorbed onto the active carbon surface, From inspection of a scanning electron micrograph and Raman spectra of the pyrolysed a scorbic acid, it was clear that a carbon film formed on the platform s urface, It was concluded that use of a short ramp and hold time, even at 1500 degrees C, for the pyrolysis of ascorbic acid leads to the for mation of active amorphous carbon enriched in micro-sized pores (r<25 nm), where adsorption of gold atoms that give rise to the second absor ption signal occurs, Furthermore, the micro-sized pores are almost des troyed by treatment at temperatures higher than 1800 degrees C, result ing in graphitization of the carbon residue.