Effects of Conditions for Pyrolysis of Ascorbic Acid as a Chemical Modifier on the Vaporization Mechanism of Gold in Electrothermal Atomic Absorption Spectrometry

Abstract

The effect of the ramp and hold times and the temperature of the pyrolysis step of ascorbic acid as a chemical modifier on AA signals for gold have been investigated. Three methods of pyrolysis for ascorbic acid were used: (1) pyrolysis of ascorbic acid before deposition of a gold solution on the platform surface; (2) pyrolysis of ascorbic acid after 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 absence of ascorbic acid, in pyrolysis method (1) a double peak appeared even at a high pyrolysis temperature of 1500 °C. However, as the ramp time increased from 5 to 80 s from a drying temperature of 120 up to 1500 °C, the first peak increased and concomitantly the second peak decreased with an isosbestic point; the integrated absorbance remained constant. The concentration dependence of the gold signals indicates that a fractional-order of release is shown for the first peak and a first-order process is obtained for the second, indicative of gold atoms adsorbed onto the active carbon surface. From inspection of a scanning electron micrograph and Raman spectra of the pyrolysed ascorbic acid, it was clear that a carbon film formed on the platform surface. It was concluded that use of a short ramp and hold time, even at 1500 °C, for the pyrolysis of ascorbic acid leads to the formation of active amorphous carbon enriched in micro-sized pores (r<25 nm), where adsorption of gold atoms that give rise to the second absorption signal occurs. Furthermore, the micro-sized pores are almost destroyed by treatment at temperatures higher than 1800 °C, resulting in graphitization of the carbon residue.