Studies on Tellurium Atomization in Graphite Furnaces

J. PEDRO; J. STRIPEKIS; A. BONIVARDI; M. TUDINO

Rio de Janerio, Brasil

Simposio; 8th Rio Symposium on Atomic Spectrometry; 2004

The determination of micro-amounts of tellurium is usually performed by hydride generation atomic absorption spectrometry (HG AAS). Electrothermal atomic absorption spectrometry (ET AAS) is also an interesting alternative but the number of papers involving ET AAS is scarce probably due to the limited information available about the mechanisms of formation of tellurium atomic vapor in graphite furnaces. This situation makes difficult the development of successful heating programs. In order to study the atomization of tellurium, solutions of Te(IV) were injected in different graphite surfaces (pyrolytic and polycrystalline) in the absence and presence of different chemical modifiers. Several noble metals were employed as solutions directly added to the sample or as permanent modifiers of the graphite surface. The pyrolysis curves obtained without modifier show differences in thermal stability when compared to those obtained with chemical modification. Curves A vs. t appear distorted by the presence of shoulders suggesting the coexistence of two mechanisms for atomization: gaseous dissociation of tellurium oxides and desorption of metallic tellurium. The addition of a solution of thioacetamide together with the Te(IV) solution produces an increment of sensitivity and a symmetric peak (Gaussian form). This analytically desired change should be explained through a different atomization mechanism: thioacetamide promotes the reduction of tellurium oxides into metallic tellurium and thus, tellurium desorption from the graphite furnace is the only mechanism responsible for obtaining the atomic vapor. The influence of different noble metals on the tellurium signal was also assessed. Solutions of Pd, Rh and Ir were employed in the classical way or as permanent modifiers of the graphite surface. Obtained results involving: optimal mass of modifier, maximal pyrolysis temperature obtainable without losses of analyte and sensitivity (height, area and form of the atomization peak) will be presented and fully discussed.The different performances will be explained in terms of the chemical properties of the different noble metals assayed and the interactions analyte-modifier-graphite surface.