ROLE OF OXYGEN IN THE DETERMINATION OF OXIDE FORMING ELEMENTS BY ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY .2. EFFECT OF OXYGEN ON THE REACTIONS OF THALLIUM, BISMUTH AND LEAD IN UNCOATED FURNACES, PYROLYTIC COATED FURNACES AND ON PLATFORMS

The chemical processes occurring in the graphite furnaces of atomic absorption spectrometers were studied when the oxide forming elements, Tl, Bi and Pb were being determined. Furnaces fabricated from uncoated electrographite, pyrolytic graphite coated graphite furnaces, uncoated furnace coated platforms and uncoated furnace uncoated platforms were tested. Pyrolysis curves were measured in all the furnace types, with flowing argon and under gas stop. The reactions occurring for Tl and Bi are reduction reactions via volatile suboxides to the elements, whereas Pb is directly reduced to the metal. The volatile suboxides are responsible for material losses during pretreatment. Under gas stop conditions the losses may be reduced by further reduction of the suboxides by additional collisions with the graphite surface. Separate atomization from the furnace wall and from the platform after thermal pretreatment gave an indication of the location and the kinetics of the different reaction steps. The oxygen conditioning of the uncoated furnaces and the uncoated platforms resulted in the thermal stabilization of the elements under study. This can be explained by the intercalation of the metals into the graphite layers.