ELECTROTHERMAL VOLATILIZATION FLAME AND GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRIC INVESTIGATIONS ON THE DETERMINATION OF CALCIUM IN GALLIUM

Electrothermal volatilization flame atomic absorption spectrometry (ET V-FAAS) was used for element-specific detection in the vapour evolved during heating of a graphite furnace (GF). The studies, combined with graphite furnace atomic absorption spectrometry (GFAAS) and GF molecul ar absorption spectrometry (GFMAS) were intended to provide a better u nderstanding of the volatilization mechanisms of calcium and gallium. Both elements were applied as minor and major components in nitric aci d, hydrochloric acid and ammonium chloride media. Low heating rates (2 0-50 degrees C/s) were used in the volatilization studies and the figu res of merit were compared with those obtained from conventional pyrol ysis curves. It was concluded that gallium, when introduced in nitric acid solution and at the 10 ng level, was evaporated between 900 and 1 000 degrees C in the form of Ga2O. When gallium was the matrix element (10 mu g range) the major fraction vaporized in the 1120-2300 degrees C range, producing Ga2O(g) and Ga(g) as major and minor components, r espectively. A 1 ng mass of calcium vaporized rapidly above 1520 degre es C with the evolution of atomic vapour, and this process overlapped in part with the vaporization of gallium matrix. Volatilization of gal lium in the pyrolysis stage could be performed to a degree sufficient to eliminate the interference of up to 3 g/l gallium in nitric acid me dium on the determination of calcium using Zeeman-effect background co rrection. A detection limit of 0.06 mu g/g Ca in gallium was found if random contamination during dissolution was avoided. Microgram amounts of calcium introduced in HCl solution vaporized as CaCl2 and CaO, the oxide being formed by a heterogeneous phase hydrolysis. The addition of excess NH4Cl did not retard the hydrolysis of calcium chloride, whi le this additive promoted the vaporization of gallium in the form of c hloride.