Superficial and underneath mapping of graphite furnace layers covered with Pd, Rh and Ir as permanent modifiers in ETAAS determination of Te.

JUANA PEDRO; JORGE STRIPEIKIS; ADRIAN BONIVARDI; MABEL TUDINO

Simposio; ELEVENTH RIO SYMPOSIUM ON ATOMIC SPECTROMETRY; 2010

RIO SYMPOSIUM ON ATOMIC SPECTROMETRY

In a previous work we studied the use of permanent modifiers of platinum group (PGM) in order to evaluate improvements on the determination of tellurium by graphite furnace atomic absorption spectrometry. Coverings of Pd, Rh and Ir were assayed, giving Ir the best analytical results [1]. Moreover, important differences on the shape of the analytical signal were observed which should be explained through the observation of the covered graphite surfaces. With the main objective of explaining the differences mentioned above, the modified surfaces were analyzed by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The graphite platforms were covered with the different modifiers by pyrolysis and, in the case of Pd, the physical vaporization/deposition technique (PVD) was also tested. Experiments were designed in order to evaluate the distribution of the different PGMs along the superficial and sub-superficial areas of the graphite platform. Regarding Pd, Pd-PVD micrographs showed a uniform film of 100% Pd covering the whole surface of graphite, which was not observed with the pyrolytic deposition. In the first case no signal was obtained which should be attributed to the lack of contribution of graphite sites to the stabilization mechanism. In the second case, graphite sites exposed themselves on the superficial area yielding an increment on the analytical signal. These findings will be fully discussed. Similar micrographs were obtained for the electrothermal deposition of Ir and Rh and then, compared to those of Pd. Ir showed the best distribution on the graphite surface, forming islands smaller than in the case of Pd. Rh allocated forming segregated channels which gave a non uniform distribution together with the presence of large areas without chemical modifier. The correlation between the surface studies and the tellurium analytical signal (height and shape) will be discussed as well as the most probable mechanisms for tellurium stabilization.   [1] Termal stabilization of tellurium in mineral acid solutions: Use of permanent  modifiers for its determination in sulfur by GFAAS. J. Pedro, J. Stripeikis, A. Bonivardi & M. Tudino. Talanta, 69(1), 199-203 (2006).