Surface investigation on chemically modified platforms for electrothermal atomic absorption spectrometry

In order to understand better the mechanism of action of a permanent modifier based on Ir deposited on graphite furnaces pretreated by carbide-forming elements (W and Zr) an XPS investigation was carried out on samples submitted to thermal treatment simulating the exptl. conditions used in electrothermal at. absorption spectrometry (ETAAS). Platforms pretreated by W and Zr were analyzed: evidence of tungsten carbide and oxides was found in the former whereas mainly oxygenated species were found on Zr-treated platforms. In both cases the existence of species anchored to graphite substrate via O and Cl bridges was hypothesized to account for the binding energy values obtained by a curve-fitting process. The deposition of Ir on either W- or Zr- platforms significantly modifies their surface chem. compn. and, in particular, promotes the formation of W and Zr species at higher oxidn. state (for example, WC is oxidized to WO3). At the same time, a substantial decrease of the overall signal intensities was recorded, most likely due to modifiers removal from the platform surface and/or their migration into the platform subsurface region as a consequence of the thermal treatments required by the platform-modifying procedure. Possible correlations between the chem. compn. of surface modifier species that are formed on graphite substrate and previous findings obtained by ETAAS expts. and morphol. and elemental investigations were pointed out. Confirmation was obtained that in all the modifier systems studied (W, Zr, W/Ir, Zr/Ir) there are at least 2 eventual forms of the modifier at the end of the preatomization treatment, with a prevalence of O-contg. species. Thus, differences in thermal stabilization as well as in the ETAAS patterns of analyte atom generation can be ascribed reasonably to the presence of active centers that are chem. different and characterized by different morphologies. [on SciFinder (R)