Systematic Study about the Elemental Interference in Phosphotungstate Structure in Ormosil Films
Author(s):
Orlando Elguera Ysnaga
XRF measurements in ormosil, performed at laboratory equipment, showed us the self-absorption effect of the Tungsten fluorescence by Phosphorous in the phosphotungstates. However, in the extent that increase the Zn concentration in ormosil, the Phosphorous absorption effect decreases gradually. EDXRF measurements of ormosil films containing phosphotungstates ([PW12O40]-3) showed that there is an inverse correlation between the concentrations of Phosphorous and Tungsten, or that the concentration of Phosphorous was overestimated and the concentration of Tungsten has been underestimated. In order to explain this phenomenon, they were performed studies of mixed solutions of phosphates and tungstates, Studies done with molybdates solutions showed the same trend than tungstates ones, analogously. Thus, we concluded that the structure of polyoxometalates (phosphotungstates and phosphomolybdates), containing Phosphorous as central atom, has influence on XRF analysis because the results obtained in tungstates and molybdates solutions. In the extent that the Phosphorous atoms are closer to Tungsten and/or Molybdenum ones, the probability of interaction between them will enhance. Thus, the interference of Phosphorous in X-ray Fluorescence of Tungsten and/or Molybdenum will be increase, thereby simulating the existing proximity of these elements in the molecules of the phosphotungstates and phosphomolybdates. Self-absorption/ interference studies of Tungsten X-ray Fluorescence by the others constituents in the ormosil films led us to approach the XRF quantification not only from the atomic perspective, but also to supplement with the molecular approach. We proposed an interference model that takes into account the self-absorption of the Tungsten fluorescence by Phosphorous, the cage-shaped molecular structure (Keggin) of the polyanions and their distribution in the structure of ormosil. Considering that the Tungsten and Phosphorous main emission lines do not overlapping by EDXRF, were performed measures by WDXRF finding the superposition of Tungsten W-LG4 and Phosphorous P-Kα lines, but that is not enough to justify our experimental observations. Therefore, we regarded an additional explanation, the X-ray Resonant Raman Scattering produced by the O2- anions network in phosphotungstates, followed of the Phosphorous absorption of the inelastic scattered photons for the O2- anions. Thus, the interference does not occur primarily by direct self-absorption, but by a combination of inelastic scattering process followed by absorption. The increase in X-ray fluorescence of Phosphorous in phosphotungstates could be due to various phenomenon such as Secondary Fluorescence, X-ray Resonant Raman Scattering and by the contribution of radiative satellites Auger lines. XANES and EXAFS measurements given evidence of intermolecular and intramolecular interaction between Zinc and Tungsten species.
