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- Author or Editor: B. Markert x
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Abstract
Neutron activation analysis was applied to the determination of theelements Al, As, Ba, Br, Cl, Co, Cr, Fe, K, Mg, Mn, Mo, Na, Rb, Sb, Sc, Ti,Th, V, Zn, and the rare earths La, Ce, Nd, Sm, Eu, Tb and Yb in the epiphyticbromeliad Tillandsia usneoides. The samples were collected at an unpollutedarea and exposed in different sites of the city of S
o Paulo, Brazil,and in a control site outside So Paulo. The results obtained showedan accumulation of Al, As, Cr, Fe, Mo, Sb, Ti, V and Zn elements in Tillandsiausneoides exposed in polluted sites, indicating a promising potential of thisspecies as a biomonitor of air pollution in So Paulo.
Abstract
Biomonitoring is a method of observing the impact of external factors on ecosystems and their development over a long period, or of ascertaining differences between one location and another. A monitor suitable for the subject of the investigation must be chosen, and the comparability of the element concentrations thus determined (both within the investigation and with other studies) must be ensured by adhering to precise sampling guidelines. In addition to the small amount of equipment required, the advantages of biomonitoring lie in the ecosystem approach that permits conclusions going beyond the biomonitor itself. Attention has to be given to the factors that may affect element concentrations and which can lead to incorrect statements if ignored.
Abstract
The as-prepared electron-doped system Nd2–xCexCuO4 (where x
0.16) is not superconducting. It becomes superconducting only after removal of a minuscule amount of extraneous oxygen (0.02 of O per unit formula). Mössbauer effect studies were carried out for oxygenated and deoxygenated Nd2–xCexCu(57Co)O4 with x=0.14, 0.16, and 0.18. The spectra show evidence of oxygen anions attaching to the probe57Co in apical positions, to form 5- and 6-coordinated species. A conventional procedure for deoxygenation brings about little change in the Mössbauer spectra both above and below the optimal superconducting concentration; however, for x=0.16, a dramatic change is observed—a major fraction of the magnetically split five-coordinated species manifests itself as a paramagnetically relaxed doublet upon deoxygenation, which costitutes a microscopic measure of the superconducting volume fraction. This apparently anomalous behavior at x0.16, where the extraneous oxygen is more readily desorbed, may be related to an electronic and/or local structural change in the CuO2 plane.
