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David London - One of the best experts on this subject based on the ideXlab platform.
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effect of current density on the Electron Microprobe Analysis of alkali aluminosilicate glasses
2005Co-Authors: George B Morgan, David LondonAbstract:In a previous work (Morgan and London 1996), we proposed an optimized procedure for Electron Microprobe Analysis (EMPA) of rhyolitic glasses using a broad (20 μm diameter), low-current (2 nA) fixed beam. Some important applications for EMPA of glass, such as vitreous inclusions in minerals and experimental run products, require smaller beam diameters that produce greater area] current densities (expressed as nA/μm 2 ). For these situations, we have assessed the effect of areal current density on the migration of Na and its concomitant effects on other elements and their ratios during EMPA of granitic glasses. Anhydrous and hydrous glasses of a haplogranite composition (Ab 3 8 . 2 3 Or 2 9 . 3 1 Qtz 3 3 . 3 7 C 0 . 1 0 ) were analyzed at 20 kV accelerating potential, using 2-50 nA beam currents, fixed beam diameters of 2-20 μm, and counting times scaled to yield similar analytical uncertainty at each condition (∼2.6% relative for Na 2 O). There is almost no loss of Na (≤1.7-2.7% relative) using a current density of 0.006 nA/μm 2 , minor (7-9%) Na loss for current densities up to 0.1 nA/μm 2 , and increasing Na loss with higher current densities that becomes severe at >0.5 nA/μm 2 (e.g., 48-63% relative loss from hydrous glass at 50 nA and 2 μm during 3-6 s of irradiation). Sodium migration is more pronounced in hydrous glasses than in anhydrous ones, with significant loss from hydrous glass occurring during the first second of irradiation. The migration of Na results in increased concentrations of Al and Si, but little or no change in the concentration of K; if not fully corrected for, these effects produce systematic errors in important elemental ratios. With current densities 100% of the Na 2 O value obtained). The correlation of analytical condition (beam current and diameter) with current density and EMPA results provided here allows analysts to select beam conditions that optimize the quality of analyses. When current densities >∼0.01 nA/μm 2 must be used (e.g., with beam spot sizes <20 μm), the results can lead to improved estimates of the systematic errors due to alkali migration. Natural and some experimental glasses contain a variety of other minor components among which Ca and Fe are important, and so the discussion of analytical methods is extended to more complex compositions. For example, Na migration is accelerated as glass structures become less polymerized than those of simple tectosilicate stoichiometry (e.g., due to increasing alkalinity and/or the presence of fluxing components such as F, Cl, B). Analysis using 20 kV accelerating voltage, as opposed to 15 kV, both slightly decreases Na migration and improves limits of detection and statistical accuracies for minor components such as Fe while providing reasonable beam penetration depths.
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optimizing the Electron Microprobe Analysis of hydrous alkali aluminosilicate glasses
1996Co-Authors: George B Morgan, David LondonAbstract:The time-dependent loss of NaKa X-ray intensity during Electron-beam irradiation of hydrous alkali alumino silicate glasses is apparently more significant during the initial few seconds of beam exposure than it is for anhydrous glasses, and it is pronounced for incident beam currents > 2-5 nA (using 15-20 /.Lmbeam diameters). Exponential fits of NaKa intensity vs. time show a progressive decrease in the apparent zero-time intercepts for incident beams from 2 to 20 nA, and thus methods for correcting Na concentrations solely on the basis of curve fitting and extrapolation to zero-time values may underestimate Na contents by almost 10% (relative) for higher beam currents. Similar exponential fits to the intensity-time data for AlKa and SiKa show that "grow-in" is greater for Al than for Si. For incident currents ~ 5 nA, the magnitudes of all intensity changes also increase with total H20 content of glass. On the basis of these observations, the optimal conditions for Analysis of hydrous alkali aluminosilicate glasses include a 2 nA beam with 20 /.Lmdiameter and counting times of 20-40 s for the Analysis of alkali aluminosilicate components, with Na and Al analyzed first (simultaneously, if possible). These methods minimize Na loss and grow-in for Al and Si to the point that little or no correction is needed, provide good statistical accuracy, and work with a wide variety of standard materials (i.e., glass standards with compositions and H20 contents comparable to the unknowns are not needed). For complete Analysis of more complex multicomponent systems, two beam conditions are recommended: an initial 2 nA, 20 /.Lmdiameter beam for Analysis of alkali alumino silicate components, followed by a 20 nA, 20 /.Lmdiameter beam for Analysis of all other components. With the use of these methods, the H20 contents of hydrous glasses (H20 as the only unknown) can be determined by difference with uncertainties mostly 5 nA, corrections for Na loss ignoring Al (and Si) grow-in underestimate H20 contents by about 10-50% of concentration.
Mortimer M. Civan - One of the best experts on this subject based on the ideXlab platform.
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Electron Microprobe Analysis of rabbit ciliary epithelium indicates enhanced secretion posteriorly and enhanced absorption anteriorly
2007Co-Authors: C. W. Mclaughlin, Sylvia Zellhuber-mcmillan, Anthony D. C. Macknight, Mortimer M. CivanAbstract:The rate of aqueous humor formation sequentially across the pigmented (PE) and nonpigmented (NPE) ciliary epithelial cell layers may not be uniform over the epithelial surface. Because of the tissu...
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Electron Microprobe Analysis of ouabain exposed ciliary epithelium pe npe cell couplets form the functional units
2004Co-Authors: C. W. Mclaughlin, Anthony D. C. Macknight, Sylvia Zellhubermcmillan, Mortimer M. CivanAbstract:Aqueous humor is secreted by the bilayered ciliary epithelium. Solutes and water enter the pigmented ciliary epithelial (PE) cell layer, cross gap junctions into the nonpigmented ciliary epithelial...
George B Morgan - One of the best experts on this subject based on the ideXlab platform.
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effect of current density on the Electron Microprobe Analysis of alkali aluminosilicate glasses
2005Co-Authors: George B Morgan, David LondonAbstract:In a previous work (Morgan and London 1996), we proposed an optimized procedure for Electron Microprobe Analysis (EMPA) of rhyolitic glasses using a broad (20 μm diameter), low-current (2 nA) fixed beam. Some important applications for EMPA of glass, such as vitreous inclusions in minerals and experimental run products, require smaller beam diameters that produce greater area] current densities (expressed as nA/μm 2 ). For these situations, we have assessed the effect of areal current density on the migration of Na and its concomitant effects on other elements and their ratios during EMPA of granitic glasses. Anhydrous and hydrous glasses of a haplogranite composition (Ab 3 8 . 2 3 Or 2 9 . 3 1 Qtz 3 3 . 3 7 C 0 . 1 0 ) were analyzed at 20 kV accelerating potential, using 2-50 nA beam currents, fixed beam diameters of 2-20 μm, and counting times scaled to yield similar analytical uncertainty at each condition (∼2.6% relative for Na 2 O). There is almost no loss of Na (≤1.7-2.7% relative) using a current density of 0.006 nA/μm 2 , minor (7-9%) Na loss for current densities up to 0.1 nA/μm 2 , and increasing Na loss with higher current densities that becomes severe at >0.5 nA/μm 2 (e.g., 48-63% relative loss from hydrous glass at 50 nA and 2 μm during 3-6 s of irradiation). Sodium migration is more pronounced in hydrous glasses than in anhydrous ones, with significant loss from hydrous glass occurring during the first second of irradiation. The migration of Na results in increased concentrations of Al and Si, but little or no change in the concentration of K; if not fully corrected for, these effects produce systematic errors in important elemental ratios. With current densities 100% of the Na 2 O value obtained). The correlation of analytical condition (beam current and diameter) with current density and EMPA results provided here allows analysts to select beam conditions that optimize the quality of analyses. When current densities >∼0.01 nA/μm 2 must be used (e.g., with beam spot sizes <20 μm), the results can lead to improved estimates of the systematic errors due to alkali migration. Natural and some experimental glasses contain a variety of other minor components among which Ca and Fe are important, and so the discussion of analytical methods is extended to more complex compositions. For example, Na migration is accelerated as glass structures become less polymerized than those of simple tectosilicate stoichiometry (e.g., due to increasing alkalinity and/or the presence of fluxing components such as F, Cl, B). Analysis using 20 kV accelerating voltage, as opposed to 15 kV, both slightly decreases Na migration and improves limits of detection and statistical accuracies for minor components such as Fe while providing reasonable beam penetration depths.
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optimizing the Electron Microprobe Analysis of hydrous alkali aluminosilicate glasses
1996Co-Authors: George B Morgan, David LondonAbstract:The time-dependent loss of NaKa X-ray intensity during Electron-beam irradiation of hydrous alkali alumino silicate glasses is apparently more significant during the initial few seconds of beam exposure than it is for anhydrous glasses, and it is pronounced for incident beam currents > 2-5 nA (using 15-20 /.Lmbeam diameters). Exponential fits of NaKa intensity vs. time show a progressive decrease in the apparent zero-time intercepts for incident beams from 2 to 20 nA, and thus methods for correcting Na concentrations solely on the basis of curve fitting and extrapolation to zero-time values may underestimate Na contents by almost 10% (relative) for higher beam currents. Similar exponential fits to the intensity-time data for AlKa and SiKa show that "grow-in" is greater for Al than for Si. For incident currents ~ 5 nA, the magnitudes of all intensity changes also increase with total H20 content of glass. On the basis of these observations, the optimal conditions for Analysis of hydrous alkali aluminosilicate glasses include a 2 nA beam with 20 /.Lmdiameter and counting times of 20-40 s for the Analysis of alkali aluminosilicate components, with Na and Al analyzed first (simultaneously, if possible). These methods minimize Na loss and grow-in for Al and Si to the point that little or no correction is needed, provide good statistical accuracy, and work with a wide variety of standard materials (i.e., glass standards with compositions and H20 contents comparable to the unknowns are not needed). For complete Analysis of more complex multicomponent systems, two beam conditions are recommended: an initial 2 nA, 20 /.Lmdiameter beam for Analysis of alkali alumino silicate components, followed by a 20 nA, 20 /.Lmdiameter beam for Analysis of all other components. With the use of these methods, the H20 contents of hydrous glasses (H20 as the only unknown) can be determined by difference with uncertainties mostly 5 nA, corrections for Na loss ignoring Al (and Si) grow-in underestimate H20 contents by about 10-50% of concentration.
L A Boatner - One of the best experts on this subject based on the ideXlab platform.
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a re examination of the rare earth element orthophosphate standards in use for Electron Microprobe Analysis
2003Co-Authors: John J Donovan, L A Boatner, John M Hanchar, Phillip M Picolli, Marc D Schrier, Eugene JarosewichAbstract:Nous re-examinons quatorze etalons. des orthophosphates des terres rares Sc et Y. synthetises dans les annees 80 dans le laboratoire national de Oak Ridge, et distribues a grande echelle par le departement des sciences minerales du Smithsonian Institution. Ces materiaux s'averent contamines de facon importante par le plomb. Cette impurete provient de l'utilisation de Pb 2 P 2 0 7 comme fondant, derive de la decomposition thermique de PbHP0 4 . Le pyrophosphate de plomb est utilise pour dissoudre les materiaux de depart (les oxydes) a temperature elevee (∼1360°C) precedant la synthese Parce que ces etalons de terres rares sont tres stables sous le faisceau d'Electrons, et qu'ils sont consideres homogenes, ils ont eu un impact enorme dans les travaux analytiques portant sur les terres rares, le scandium, l'yttrium et le phosphor faisant appel a la microsonde Electronique. Les orthophosphates monocliniques, adoptant la structure de phases du groupe de la monazite, acceptent davantage de plomb dans leur structure que les orthophosphates tetragonaux, adoptant la structure du groupe du xenotime. Nous decrivons le portee de cette contamination, et noux expliquons l'impact de cette impurete sur la stoechiometrie des etalons du au remplacement des terres rares par le plomb dans la structure cristalline.
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rare earth element reference samples for Electron Microprobe Analysis
1991Co-Authors: Eugene Jarosewich, L A BoatnerAbstract:Fourteen rare-earth (plus scandium and yttrium) orthophosphates have been synthesized and characterized for use as Electron Microprobe reference samples. These reference samples are single-element phosphates that are homogeneous, stable under the Electron beam, and free of interfering elements. They have a stoichiometric composition (RPO4) with the element of interest (R) ranging from 32 to 64 weight percent.
N. Welter - One of the best experts on this subject based on the ideXlab platform.
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Chemical composition and colouring agents of Roman mosaic and millefiori glass, studied by Electron Microprobe Analysis and Raman microspectroscopy
2009Co-Authors: V. Gedzevičiūtė, N. Welter, U. Schüssler, C. WeissAbstract:About 100 fragments of Roman mosaic and millefiori glass were stylistically attributed to a Hellenistic type, a Ptolemaic and Romano-Egyptian period type and an early imperial period type. Twelve representative fragments were studied by Electron Microprobe Analysis and Raman microspectroscopy. Eleven of them display a Na-pronounced recipe with low K, Mg and P contents, typical for the Roman period. Minor differences in composition are unsystematic, not reflecting the stylistic classification. Ionic colouring agents are Mn^3+ for violet, Cu^2+ for light blue, Co^2+ for deep blue and Fe^3+ for brown translucent colours. Calcium antimonates, lead antimonate and cuprite are the colourants responsible for white, yellow and red colours, respectively, and additionally serve as opacifiers. Mixing of ionic colouring agents and opacifying colourants led to a more differentiated palette of colours. Pb was used as yellow colouring agent, as a flux material and as a stabiliser for the colourant crystals. The remaining fragment consisting of a K-pronounced but still Na-bearing glass matrix was most likely produced during the Middle Ages or later.
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characterisation of inorganic pigments in ancient glass beads by means of raman microspectroscopy Microprobe Analysis and x ray diffractometry
2007Co-Authors: N. Welter, U. Schüssler, W KieferAbstract:Ancient coloured glass beads from Sri Lanka and Oman were analysed by Raman microspectroscopy for non-destructive identification of inorganic pigments in the glass. Calcium phosphate (Ca3(PO4)2), cassiterite (SnO2), cuprite (Cu2O) and a Pb(Sn,Si)O3-type lead tin oxide were found to be used as colouring agents. Moreover, a distinction between lead-based and alkali-based glass matrices could be made. Electron Microprobe Analysis and X-ray diffractometry were performed to show the capability of Raman microspectroscopy in comparison to these methods for answering archaeometric questions. Copyright © 2006 John Wiley & Sons, Ltd.