PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Tungsten Compounds”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 127 records · Page 7Linked to original sources
PubMed2005

Photoelectron spectroscopy of doubly and singly charged group VIB dimetalate anions: M2O72-, MM'O72-, and M2O7- (M, M' = Cr, Mo, W).

We produced both doubly and singly charged Group VIB dimetalate species-M(2)O(7)(2-), MM'O(7)(2-), and M(2)O(7)(-) (M, M' = Cr, Mo, W)-using two different experimental techniques (electrospray ionization for the doubly charged anions and laser vaporization for the singly charged anions) and investigated their electronic and geometric structures using photoelectron spectroscopy and density functional calculations. Distinct changes in the electronic and geometric structures were observed as a function of the metal and charge state. The electron binding energies of the heteronuclear dianions MM'O(7)(2-) were observed to be roughly the average of those of their homonuclear counterparts (M(2)O(7)(2-) and M'(2)O(7)(2-)). Density functional calculations indicated that W(2)O(7)(2-), W(2)O(7)(-), and W(2)O(7) possess different ground-state structures: the dianion is highly symmetric (D(3d),(1)A(1g)) with a single bridging oxo ligand, the monoanion is a doublet (C(1), (2)A) with two bridging oxo ligands and a radical terminal oxo ligand, whereas the neutral is a singlet (C(1), (1)A) with two bridging oxo ligands and a terminal peroxo ligand. The combined experimental and theoretical study provides insights into the evolution of geometric and electronic structures as a function of charge state. The clusters identified might provide insights into the possible structures of reactive species present in early transition-metal oxide catalysts that are relevant to their reactivity and catalytic function.

Anions↗
PubMed2005

Investigation of the reactivity of arylamines, organo-hydrazines and tolylisocyanate towards [PW12-xMxO40]n- Keggin anions.

Reaction of K7[A,alpha-PW9Mo2O39] with Na2MoO4.2H2O in a mixture of water/dioxane/hydrochloric acid and further precipitation with (Bu4N)Br provided (Bu4N)3[A,alpha-PW9Mo3O40](3). Analogous reaction with K7-xNax[alpha-PW11O39] is an alternative to the synthesis of (Bu4N)3[alpha-PW11O39{MoVIO}]2. Multinuclear NMR and ESI mass spectrometry have been used to interpret the reaction of (Bu4N)x[alpha-PW11O39{ReO}](x=3 1; x=4 1I), (Bu4N)x[alpha-PW11O39{MoO}](x=3 2; x=4 2I) and (Bu4N)3[A,alpha-PW9Mo3O40]3 by organohydrazines, arylamines, tolylisocyanate and tetraphenylphosphine imide.

Electron Spin Resonance Spectroscopy↗
PubMed1994

Image quality and patient dose for different screen-film combinations.

Comparisons of image quality and estimations of dose reductions were made for several new screen-film combinations including conventional and rare-earth systems under conditions similar to abdomen radiography. The evaluation was carried out using an ANSI type phantom and the TOR(CDR) image quality object developed by the University of Leeds. The parameters employed for the comparison were: entrance dose, high and low contrast threshold sensitivity and resolution. The advantages and limitations of the method are also discussed. Results show that images with high quality can be obtained with comparatively low patient doses, and that there are still wide differences in image quality for similar speed systems, depending on the manufacturer and on the chosen combination.

Calcium Compounds↗
PubMed1984

Plain radiography with a rare-earth screen: comparison with calcium tungstate screen.

Plain radiographic examinations were performed on 388 patients (adult and pediatric) using either a calcium tungstate or rare-earth screen. Radiographs were compared for overall image quality, mottle, contrast, density, and detail. The rare-earth screen produced radiographs with significantly more mottle than did the standard screen. However, this was almost never of diagnostic importance. A small but significant decrease in overall image quality also was found with the rare-earth screen. This was the result of minor differences between the grading of optimal and good. A major advantage of rare-earth screens is a 50% or more reduction in radiation exposure. The results here show that rare-earth screens may be used in plain radiographic examinations without a clinically important decrease in image quality.

Adult↗
PubMed1986

A comparative evaluation of rare-earth screen-film systems. System speed, contrast, sensitometry, RMS noise, square-wave response function, and contrast-dose-detail analysis.

We evaluated the physical characteristics and contrast-dose-detail performance of 11 rare-earth and three calcium tungstate screen-film systems. Measurements included system speed, contrast, sensitometry, RMS noise, square-wave response function, and contrast-dose-detail analysis. The major differences in physical characteristics among systems were system speed and RMS noise. Square-wave response differences were more subtle. For contrast-dose-detail analysis, the rare-earth screen-film systems and the calcium tungstate system responses were significant over a limited subject contrast range as a function of detail diameter. Relative dose efficiency in the noise-limited region is a function of the properties of the screen only and is independent of the film.

Calcium Compounds↗
PubMed1984

Rare earth screens for panoramic radiography.

Six panoramic radiographs, each made with a different screen/film combination, were compared for "periapical visibility" by fourteen dentists. Two combinations (a par-speed and a high-speed) used calcium tungstate screens, and three used rare earth screen systems four times faster than par-speed. One other combination consisted of a single rare earth screen plus a single-coated detail film. The radiographs produced by the faster rare earth screen/film combinations were found to be similar in quality to those produced by calcium tungstate screens. Rare earth screens could be used for panoramic radiography while maintaining quality and reducing exposure.

Adult↗
PubMed1992

27Al and 23Na double-rotation NMR of sodalites.

The 27Al NMR spectra of calcium tungstate aluminate sodalite (CAW), Ca8[Al12O24](WO4)2, and the 23Na NMR spectra of sodium aluminosilicate sodalites of general composition Na9[Si6Al6O24]A2 with A = B(OH)4- (SBS), SCN- (SRS) and A2 = SO4(2-) (SSS), MoO4(2-) (SMS) have been measured using magic-angle spinning (MAS) and double-rotation (DOR) techniques. Rotor synchronized pulse excitation is applied in the DOR experiments. Dramatic line narrowing is observed in the DOR spectra of all samples. The 27Al DOR NMR spectra of CAW measured at 9.4 and 11.7 T and spinning rates of 800-1150 Hz of the outer and 5 kHz of the inner rotor show seven sharp central lines accompanied by a manifold of spinning sidebands. These lines correspond to the seven crystallographically inequivalent Al sites of the CAW framework derived from X-ray structure analysis. From the difference of the line positions in the 9.4 and 11.7 T spectra the quadrupole coupling constant, QCC, quadrupole induced shift, sigma qs, and isotropic chemical shift, delta cs, of each Al site have been calculated. QCC values in the range of 5 to 9 MHz are obtained which reflect the strong tetragonal distortion of the AlO4 tetrahedra in CAW. delta cs shows only small changes in the range between 74.4 and 77.2 ppm. A tentative assignment of all lines to the distinct Al sites is derived from the correlation between QCC and a "shear strain parameter" describing quantitatively the distortion of the AlO4 tetrahedra.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum↗
PubMed2006

Modeling the competitive effect of phosphate, sulfate, silicate, and tungstate anions on the adsorption of molybdate onto goethite.

The mobility of Mo in soils and sediments depends on several factors including soil mineralogy and the presence of other oxyanions that compete with Mo for the adsorbent's retention sites. Batch experiments addressing Mo adsorption onto goethite were conducted with phosphate, sulfate, silicate, and tungstate as competing anions in order to produce competitive two anions adsorption envelopes, as well as competitive two anions adsorption isotherms. Tungstate and phosphate appear to be the strongest competitors of Mo for the adsorption sites of goethite, whereas little competitive effects were observed in the case of silicate and sulfate. Mo adsorption isotherm from a phosphate solution was similar to the one from a tungstate solution. The charge distribution multi-site complexation (CD-MUSIC) model was used to predict competitive adsorption between MoO(4)(2-) and other anions (i.e., phosphate, sulfate, silicate and tungstate) using model parameters obtained from the fitting of single ion adsorption envelopes. CD-MUSIC results strongly agree with the experimental adsorption envelopes of molybdate over the pH range from 3.5 to 10. Furthermore, CD-MUSIC prediction of the molybdate adsorption isotherm show a satisfactory fit of the experimental results. Modeling results suggest that the diprotonated monodentate complexes, FeOW(OH)(5)(-0.5) and FeOMo(OH)(5)(-0.5), were respectively the dominant complexes of adsorbed W and Mo on goethite 110 faces at low pH. The model suggests that Mo and W are retained mainly by the formation of monodentate complexes on the goethite surface. Our results indicate that surface complexation modeling may have applications in predicting competitive adsorption in more complex systems containing multiple competing ions.

Adsorption↗
PubMed2004

Ferrihydrite-dependent growth of Sulfurospirillum deleyianum through electron transfer via sulfur cycling.

Observations in enrichment cultures of ferric iron-reducing bacteria indicated that ferrihydrite was reduced to ferrous iron minerals via sulfur cycling with sulfide as the reductant. Ferric iron reduction via sulfur cycling was investigated in more detail with Sulfurospirillum deleyianum, which can utilize sulfur or thiosulfate as an electron acceptor. In the presence of cysteine (0.5 or 2 mM) as the sole sulfur source, no (microbial) reduction of ferrihydrite or ferric citrate was observed, indicating that S. deleyianum is unable to use ferric iron as an immediate electron acceptor. However, with thiosulfate at a low concentration (0.05 mM), growth with ferrihydrite (6 mM) was possible and sulfur was cycled up to 60 times. Also, spatially distant ferrihydrite in agar cultures was reduced via diffusible sulfur species. Due to the low concentrations of thiosulfate, S. deleyianum produced only small amounts of sulfide. Obviously, sulfide delivered electrons to ferrihydrite with no or only little precipitation of black iron sulfides. Ferrous iron and oxidized sulfur species were produced instead, and the latter served again as the electron acceptor. These oxidized sulfur species have not yet been identified. However, sulfate and sulfite cannot be major products of ferrihydrite-dependent sulfide oxidation, since neither compound can serve as an electron acceptor for S. deleyianum. Instead, sulfur (elemental S or polysulfides) and/or thiosulfate as oxidized products could complete a sulfur cycle-mediated reduction of ferrihydrite.

Electron Transport↗
PubMed1982

Clinical comparison of high-speed rare-earth screen and par-speed screen for diagnostic efficacy and radiation dosage.

One hundred patients underwent excretory urography and a comparison was made of ten-minute, well-collimated images that were obtained with both par-speed and rare-earth screens, the latter being 6.5 times faster than the par-speed calcium tungstate screens. Radiation dose was greatly reduced with the rare-earth screens. There were fewer inferior examinations, even though fine detail was imaged poorly, and there was a slightly increased quantum mottle, which was only a minor problem at this low 65 kVp. Since quantum mottle increases with kVp, however, our results are not applicable to higher kVp examinations. Despite reduced detail and increased mottle, the overall image quality obtained with the rare-earth screen was superior to the image quality obtained with the par-speed screen.

Body Burden↗
PubMed1988

The use of [185W] thiotungstates for tracer studies in secondary hypocuprosis in cattle.

[185W] thiotungstates were synthesized in-vitro and purified by techniques developed for thiomolybdates. [185W] trithiotungstate (15 mg W) injected intravenously into cattle was mainly associated with albumin albumin. The presence of the compound in plasma when injected in larger amounts (0.2 to 1 gW) caused an increase in albumin-bound copper and transiently depressed ceruloplasmin diamine oxidase activity. It is suggested that the long-lived, less costly 185W could prove a convenient alternative to 99Mo for tracer studies with labelled thio-compounds in animals.

Animals↗
PubMed2006

Polyoxometalate catalysts: toward the development of green H2O2-based epoxidation systems.

This paper describes the development of green, efficient H(2)O(2)-based epoxidation systems with three kinds of polyoxometalates: (i) a dinuclear peroxotungstate [W(2)O(3)(O(2))(4)(H(2)O)(2)](2-) (I), (ii) a divacant lacunary polyoxotungstate [gamma-SiW(10)O(34)(H(2)O)(2)]4 (II), (iii) and a divanadium-substituted polyoxotungstate [gamma-1,2-H(2)SiV(2)W(10)O(40)](4-) (III). The highly chemo-, regio-, and diastereoselective epoxidation of various allylic alcohols with only 1 equiv H(2)O(2) in water can be efficiently catalyzed by potassium salt of I (K-I). The catalyst K-I can be recycled with the retention of the catalytic performance. Protonation of a divacant lacunary polyoxotungstate [gamma-SiW(10)O(36)](8-) gives [gamma-SiW(10)O(34)(H(2)O)(2)](4-) (II) with two aquo ligands. The tetra-n-butylammonium salt of II (TBA-II) catalyzes epoxidation of common olefins including propylene with >or=99% selectivity to epoxide and >or=99% efficiency of H(2)O(2) utilization. The bis(mu-hydroxo)bridged dioxovanadium site in [gamma-1,2-H(2)SiV(2)W(10)O(40)](4-) (III) can also efficiently catalyze epoxidation of a variety of olefins with 1 equiv H(2)O(2). Notably, the system with III shows unique stereospecificity, diastereoselectivity, and regioselectivity for the epoxidation of cis/trans olefins, 3-substituted cyclohexenes, and nonconjugated dienes, respectively, which are quite different from those reported for epoxidation systems up to now. Furthermore, the heterogenization of the mentioned polyoxometalates can be achieved by using ionic liquid-modified SiO(2) as a support without loss of catalytic performance.

Alkenes↗
PubMed1980

[A new rare earth intensifying screen: lanthanum oxybromide (Quanta III - Dupont): relative speed, resolution and image quality (author's transl)].

Some physical and diagnostic characteristics of a new rare earth screen (lanthanum oxybromide: Quanta III) were evaluated and compared to those of conventional calcium tungstate (Hi-Plus-Dupont). The speed, resolution and clinical acceptance were analyzed using the same film (Cronex 4 - Dupont). This new system allows a significant X-ray exposure reduction with little or no loss of overall diagnostic performance.

Calcium↗
PubMed1987

The evaluation of high-speed screen/film combinations in cephalometric radiography.

Cephalometric radiographs of a tissue equivalent plastic encased dry skull, each made with eight different screen/film combinations, were compared for usefulness in making orthodontic analyses. The eight screen/film combinations chosen ranged in relative speed from 80 to 600. Only one grid cassette was used, and it was a Par speed screen/film combination. Ten orthodontists identified 13 selected anatomic landmarks and rated the adequacy of their radiographic images. The density of each cephalometric radiograph was analyzed for each anatomic landmark. The results showed a difference in density readings for the various landmarks visualized. The optical density preferences of the 13 anatomic landmarks ranged between 0.87 and 2.09. The study leads to the conclusion that cephalometric radiographs can be taken with screen/film combinations that provide maximum useful information with a minimum dose level to the patient.

Calcium Compounds↗
PubMed1988

Exposure reduction in cephalometric radiology: a comprehensive approach.

Various methods have been proposed and investigated for the purpose of reducing radiation exposures in cephalometric radiography. The purpose of this investigation was to use various dose-reducing methods from four major categories--(1) rare-earth intensifying screens, (2) rare-earth filtration, (3) prepatient soft-tissue enhancement methods, and (4) films varying in speed and latitude--to determine optimal combinations on the basis of exposure reduction and image quality achieved. In laboratory tests and clinical trials, radiation doses were compared along with various tests of image quality for the standard system currently used at this institution and for experimental systems. Image quality was assessed by standard panel-of-expert methods and more quantitative methods involving optical densitometry, contrast indices, and landmark envelopes of error. Results demonstrated that significant exposure reductions were achievable, often with improved image quality. The degree of exposure reduction was dependent largely upon the type of intensifying screen and to a lesser extent on beam filtration or film types. The greatest reduction in exposures were achieved with techniques using rare-earth beam filtration. Image quality was highest with a new wide-latitude type film. Because such significant reduction in radiation exposure for cephalometrics is possible with new techniques, we conclude that a change from the previous standard should occur. In establishing a new technique, orthodontists should consider adopting rare-earth screens and beam filters, flat-grained films, prepatient soft-tissue enhancement methods, and the elimination of grids.

Calcium Compounds↗
PubMed1979

X-ray absorption, speed, and luminescent efficiency of rare earth and other intensifying screens.

Seven new and two conventional screens were investigated. Measurements were performed at different energies of the photon fraction interacting in the screens, and energy losses due to escape of K-fluorescent radiations were calculated. The speed of the screens was dependent on the energy; this was primarily due to the fact that x-ray absorption is dependent on energy. Rare earth and other new screens absorb a maximum of 1.5 times as much energy and emit twice as much light as comparable calcium tungstate screens. Reabsorbed K-fluorescent radiations might contribute to image unsharpness in the new screens.

Calcium Compounds↗