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101.
102.
Suresh K. Aggarwal Michael Kinter Michael R. Wills John Savory David A. Herold 《Analytica chimica acta》1989
A gas chromatographic-mass spectrometric (GC-MS) method involving thermally stable, volatile chelates was investigated for measurements of isotope ratios of chromium, nickel, zinc and copper. The chelating agents acetylacetone, trifluoroacetylacetone, sodium diethyldithiocarbamate and lithium bis (trifluoroethyl)dithiocarbamate [Li(FDEDTC])] were used. Experimental conditions for the preparation of chelates and the mass spectrometric operating parameters for precise and accurate measurement of isotope ratios were optimized using a general-purpose mass spectrometer. Imprecision values of 1–4% were obtained for measurements of different isotope ratios using chelates containing about 10 ng of metal. The capability of this technique for the accurate determination of natural and altered isotope ratios was also evaluated for these elements using Li(FDEDTC) as a chelating agent. This GC-MS method obviates the need for a more specialized mass spectrometer such as a thermal ionization or inductively coupled plasma mass spectrometer for trace metal determination. The technique gives detection limits down to parts per 109 levels and offers considerable potential for isotope dilution measurements. 相似文献
103.
R L Frost R A Wills J T Kloprogge W N Martens 《Journal of Thermal Analysis and Calorimetry》2006,83(1):213-218
Thermogravimetry combined
with mass spectrometry has been used to study the thermal decomposition of
a synthetic hydronium jarosite. Five mass loss steps are observed at 262,
294, 385, 557 and 619°C. The mass loss step at 557°C is sharp and
marks a sharp loss of sulphate as SO3 from the hydronium
jarosite. Mass spectrometry through evolved gases confirms the first three
mass loss steps to dehydroxylation, the fourth to a mass loss of the hydrated
proton and a sulphate and the final step to the loss of the remaining sulphate.
Changes in the molecular structure of the hydronium jarosite were followed
by infrared emission spectroscopy. This technique allows the infrared spectrum
at the elevated temperatures to be obtained. Infrared emission spectroscopy
confirms the dehydroxylation has taken place by 400 and the sulphate loss
by 650°C. Jarosites are a group of minerals formed in evaporite deposits
and form a component of the efflorescence. The minerals can function as cation
and heavy metal collectors. Hydronium jarosite has the potential to act as
a cation collector by the replacement of the proton with a heavy metal cation. 相似文献
104.
Mutter R Campbell IB Martin De La Nava EM Merritt AT Wills M 《The Journal of organic chemistry》2001,66(10):3284-3290
The palladium-catalyzed reaction of pinacolone with tert-butyldimethyl(3-(2-bromophenyl)allyloxy)silane results in direct formation of 1-vinyl-3-tert-butyl-1H-isochromene. This is the result of a ketone arylation followed by an intramolecular cyclization of the enolate with the allylic system. The use of a lithium diamide base appears to be essential for success. The tert-butyldimethylsilyl protecting group is also an essential choice as it furnishes the appropriate reactivity to promote allylic substitution after the aryl coupling process. The use of more effective leaving groups, such as acetate, results in reaction of the allylic group, and no aryl coupling is observed. Through the appropriate selection of phosphine ligand and solvent, either the cyclized isochromene product or the noncyclized intermediate may be formed selectively. A short combinatorial study of the scope and limitations of the reaction, involving 24 ketones, is described. 相似文献
105.
Wills NJ Park J Wen J Kesavan S Kraus GA Petrich JW Carpenter S 《Photochemistry and photobiology》2001,74(2):216-220
A series of hypericin analogs were found to differ in their cytotoxic activity induced by ambient light levels. These analogs vary in their ability to partition into cells, to generate singlet oxygen as well as in other photophysical properties. The data suggest that the biological activity of hypericin is due to a combination of factors whose roles may vary under different circumstances. 相似文献
106.
J. M. Wills 《Geometriae Dedicata》1991,40(2):237-244
For the lattice point enumerator of a lattice
and a convex body K we give bounds in terms of the intrinsic volumes of K and of minimal determinants of
. The intrinsic volumes are the normalized Minkowski quermassintegrals and the minimal determinants are analogous functionals of
. 相似文献
107.
108.
Cheung FK Hayes AM Hannedouche J Yim AS Wills M 《The Journal of organic chemistry》2005,70(8):3188-3197
Stereochemically well-defined ruthenium(II) catalysts have been applied to the asymmetric transfer hydrogenation of a series of ketones. In one case, statistical experimental design was employed to optimize the enantiomeric excess of the product. In the case of the TsDPEN-based systems, the replacement of trans-1,2-diphenyl substitution with cis-, or deletion of one of the phenyl groups, results in significant deterioration of the enantiomeric excess. A new method is described for the synthesis of tethered amino alcohol-containing catalysts. 相似文献
109.
J. M. Wills 《Geometriae Dedicata》1997,65(1):117-126
The packing density of large lattice packings of spheres in Euclidean E
d
measured by the parametric density depends on the parameter and on the shape of the convex hull P of the sphere centers; in particular on the isoperimetric coefficient of P and on the second term in the Ehrhart polynomial of the lattice polytope P. We show in E
d
, d 2, that flat or spherelike polytopes generate less dense packings, whereas polytopes with suitably chosen large facets generate dense packings. This indicates that large lattice packings in E
3 of high parametric density may be good models for real crystals. 相似文献
110.