Home  About us  Contact
 

Larger Component (larger + component)

Distribution by Scientific Domains
Chemistry75%

Selected Abstracts

Separation of light gas mixtures using SAPO-34 membranes

AICHE JOURNAL, Issue 4 2000
Joseph C. Poshusta
Continuous SAPO-34 membranes were prepared on porous alumina tubular supports, and shown to be useful for light gas separations at low and high temperatures. Single-gas permeances of CO2, N2 and CH4 decreased with increasing kinetic diameter. For the best membrane at 300 K, the He and H2 permeances were less than that of CO2, because He, H2, and CO2 were small compared to the SAPO-34 pore, and differences in the heat of adsorption determined the permeance order. The smaller component permeated the fastest in CO2/CH4, CO2/N2, N2/CH4, H2/CH4 and H2/N2 mixtures between 300 and 470 K. For H2/CO2 mixtures, which were separated by competitive adsorption at room temperature, the larger component permeated faster below 400 K. The CO2/CH4 selectivity at room temperature was 36 and decreased with temperature. The H2/CH4 mixture selectivity was 8 and constant with temperature up to 480 K. Calcination, slow temperature cycles, and exposure to water vapor had no permanent effect on membrane performance, but temperature changes of approximately 30 K/min decreased the membrane's effectiveness. [source]

Shipwrecks and founder effects: Divergent demographic histories reflected in Caribbean mtDNA

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 4 2005
Antonio Salas
Abstract During the period of the Atlantic slave trade (15th,19th centuries), millions of people were forced to move from Africa to many American destinations, changing dramatically the human landscape of the Americas. Here, we analyze mitochondrial DNA from two different American populations with African ancestry, with hitherto unknown European and Native American components. On the basis of historical records, African-Americans from Chocó (Colombia) and the Garífunas (or "Black Carib") of Honduras are likely to have had very different demographic histories, with a significant founder effect in the formation of the latter. Both the common features and differences are reflected in their mtDNA composition. Both show a minor component (,16%) from Native Central/South Americans and a larger component (,84%) from sub-Saharan Africans. The latter component is very diverse in the African-Americans from Chocó, similar to that of sub-Saharan Africans, but much less so in the Garífunas, with several mtDNA types elevated to high frequency, suggesting the action of genetic drift. Am J Phys Anthropol, 2005. © 2005 Wiley-Liss, Inc. [source]

HPLC of humic substances fractionated by Flow FFF

JOURNAL OF SEPARATION SCIENCE, JSS, Issue 5 2003
Maria-Anna Benincasa
Abstract This communication reports a study of the effect of ionic strength and electrolyte composition on fractions, separately collected by flow FFF, of a mixture of humic substances. Reverse phase HPLC analysis of three early eluting fractions suggests that the components released by the column behave as organic acids. The baseline-resolved peaks of the first two fractions, subject to higher retention in solutions of lower pH and/or higher polarity, substantiate this suggestion. The fraction with larger components, as measured by flow FFF, also appears to contain acidic species. Their retention level, however, may not be accurately modulated by varying the mobile phase properties as these species are either totally retained in acidic phases or released before the void peak at pH , 4.2. Besides showing the effective separation achieved in the flow FFF channel, this study reveals the pronounced difference in the physicochemical properties of some components of a humic mixture even with very close particle size. [source]

The Breakdown of the Minimum Polarizability Principle in Vibrational Motions as an Indicator of the Most Aromatic Center

CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2005
Miquel Torrent-Sucarrat Dr.
Abstract The vibrational motions that disobey the minimum polarizability principle (MPP) in ,-conjugated molecules are distortions of the equilibrium geometry that produce a reduction in the polarizability due to the localization of , electrons. For aromatic species, this electronic localization is responsible for the subsequent reduction in the aromaticity of the system. In the present work, we diagonalize the Hessian matrix of the polarizability with respect to the vibrational nontotally symmetric normal coordinates, to calculate the nontotally symmetric distortions that produce the maximum breakdown of the MPP in a series of twenty polycyclic aromatic hydrocarbons. It is shown that the nuclear displacements that break the MPP have larger components in those rings that possess the highest local aromaticity. Thus, these vibrational motions can be used as an indicator of local aromaticity. [source]