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941.
The behaviour of iron in the Bayer process has been investigated by neutron activation analysis and Ge(Li) spectrometry, using
bauxite samples of various origin. The amount of iron impurities in alumina was found to be independent of the origin of the
bauxites. Alumina is contaminated by iron in the process of decomposition of aluminate liquors; regardless whether iron is
present in ‘dissolved’ form or as a floating impurity, it will pass into the alumina almost completely. 相似文献
942.
Effective heat dissipation is critical for reproducible and efficient separations in electrically driven separation systems. Flow rate, retention kinetics, and analyte diffusion rates are some of the characteristics that are affected by variation in the temperature of the mobile phase inside the column. In this study, we examine the issue of Joule heating in packed capillary columns used in capillary electrochromatography (CEC). As almost all commonly used CEC packings are poor thermal conductors, it is assumed that the packing particles do not conduct heat and heat transfer is solely through the mobile phase flowing through the system. The electrical conductivity of various mobile phases was measured at different temperatures by a conductivity meter and the temperature coefficient for each mobile phase was calculated. This was followed by measurement of the electrical current at several applied voltages to calculate the conductivity of the solution within the column as a function of the applied voltage. An overall increase in the conductivity is attributed to Joule heating within the column, while a constant conductivity means good heat dissipation. A plot of conductivity versus applied voltage was used as the indicator of poor heat dissipation. Using theories that have been proposed earlier for modeling of Joule heating effects in capillary electrophoresis (CE), we estimated the temperature within CEC columns. Under mobile and stationary phase conditions typically used in CEC, heat dissipation was found to be not always efficient. Elevated temperatures within the columns in excess of 23 degrees C above ambient temperature were calculated for packed columns, and about 35 degrees C for an open column, under a given set of conditions. The results agree with recently published experimental findings with nuclear magnetic resonance (NMR) thermometry, and Raman spectroscopic measurements. 相似文献
943.
T Gunnlaugsson D A Mac Dónaill D Parker 《Journal of the American Chemical Society》2001,123(51):12866-12876
The Eu(III) tetraazamacrocyclic complexes [Eu.1] and [Eu.2], and the Tb(III) and Yb(III) complexes [Tb.1] and [Yb.2], have been synthesized as luminescent molecular-level devices. The Eu complexes exhibit unique dual pH switching behavior in water under ambient conditions. The delayed Eu emission is reversibly switched on in acid, with an enhancement factor of several hundred for [Eu.1]. These observations are consistent with the protonation of the quinoline aryl nitrogen moiety (pK(a) approximately equal to 5.9 for [Eu.1]). The fluorescence emission spectra of these complexes are unaffected by acid, but pronounced changes occur in alkaline solution due to the deprotonation of the aryl amide nitrogen (pK(a) approximately 9.4 for [Eu.1]). [Tb.1] shows a more intriguing pH dependence; Tb emission is switched "on" only in the presence of H+ and in the absence of molecular oxygen, whereas the fluorescence emission properties are similar to those observed with [Eu.1]. This behavior can be conveniently described as a molecular-level logic gate, corresponding to a two-input INHIBIT function, A wedge B'. The analogous [Yb.2] complex shows no such pH or O(2) dependence. 相似文献
944.
A reaction–diffusion system consisting of one, two or three chemical species and taking place in an arbitrary number of spatial
dimensions cannot exhibit Turing instability if none of the reaction steps express cross‐inhibition. A corollary of this result
– obtained by elementary calculations – underlines the importance of nonlinearity in the formation of stationary structures,
a kind of self‐organization on a chemical basis. Relations to global stability of reaction–diffusion systems, and results
on multispecies systems are also mentioned. The statements are not restricted to mass action type models. As a by‐product,
the solution of a basic inverse problem of formal kinetics is also presented which extends a previous result by Hárs and Tóth
(1981) to models with arbitrary – including rational – functions as reaction rates so often occurring, e.g., in enzyme kinetics.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
945.
Heider Nasser ákos Rédey Tatiana Yuzhakova Zsolt Németh Tóth Tamás Ollár 《Reaction Kinetics and Catalysis Letters》2007,92(2):329-335
CO adsorption at low temperature has been used to probe Lewis acid sites created upon dehydroxylation of γ-Al2O3 and reduction of Mo/Al2O3 catalysts, using Fourier Transform Infrared spectroscopy (FTIR).
Carbon-monoxide adsorption on γ-Al2O3 and Mo/Al2O3 catalysts dehydroxylated and reduced at different temperatures was studied at 78 K by IR spectroscopy. However, our results
indicate that there is an approximately linear correlation between the increase either of dehydroxylation or the extent of
reduction of the catalysts and the increasing absorbance of CO due to CO adsorption on Lewis acid sites created upon dehydroxylation
of γ-Al2O3 and reduction of Mo/Al2O3. 相似文献
946.
The behaviour of oxazepam in adsorptive stripping voltammetry was studied taking into account those conditions which have an influence on the accumulation step (electrolyte, pH, time, potential, drop size and stirring rate), rest time and stripping step (pulse amplitude and scan rate). Oxazepam can be determined at a hanging mercury drop electrode by differential-pulse voltammetry in 0.008 M Britton-Robinson buffer at pH 2.0 with a ?0.50 V accumulation potential. Its detection limit was found to be 3.6 × 10?10 M (30-s accumulation) and the relative standard deviation for oxazepam concentrations in the range 2.8 × 10?8?4.0 × 10?7 M is lower than 2.8% (80-s accumulation). In addition, a procedure using adsorptive stripping voltammetry was developed to study the interactions occurring between human albumin and the camazepam metabolic series (camazepam, temazepam and oxazepam). The interactions decreased in the order temazepam ? oxazepam ? camazepam and the groups and structural modifications favouring interaction were determined. 相似文献
947.
The complexes of 4-chloro-2-methoxybenzoic acid anion with Mn2+,
Co2+, Ni2+, Cu2+
and Zn2+ were obtained as polycrystalline solids
with general formula M(C8H6ClO3)2·nH2O and colours typical for M(II) ions (Mn – slightly pink, Co –
pink, Ni – slightly green, Cu – turquoise and Zn – white).
The results of elemental, thermal and spectral analyses suggest that compounds
of Mn(II), Cu(II) and Zn(II) are tetrahydrates whereas those of Co(II) and
Ni(II) are pentahydrates. The carboxylate groups in these complexes are monodentate.
The hydrates of 4-chloro-2-methoxybenzoates of Mn(II), Co(II), Ni(II), Cu(II)
and Zn(II) heated in air to 1273 K are dehydrated in one step in the range
of 323–411 K and form anhydrous salts which next in the range of 433–1212
K are decomposed to the following oxides: Mn3O4,
CoO, NiO and ZnO. The final products of decomposition of Cu(II) complex are
CuO and Cu. The solubility value in water at 293 K for all complexes is in
the order of 10–3 mol dm–3.
The plots of χM
vs.
temperature of 4-chloro-2-methoxybenzoates of Mn(II), Co(II), Ni(II) and Cu(II)
follow the Curie–Weiss law. The magnetic moment values of Mn2+,
Co2+, Ni2+ and Cu2+
ions in these complexes were determined in the range of 76−303 K and
they change from: 5.88–6.04 μB for Mn(C8H6ClO3)2·4H2O, 3.96–4.75
μB for Co(C8H6ClO3)2·5H2O, 2.32–3.02 μB for Ni(C8H6ClO3)2·5H2O and 1.77–1.94
μB for Cu(C8H6ClO3)2·4H2O. 相似文献
948.
Gabriela Flores-Ramírez Manuel Rivera Alfredo Morales-Pablos Joel Osuna Xavier Soberón Paul Gaytán 《BMC chemical biology》2007,7(1):1
Background
The effect of single and multiple amino acid substitutions in the green fluorescent protein (GFP) from Aequorea victoria has been extensively explored, yielding several proteins of diverse spectral properties. However, the role of amino acid deletions in this protein -as with most proteins- is still unknown, due to the technical difficulties involved in generating combinatorial in-phase amino acid deletions on a target region. 相似文献949.
A room-temperature flow injection spectrofluorimetric method is presented for the determination of Zn(II), based on the use of salicylaldehyde thiocarbohydrazone in the presence of Triton X-100 and sodium acetate-acetic acid buffer. Various physical and chemical variables affecting the reaction in the flow system were evaluated. The proposed method is very selective. The calibration graph is linear over the range 10-1000 ng/ml, with a detection limit of 5 ng/ml and a relative standard deviation at the 50 ng/ml level of 1.8 %. The method was successfully applied to the determination of Zn(II) in drinking waters and biological samples. 相似文献
950.
Supercritical fluid extraction of polycyclic aromatic hydrocarbons from liver samples and determination by HPLC-FL 总被引:3,自引:0,他引:3
González Amigo S García Falcón MS Lage Yusty MA Simal Lozano J 《Fresenius' Journal of Analytical Chemistry》2000,367(6):572-578
An extraction/clean-up procedure by SFE was developed for isolating PAHs from liver samples for subsequent HPLC-FL determination of ten PAHs in the enriched extract. Recoveries (90-115%) and RSD % (< or =7.7) were satisfactory. When applied to 11 samples of bird of prey (Tyto alba) protected species and classified of special interest, from the Galicia (Northwest to Spain), benzo[ghi]perylene and indeno[1,2,3-cd]pyrene were undetectable; chrysene and benzo[a]pyrene are only detected in one sample; benzo[a]anthracene and benzo[k]fluoranthene are only quantified in one sample and benzo[b]fluoranthene in two samples. The other PAHs, anthracene, fluoranthene and pyrene are present in almost all the samples. 相似文献