Overloaded elution band profiles of ionizable compounds in reversed‐phase liquid chromatography: Influence of the competition between the neutral and the ionic species

The parameters that affect the shape of the band profiles of acido‐basic compounds under moderately overloaded conditions (sample size less than 500 nmol for a conventional column) in RPLC are discussed. Only analytes that have a single pK_a are considered. In the buffer mobile phase used for their elution, their dissociation may, under certain conditions, cause a significant pH perturbation during the passage of the band. Two consecutive injections (3.3 and 10 μL) of each one of three sample solutions (0.5, 5, and 50 mM) of ten compounds were injected on five C₁₈‐bonded packing materials, including the 5 μm Xterra‐C₁₈ (121 Å), 5 μm Gemini‐C₁₈ (110 Å), 5 μm Luna‐C₁₈(2) (93 Å), 3.5 μm Extend‐C₁₈ (80 Å), and 2.7 μm Halo‐C₁₈ (90 Å). The mobile phase was an aqueous solution of methanol buffered at a constant _W^WpH of 6, with a phosphate buffer. The total concentration of the phosphate groups was constant at 50 mM. The methanol concentration was adjusted to keep all the retention factors between 1 and 10. The compounds injected were phenol, caffeine, 3‐phenyl 1‐propanol, 2‐phenyl butyric acid, amphetamine, aniline, benzylamine, p‐toluidine, procainamidium chloride, and propranololium chloride. Depending on the relative values of the analyte pK_a and the buffer solution pH, these analytes elute as the neutral, the cationic, or the anionic species. The influence of structural parameters such as the charge, the size, and the hydrophobicity of the analytes on the shape of its overloaded band profile is discussed. Simple but general rules predict these shapes. An original adsorption model is proposed that accounts for the unusual peak shapes observed when the analyte is partially dissociated in the buffer solution during its elution.     

Kinetics and mechanism of free-surface vaporization of zinc, cadmium and mercury oxides analyzed by the third-law method

On the basis of critical comparison of experimental and theoretical values of the E parameter and investigation of the retardation effect of oxygen on the evaporation rate of ZnO, CdO and HgO, it was concluded that the dissociative evaporation of ZnO and HgO proceeds with releasing of atomic oxygen (O) as a primary product of decomposition. By contrast, the mechanism of dissociative evaporation of CdO corresponds to the equilibrium reaction with releasing of molecular oxygen (O₂) as a primary product of decomposition. As was shown, this difference in mechanisms is not related with interatomic OO distances in these oxides. From the analysis of crystal structure for 12 different oxides, which evaporate with releasing of atomic oxygen, and for 13 compounds, which evaporate with releasing of molecular oxygen, it was revealed that the first mechanism is observed for all oxides with the cubic crystal structure. It was proposed that a decisive role in this difference belongs to a local symmetry in the position of O atoms.     

燃煤过程痕量元素动力学机理的研究进展

由于痕量元素在煤中的含量低微、检测困难,加之其原子量一般较大,可能的反应途径多,使得相关的反应机理研究难度极大。本文结合作者的研究成果,介绍了煤燃烧过程中痕量元素化学反应动力学的国内外研究进展,包括痕量元素化学动力学机理的建立;相关的典型实验、计算模拟及其实验验证、动力学机理模型的简化;痕量元素反应动力学机理的完善和发展,包括采用简单碰撞理论、活化络合物理论(亦称过渡态理论,或绝对反应速率理论)对痕量元素化学反应动力学机理的修正;最后指出煤燃烧过程中痕量元素动力学研究的若干方向是: (1)痕量元素反应动力学模型数据库的建立;(2)煤燃烧过程中,主量元素和次量元素的动力学机理的完善; (3)各痕量元素之间动力学研究的开展; (4)实际燃烧过程中痕量元素动力学行为的研究。     

Numerical analysis of reversible A B C reaction-diffusion systems

We develop an effective numerical method of studying large-time properties of reversible reaction-diffusion systems of type A + B ↔ C with initially separated reactants. Using it we find that there are three types of asymptotic reaction zones. In particular we show that the reaction rate can be locally negative and concentrations of species A and B can be nonmonotonic functions of the space coordinate x, locally significantly exceeding their initial values. Received 6 June 2002 / Received in final form 20 January 2003 Published online 7 May 2003     

A Monte Carlo simulation of the heterogeneous adsorption of hydrogen ions on metal oxides: Effect of inert electrolyte

Charging of the surface of an oxide caused by the adsorption of hydrogen ions and ions of inert 1:1 electrolyte was investigated by using grand canonical Monte Carlo simulation technique. In particular, adsorption isotherms of protons as well as of ions of the electrolyte together with the resulting charge density of the surface were obtained for different system parameters. Also, the effect of the surface energetic heterogeneity and the concentration of the background electrolyte on the isotherms and the charge density curves was examined. Furthermore, lateral interactions in the mixed adsorbed phase were taken into account in the modeling of the system behavior. The obtained results, in general, suggest that the three factors mentioned above may have substantial influence on the charging mechanism at the liquid/oxide interface.     

固定化单宁对酒类营养成分的吸附性能研究

采用固定化单宁对酒中可能存在的营养成分蛋白质、氨基酸、糖类、有机酸、乙醇和铁离子等进行吸咐试验。结果表明：固定化单宁对不同营养成分的吸附率差别明显，它通过氢键和疏水作用对蛋白质有较大的吸附率，通过多个酚羟基与中心铁离子形成螯合物而对其有较大的吸附率，对氨基酸、糖、有机酸和乙醇等营养成分或风味物质吸附率不大，固定化单宁可以作为一种较为理想的酒类处理剂。     

Simplified and complex modeling of self-sustained discharge-pumped,Ne-buffered XeCl laser kinetics

Two simple kinetic models of a Ne-buffered XeCl laser discharge are presented based on different simplifications of the chemical kinetics of a complex model that recently appeared in the literature. When applied to the study of a small-volume XeCl laser, the results of both simple and complex models are in satisfactory agreement with the experiments. Shifting to a larger-volume laser, both models show problems due to loss of stability of the discharge. A one-dimensional modelling is performed, and it is found that different assumptions on the kinetics of the NeXe⁺ ion in the discharge lead to completely different results as regards the effect of a preionization-triggered instability.     

Insertion of benzothiazolium compounds into montmorillonite interlayers

Benzothiazolium compounds exhibit pronounced antimicrobial activities and stimulation effects on plant growth. When applied to fields they can pollute soil colloids. In contact with the soil they can potentially interact with a clay fraction giving rise to clay organocomplexes. Model intercalation complexes were prepared using monoionic montmorillonite and a variety of water-soluble benzothia zolium salts. It was found that the adsorption into the silicate interlayer space proceeds via a cation exchange process. Substantial differences were observed in the extent of the reaction between non-substituted andN-substituted species. The sulphur atoms present in the benzothiazolium molecules repel the surface oxygen atom: this is considered to be the factor responsible for prevention of insertion of the guest cations perpendicular to the layers.     

Determination of the rate of rapid lipid transfer induced by poly(ethylene glycol) using the SLM Fourier transform phase and modulation spectrofluorometer

Rate constants were determined for the transfer of the fluorescent lipid probe 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5-hexatrienyl)phenyl]ethyl] oxy]carbonyl]-3-sn-phosphatidylcholine (DPHpPC) between large, unilamellar extrusion vesicles composed either of dipalmitoyl phosphatidylcholine (DPPC) or of DPPC mixed with a small amount (0.5 mol%) of lyso phosphatidylcholine (Lyso PC). Transfer of the lipid probe in the presence of varying concentrations of poly(ethylene glycol) (PEG) was monitored using the SLM 48000-MHF Multi-Harmonic Fourier Transform phase and modulation spectrofluorometer to collect multifrequency phase and modulation fluorescence data sets on a subsecond time scale. The unique ability of this instrument to yield accurate fluorescence lifetime data on this time scale allowed transfer to be detected in terms of a time-dependent change in the fluorescent lifetime distribution associated with the lipid-like DPHpPC probe. This probe demonstrates two short fluoresence decay times (ca. 1.1–1.4 and 4.3–4.8 ns) in a probe-rich environment but a single long lifetime (ca. 7 ns) in a probe-poor environment. A simple two-state model for initial lipid transfer was used to analyze the multifrequency data sets collected over a 4-s time frame to obtain the time rate of change of the concentrations of donor and acceptor probe populations following rapid mixing of vesicles with PEG. The ability to measure fluorescence lifetimes on this time scale has allowed us to show that the of rate of lipid transfer increased dramatically at 35% PEG in both fusing and nonfusing vesicle systems. These results are interpreted in terms of a distinct interbilayer structure associated with intimate bilayer contact induced by high and potentially fusogenic concentrations of PEG.     

Form of the Spectra of Nucleus Dimensions in First-Order Phase Transitions

We study a generalization of the analytic theory of first-order phase transitions to the cases of arbitrary droplet growth, of nonisothermal processes, and of heterogeneous centers in the system. We show that in all these cases, the spectra of droplet dimensions are similar. The same forms of the spectra are also obtained for the stationary condensation process in a spatially inhomogeneous system.