Matrix effect on the enantiorecognition ability of adsorbents based on heptakis(6-amino-6-deoxy)-β-cyclodextrin

Four adsorbents were synthesized by immobilizing a chiral selector, heptakis(6-amino-6-deoxy)-β-cyclodextrin, on different silica matrices, such as LiChrosorb Si 100, 10 μm; Silasorb SPH Amin, 5 μm; Kromasil 100-5-Sil, 5 μm; and Kromasil 300-5-Sil, 5 μm. The surface of adsorbents was examined by low-temperature nitrogen adsorption, and the structural properties of substrates and adsorbents were studied by electron microscopy. A matrix effect on the enantiorecognition ability of adsorbents was studied in the reversed-phase and polar-organic modes of high performance liquid chromatography using some amino acid derivatives (N-carbobenzyloxy-and tert-butoxycarbonyl-) and profens as examples. The adsorbent based on Kromasil 100-5-Sil possesses the best chromatographic properties. It was shown that the enantiomeric composition and the concentration of the active component in Ibuprofen can be determined.     

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E _e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p _3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E _e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E _e ≤ 66 eV. Deposition of Sm atoms at room temperature (～300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.     

Cross sections for electron-stimulated desorption of alkali-metal atoms from adlayers on oxidized tungsten

Earlier cross-section measurements of electron-stimulated desorption (ESD) of Li, Na, K, and Cs atoms from adlayers on oxidized tungsten are analyzed with respect to substrate temperature and degree of oxidation. It is conjectured that the ESD cross sections are determined by the ratio of the rates of neutral alkali-metal re-ionization on the surface and of relaxation of the O⁺ charge in the substrate. A comparison with experiment revealed the dependence of the re-ionization rate of an alkali-metal adatom on its size and mass, as well as the dependence of the O⁺ charge relaxation rate on substrate temperature and degree of oxidation. The relation of the charge relaxation time to the substrate band structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1491–1497 (August 1997)     

Specific features of photoelectron emission from palladium clusters on graphite

Subthreshold photoelectron emission was observed to emerge from palladium nanoclusters formed on pyrographite surface under irradiation by photons in the energy range 3.1–6.5 eV. The average size of the palladium nanoclusters on the pyrographite surface was 50–80 nm, and the average height, 2–4 nm. Besides conventional photoemission from states below the Fermi level, photoelectron emission was observed at the energies of photons irradiating the surface 0.9 eV below the work function of the Pd surface. It is assumed that this emission is stimulated by direct electron transitions from Pd states below the Fermi level to the unfilled electron surface states formed in the Coulomb potential of image forces (image states) and, subsequently, into vacuum. This phenomenon is assumed to originate from the contact spot field generated above the surface which is nonuniform in work function. This assumption is supported by the calculations presented in the paper.     

Growth kinetics of graphene and graphite films on the \operatorname{Re} (10\bar 10) surface

Growth kinetics of graphene and graphite layers on the surface of carbon-saturated rhenium has been investigated. The experimental data agree well with the results of the mathematical analysis of diffusion processes in the metal bulk with the participation of carbon atoms and the subsequent precipitation of carbon on the surface. It has been established that graphene layers of rigorously specified thickness in the range of 1–50 layers can be grown on metal in the temperature range of 1300–1500 K.     

Microstructural analysis of biological fluids

The peculiarities of microstructural analysis as an experimental technique for studying biofluids are demonstrated. The application of the concept of permolecular system organization to the analysis of protein system structures is considered. It is shown that systematic studies of biological fluids within the frames of the selected unified approach stimulate the development of the methods of microstructural analysis of biofluids and facilitates better understanding of the observed biostructural effects.     

Carbon segregation on a tungsten surface

New measurements are performed and a comprehensive analysis of experimental data is made on the dependence of equilibrium carbon coverage θ on a tungsten surface on its temperature and the degree of carbon loading. It is shown that if the volume is free of carbon, the variation of θ for T≥1400 K can be approximately described by the balance between the carbon flows through the boundary with the activation energy for transition into the bulk E₁=4.6 eV and the segregation energy ΔE=1.7 eV. For tungsten loaded preliminarily with carbon to a content of ≈10^?2 at. %, the θ(T) relation cannot be described in terms of the equilibrium conditions with constant E₁ and ΔE, because these quantities depend on the degree of carbon loading; E₁ grows from 4.6 to 6.8 eV and ΔE grows from 1.7 to 2.3 eV with an increase in carbon content from 0 to 10^?2 at. %. These variations are attributed to the bonds becoming stronger in carbon-loaded tungsten with increasing carbon content.     

Electron-stimulated desorption of cesium atoms from cesium layers adsorbed on tungsten coated by a gold film

Physics of the Solid State - The yield and energy distributions of cesium atoms escaping in electron-stimulated desorption (ESD) from cesium layers adsorbed on tungsten coated by a gold film have...