Chiral separation of dansyl amino acids by capillary electrophoresis: Comparison of formamide andN-Methylformamide as background electrolytes

Summary Chiral separation of 12 dansyl amino acids has been achieved by capillary electrophoresis using -cyclodextrin dissolved inN-methylformamide or formamide. The viscosity and the dielectric constants of these two solvents are very different, giving them disimilar electrophoretic properties. Intense electroosmotic flow, high electrophoretic mobilities and high efficiency were achieved inN-methylformamide, but high chiral selector concentration was needed for sufficient resolution. In contrast, the separation of dansyl amino acids in formamide was characterized by longer analysis time and lower efficiency, but the resolution and selectivity of the separation were better and baseline separation could be achieved at lower -cyclodextrin concentrations.     

Pressurised hot water extraction-microporous membrane liquid-liquid extraction coupled on-line with gas chromatography-mass spectrometry in the analysis of pesticides in grapes

Pressurised hot water extraction-microporous membrane liquid-liquid extraction was coupled on-line with gas chromatography-mass spectrometry (PHWE-MMLLE-GC-MS) for the analysis of pesticides in grapes. MMLLE serves as a trapping step after PHWE. Water from PHWE is directed to the donor side of the membrane unit and the analytes are extracted to the acceptor solution on the other side. The role of MMLLE is to clean and concentrate the extract before on-line transfer to the GC via a sample loop and an on-column interface using partially concurrent solvent evaporation. The extraction conditions were investigated, and then the quantitative features such as linearity, limit of quantification (LOQ), extraction yield and enrichment factors. LOQs in the range 0.3-1.8 microg kg(-1) were achieved. Procymidone and tetradifon were found in the skins of the grapes. The results were in good agreement with those obtained by liquid-solid and ultrasonic extractions.     

Progress in the lithium insertion mechanism in Cu3P

Copper phosphide, Cu₃P has been synthesized using a ceramic route, and its electrochemical behaviour versus lithium has been studied studied galvanostatic and potentiodynamic measurements and in situ X-ray diffraction analysis. The insertion/extraction mechanism proceeds with the formation of at least three different Li_xCu_3−xP (x=1, 2, 3) phases. The electrochemical behaviour of Cu₃P samples obtained from ceramic and solvothermal syntheses are compared to further understanding of the complex redox mechanism occurring during insertion/extraction. First-principle electronic structure calculations show that discharge probably begins with the formation of a solid solution Li_xCu_3−yP (x<0.5). Paper presented at the Patras Conference on Solid State Ionics-Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

HCl and DCl: a case study of different approaches for determining photo fractionation constants

The photoabsorption cross sections of HCl and DCl are calculated using the reflection principle and time dependent wavepacket propagation methods. The absorption cross sections are compared to high precision experimental absorption cross sections from the literature and the different results given by the methods are discussed. The results of the calculations emphasize the important roles that photodissociation dynamics and the change in transition dipole moment with internuclear distance play in isotopic fractionation. The wave number dependent fractionation constants have been determined. The process fractionation constant has been calculated in the Venusian atmosphere where photo-fractionation leads to enrichment in deuterium through loss of hydrogen to space. At an altitude of 70 km the process fractionation constant was found to be epsilon(p) = -344 per thousand and epsilon(p) = -256 per thousand for the experimental and the reflection principle methods, respectively. At the top of the atmosphere the process fractionation constant was evaluated to be epsilon(p) = -32 per thousand, epsilon(p) = -20 per thousand and epsilon(p) = -40 per thousand using the experimental data, the wavepacket and the reflection principle methods, respectively. Using the Rayleigh distillation formula it is concluded that HCl at the top of the Venusian atmosphere is fractionated (enriched in D) relative to the bulk composition prior to photolysis.     

Exposure of monomolecular lithographic patterns to ambient: an X-ray photoemission spectromicroscopy study

Patterned self-assembled monolayers (SAMs) of alkanethiolates (AT) on Au and Ag substrates were imaged and characterized by scanning photoelectron microscopy (SPEM). The patterns were prepared in situ by direct writing with the zone-plate-focused X-ray beam provided by the SPEM station. Whereas both AT/Au and AT/Ag behaved alike upon the irradiation, which resulted in similar contrasts in the fabricated patterns and similar microspot spectra from the irradiated areas, the intensity relationship between the patterned and nonpatterned areas changed by different pathways for the Au and Ag substrates after the exposure of the patterns to ambient. The SPEM data imply that weakly bound molecular fragments are desorbed from the irradiated areas upon air exposure in the case of Ag, whereas adsorption of airborne molecules from ambient occurs for the Au substrate. The origin of the observed differences is presumably related to the specific branching patterns of irradiation-induced modification of AT/Au and AT/Ag.     

Development of structural order during supercooling of a fragile oxide melt

The authors have studied the structural evolution of the fragile glass-forming liquid CaAl2O4 during supercooling from the stable liquid phase to the cold glass below Tg. The evolution is characterized by a sharpening of the first diffraction peak and a shortening of the average nearest-neighbor bond length around 1.25Tg, indicating an increase in the degree of both intermediate-range and short-range orders occurring close to the dynamical crossover temperature. The cooling curve developed a kink at this temperature, indicating a simultaneous change in thermodynamic properties.     

Modification of the ultrasound induced activity by the presence of an electrode in a sonoreactor working at two low frequencies (20 and 40 kHz). Part I: Active zone visualization by laser tomography

Sonoelectrochemical experiments differ from sonochemical ones by the introduction of electrodes in the sonicated reaction vessel. The aim of the study is to characterize the changes of the ultrasonic activity induced by the presence of an electrode located in front of the transducer. The scope of our investigations concerns two low frequencies vibration modes: 20 and 40 kHz. For this purpose, two laser visualization techniques have been used. The first part of the study, described in the present paper (part I), deals with the laser tomography technique which provides an accurate picture of the reactor actives zones, related to numerous cavitation events. For each frequency, two parameters were studied: the electrical power supplied to the transducer and the electrode transducer distance. At both frequencies, without electrode, we can observe distinct zones corresponding to cavitation production and stationary waves establishment. When increasing the input power, bubble clouds tend to form a unique cloud near the transducer. In presence of the electrode, bubble cavitation clouds are largely influenced by the obstacle. The second part of the paper (part II) will describe the Particle Image Velocimetry (P.I.V.) technique which allows to measure the velocity vector field in the fluid portion between the horn and the electrode.