Chiral separation of enantiomers of amino acid derivatives by high-performance liquid chromatography on a norvancomycin-bonded chiral stationary phase

A novel norvancomycin-bonded chiral stationary phase (NVC-CSP) was synthesized by using the chiral selector of norvancomycin. The chiral separation of enantiomers of several dansyl-amino acids by high-performance liquid chromatography (HPLC) in the reversed-phase mode is described. The effects of some parameters, such as organic modifier concentration, column temperature, pH and flow rate of the mobile phase, on the retention and enantioselectivity were investigated. The study showed that ionic, as well as hydrophobic interactions were engaged between the analyte and macrocycle in this chromatographic system. Increasing pH of buffers usually improved the chiral resolution for dansyl-alpha-amino-n-butyric acid (Dns-But), dansyl-methionine (Dns-Met) and dansyl-threonine (Dns-Thr), but not for dansyl-glutamic acid (Dns-Glu) which contains two carboxylic groups in its molecular structure. The natural logarithms of selectivity factors (In alpha) of all the investigated compounds depended linearly on the reciprocal of temperature (1/T), most processes of enantioseparation were controlled enthalpically. Interestingly, the process of enantioseparation for dansyl-threonine was enthalpy-controlled at pH of 3.5, while at pH of 7.0, it was entropy-controlled according to thermodynamic parameters Delta(R,S)DeltaHdegrees and Delta(R,S)DeltaSdegrees afforded by Van't Hoff plots. In order to get baseline separation for all the solutes researched, norvancomycin was also used as a chiral mobile phase additive. In combination with the NVC-CSP remarkable increases in enanselectivity were observed for all the compounds, as the result of a "synergistic" effect. (C) 2003 Elsevier B.V. All rights reserved.

An empirical equation for the calculation of retention time with extrapolation of temperature in gas chromatography

An empirical equation is proposed to accurately correlate isothermal data over a wide range of temperature With the equation ln k = A* + B*/T-lambda the retention times of different solutes tested on OV-101, SE-54 and PEG 20M capillary columns have been achieved even when lambda is assigned a constant value of 1.7 Comparison with ln k = A + B/T and in k = c + d/T+ h/T-2, shows that the proposed equation is of higher accuracy and is applicable to extrapolation calculation, especially from data at high temperature to those at low temperature. Parameters A* and B* as well as A and B are also discussed. The linear correlation of A* and B* is weaker than that of A and B.

Quantitative determination of compounds in tobacco essential oils by comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry

A quantitative analysis of the individual compounds in tobacco essential oils is performed by comprehensive two-dimensional gas chromatography (GC x GC) combined with flame ionization detector (FID). A time-of-flight mass spectrometer (TOF/MS) was coupled to GC x GC for the identification of the resolved peaks. The response of a flame ionization detector to different compound classes was calibrated using multiple internal standards. In total, 172 compounds were identified with good match and 61 compounds with high probability value were reliably quantified. For comparative purposes, the essential oil sample was also quantified by one-dimensional gas chromatography-mass spectrometry (GC/MS) with multiple internal standards method. The results showed that there was close agreement between the two analysis methods when the peak purity and match quality in one-dimensional GC/MS are high enough. (c) 2005 Elsevier B.V. All rights reserved.

High-Resolution Finite Compact Difference Schemes for Hyperbolic Conservation Laws

A finite compact (FC) difference scheme requiring only bi-diagonal matrix inversion is proposed by using the known high-resolution flux. Introducing TVD or ENO limiters in the numerical flux, several high-resolution FC-schemes of hyperbolic conservation law are developed, including the FC-TVD, third-order FC-ENO and fifth-order FC-ENO schemes. Boundary conditions formulated need only one unknown variable for third-order FC-ENO scheme and two unknown variables for fifth-order FC-ENO scheme. Numerical test results of the proposed FC-scheme were compared with traditional TVD, ENO and WENO schemes to demonstrate its high-order accuracy and high-resolution.

Conjugate Model for Heat and Mass Transfer of Porous Wall in the High Temperature Gas Flow

Heat and mass transfer of a porous permeable wall in a high temperature gas dynamical flow is considered. Numerical simulation is conducted on the ground of the conjugate mathematical model which includes filtration and heat transfer equations in a porous body and boundary layer equations on its surface. Such an approach enables one to take into account complex interaction between heat and mass transfer in the gasdynamical flow and in the structure subjected to this flow. The main attention is given to the impact of the intraporous heat transfer intensity on the transpiration cooling efficiency.

Experimental Study on Hydrodynamic Behaviors of High-Speed Gas Jets in Still Water

The present paper describes experimental investigation on the flow pattern and hydrodynamic effect of underwater gas jets from supersonic and sonic nozzles operated in correct- and imperfect expansion conditions. The flow visualizations show that jetting is the flow regime for the submerged gas injection at a high speed in the parameter range under consideration. The obtained results indicate that high-speed gas jets in still water induce large pressure pulsations upstream of the nozzle exit and the presence of shock-cell structure in the over- and under-expanded jets leads to an increase in the intensity of the jet-induced hydrodynamic pressure.

Fiber Bragg grating temperature sensor for multiplexed measurement with high resolution

A novel fiber Bragg grating temperature sensor is proposed and experimentally demonstrated with a long-period grating as a linear response edge filter to convert wavelength into intensity-encoded information for interrogation. The sensor is embedded into an aluminum substrate with a larger coefficient of thermal expansion to enhance its temperature sensitivity. A large dynamic range of 110 degreesC and a high resolution of 0.02 degreesC are obtained in the experiments. The technique can be used for multiplexed measurements with one broadband source and one long-period grating, and therefore is low Cost. (C) 2004 Society of PhotoOptical Instrumentation Engineers.

Structural role of fluoride in aluminophosphate sol-gel glasses: High-resolution double-resonance NMR studies

The local structure of Na-Al-P-O-F glasses, prepared by a novel sol-gel route, was extensively investigated by advanced solid-state NMR techniques. Al-21{F-19} rotational echo double resonance (REDOR) results indicate that the F incorporated into aluminophosphate glass is preferentially bonded to octahedral Al units and results in a significant increase in the concentration of six-coordinated aluminum. The extent of Al-F and Al-O-P connectivities are quantified consistently by analyzing Al-27{P-31} and Al-21{F-19} REDOR NMR data. Two distinct types of fluorine species were identified and characterized by various F-19{Al-27}, F-19{Na-23}, and F-19{P-31} double resonance experiments, which were able to support peak assignments to bridging (Al-F-Al, -140 ppm) and terminal (Al-F, -170 ppm) units. On the basis of the detailed quantitative dipole-dipole coupling information obtained, a comprehensive structural model for these glasses is presented, detailing the structural speciation as a function of composition.