Chiral separation of beta-adrenergic antagonists, profen non-steroidal anti-inflammatory drugs and chlorophenoxypropionic acid herbicides using teicoplanin as the chiral selector in capillary liquid chromatography

Three groups of structurally diverse chiral compounds were used to study the interaction mechanism responsible for stereoselective recognition with teicoplanin as chiral selector in capillary liquid chromatography. Teicoplanin-based chiral stationary phase (CSP) was used. The effect of the variation of mobile phase composition on retention and enantioselective separation was studied. The mobile phase composition suitable for enantioresolution of the various chiral compounds differed according to the interaction forces needed for chiral recognition. Mobile phases with high buffer portion (70-90 vol.%) were preferred for separation of enantiomers of profen non-steroidal anti-inflammatory drugs and chlorophenoxypropionic acid herbicides that require hydrophobic interactions, inclusion and pi-pi interactions for stereoselective recognition with teicoplanin. Higher concentration triethylamine in the buffer (0.5-1.0%) increased resolution of these acids. On the other hand, H-bonding and electrostatic interactions are important in stereoselective interaction mechanism of beta-adrenergic antagonists with teicoplanin. These interaction types predominate in the reversed phase separation mode with high organic modifier content (95% methanol) and in polar organic mobile phases. For this reason beta-adrenergic antagonists were best enantioresolved in the polar organic mode. The mobile phase composed of methanol/acetic acid/triethylamine, 100/0.01/0.01 (v/v/v), provided enantioresolution values of all the studied beta-adrenergic antagonists in the range 1.1-1.9. Addition of teicoplanin to the mobile phase, which was suitable for enantioseparation of certain compounds on the CSP, was also investigated. This system was used to dispose of nonstereoselective interactions of analytes with silica gel support that often participate in the interaction with CSPs. Very low concentration of teicoplanin in the mobile phase (0.1 mM) resulted in enantioselective separation of 2,2- and 2,4-chlorophenoxypropionic acids.     

Novel thermotropic esters‐based polymers with broad nematic processing windows

In this study, a new series of semiflexible liquid crystalline (LC) polyesters and poly(ester‐amide)s were synthesized and characterized. Polymers based on 4‐hydroxybenzoic acid (4‐HBA), 6‐hydroxy‐2‐naphthoic acid (HNA), suberic acid (SUA), and sebacic acid (SEA) were modified with hydroquinone (HQ) and different concentrations of 4‐acetamidophenol (AP) to obtain a polyester and two poly(ester‐amide)s, respectively. All polymers were successfully prepared using conventional melt‐condensation techniques. The polymers were characterized by inherent viscosity measurements, SEC, hot‐stage polarizing microscopy, DSC, and TGA. The mechanical behavior was investigated using DMTA and tensile testing. All linear polymers have T_gs in the range of 50–80 °C and melt between 120 and 150 °C. Our polymers display good thermooxidative stabilities (5% wt loss at ～ 400 °C) and exhibit homogeneous nematic melt behavior over a wide temperature range (ΔN ～ 250 °C). The liquid crystal phase was lost when high concentrations of nonlinear monomers such as 3‐HBA (>27 mol %) and resorcinol (RC) (>23 mol %) were incorporated. The LC polyester based on 4‐HBA/HNA/HQ/SUA/SEA could easily be processed into good quality films and fibers. The films display good mechanical properties (E′ ～ 4 GPa) and high toughness, that is, ～ 15% elongation at break, at room temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6565–6574, 2008     

System peaks observed in capillary liquid chromatography with eluents containing triethylamine

Summary Triethylamine is often added to mobile phases in reversed-phase liquid chromatography for dynamic deactivation of free silanol groups of the stationary phase. It has been observed that eluents composed of methanol and triethylamine generate two system peaks in chromatograms obtained with LiChrosorb RP-select B stationary phase, whose retention times correspond to the dead time and to the retention time of triethylamine. It has been demonstrated that the system peaks can be positive or negative depending on the experimental conditions and may be incorrectly interpreted as peaks corresponding to sample components. An approach is outlined to unambiguous identify these system peaks in chromatograms of practical samples.     

Application of allenylsilver(I) compounds in organic synthesis. A simple route to substituted allenes

Allenylsilver(I) compounds, prepared in situ by addition of alkylsilver(I)-lithium bromide complexes to butenynes, readily react with a variety of electrophiles. The produced compounds are usually almost pure allenes, but in some cases substantial amounts of the isomeric acetylenes are also formed.     

Electrochemical detectors in flowing liquid systems: an assessment of their use

Electrochemical detection in flowing liquid systems such as liquid chromatography and autoanalyzers has attracted considerable interest in recent years. Electrochemical detectors are particularly suited to such systems because of their selectivity and sensitivity and their linear response over a wide concentration range.     

Über die Verbindbarkeit von verdickbaren (n—1)-Zellen in topologischenn-Mannigfaltigkeiten

Ohne Zusammenfassung     

The development of a fully computerized system for sampled d.c. polarography with standard interfacing

A complete system, based on the online PDP-11 computer (Digital Equipment Corporation) was developed for computerized sampled d.c. polarography with direct digital control. The system includes compensation of ohmic cell resistance and processing of the polarographic data. The accuracy of the system in the determination of the various polarographic parameters is: diffusion current ± 2 %, half-wave potential ± 2 mV, and slope of the log plot ± 2 mV.