Effects of stationary-phase polarity on retention in reversed bonded phase HPLC columns

Summary Variations in retention and selectivity have been studied in cyano, phenyl and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns was correlated with polar group selectivities of mobile phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers.     

盘式触发电极触发管开关的触发特性

 对采用盘状触发电极的新型触发管开关的触发特性进行了实验研究,得到不同极性组合、不同触发电压幅值和不同触发电流条件下开关的触发特性实验结果。实验结果表明:新型触发管开关获得最佳触发特性所需的极性组合应为异极性组合,触发电压幅值范围为自击穿电压的30%~50%,为兼顾触发电流和保护触发器,触发电阻值应为50~100 Ω,验证了触发管开关击穿机制的已有认识。     

Broad-spectrum separations in metabolomics using enhanced polar LC stationary phases: a dedicated evaluation using plant metabolites

In metabolomics, major efforts are invested in the development of suitable analytical approaches. A tendency towards the use of LC-MS is nowadays very obvious. A great majority of the metabolites of interest are polar to highly polar in nature. We focus on the so-called 'extended polarity' reversed LC phases, developed specifically to allow better retention characteristics for polar compounds. Several of these phases (Atlantis dC18, Inertsil ODS-3, Zorbax XDB, Alltima HP C18) are tested for different column dimension variations (0.5, 1.0, 2.1 mm id) in a specific LC-MS metabolomics setting. Important chromatographic and mass spectrometric quality parameters such as capacity factor, separation efficiency, peak symmetry, sensitivity, and mass accuracy are taken into account. All phases show adequate retention of polar compounds and also perform well with highly aqueous mobile phase compositions. On comparing 1.0 and 2.1 mm id columns, it is clear that the potential gain in sensitivity is not achieved. Using a Lockspray device, accurate mass measurement with a Q-TOF micro is feasible within a mass range of 12 ppm if signal intensities of compound and lockmass are equated. Finally, the extended polarity RP approach in metabolomics experiments is endorsed using real plant extracts.     

Radical‐Transfer Hydroamination of Olefins with N‐Aminated Dihydropyridines

An efficient synthesis of N‐phthalimidyl, benzamidyl, acetamidyl, carbamoyl, and ureayl derivatives of dihydropyridines and the application of these reagents as precursors for N‐centered radicals are presented. These aminated dihydropyridines could be used in radical‐transfer hydroamination reactions of various electron‐rich as well as nonactivated olefins in the presence of thiols as polarity‐reversal catalysts. These reactions worked without the aid of any transition metal. Steric and electronic effects exerted by the N‐substitutents of the N‐centered radicals are discussed. In contrast to most metal‐catalyzed processes, the radical hydroamination delivered the opposite regioisomer with excellent anti‐Markovnikov selectivity. Hydroamination products were obtained as protected amines that are readily isolated.     

Oxidized multi-walled carbon nanotubes for the dispersive solid-phase extraction of quinolone antibiotics from water samples using capillary electrophoresis and large volume sample stacking with polarity switching

In this work, a new method for the determination of eleven quinolone antibiotics (moxifloxacin, lomefloxacin, danofloxacin, ciprofloxacin, levofloxacin, marbofloxacin, enrofloxacin, difloxacin, pefloxacin, oxolinic acid and flumequine) in different water samples using dispersive solid-phase extraction (dSPE) and capillary zone electrophoresis with diode-array detection was developed. Oxidized multi-walled carbon nanotubes (o-MWCNTs) were used for the first time as stationary phases for the off-line preconcentration by dSPE of the antibiotics. A 65 mM phosphate buffer at pH 8.5 was found adequate for analyte separation while large volume sample stacking with polarity switching of the analytes dissolved in water containing 10% (v/v) of acetonitrile was carried out in order to improve the sensitivity. dSPE parameters, such as sample volume and pH, o-MWCNT amount, volume and type of eluent in dSPE were optimized. Application of the developed method to the analysis of spiked Milli-Q, mineral, tap, and wastewater samples resulted in good recoveries values ranging from 62.3 to 116% with relative standard deviation values lower than 7.7% in all cases. Limits of detection were in the range of 28-94 ng/L. The proposed method is very fast, simple, repeatable, accurate and highly selective.     

Polymer-supported ionic-liquid-like phases (SILLPs): transferring ionic liquid properties to polymeric matrices

The physico-chemical properties of polymers with ionic-liquid-like moieties covalently bound to their surfaces (SILLPs) have been studied by thermal and spectroscopic techniques, as well as by direct impedance and dielectric measurements, and compared to those of the corresponding bulk ionic liquids. The effective transfer of properties from ionic liquids in solution to the supported species has thereby been demonstrated. The effects of the chemical nature of these tunable "solid solvents" on their macroscopic swelling and microwave heating, as well as the stabilities and activities of different catalytic moieties immobilized on the SILLPs, have been studied. Finally, the experimental effect observed in microwave heating can be directly correlated with the values of tan δ derived from dielectric measurements.     

Determination of solvatochromic solvent polarity parameters for the characterisation of some nematic liquid crystal in anisotropic and isotropic phases

Characterisation of liquid crystals (LCs) as solvents is needed, to obtain the polarity and solvatochromic polarity parameters of these media. Polarity parameters demonstrate the effects of LC media on the photo-physical behaviour of solute molecules in an anisotropic medium. The practical limitations in determining solvent polarity scale parameters for LCs can overcome the overlapping absorption band of LCs and solvent-sensitive standard compounds or their insolubility in LCs. In this work, we report Kamlet–Abboud–Taft polarity functions of some nematic LCs in different temperatures and phases, isotropic and anisotropic, with the solvatochromic method, using the Reichardt's dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-phenolate standard probe. In addition, a new azo and coumarin dye were used as probes to obtain some solvatochromic polarity parameters. Finally, a new polarity parameter, the LC anisotropic matrix, is introduced.     

An efficient strategy for large-scale preparation of low polarity gingerols directly from ginger crude extract by high-speed countercurrent chromatography with different rotation mode

This study presents an efficient strategy for large-scale preparation of low polarity gingerols directly from ginger crude extract by high-speed countercurrent chromatography with different rotation mode. The ultrasonic-assisted extraction conditions were optimized by response surface methodology and the results showed the major low polarity gingerols could be well enriched under the optimized extraction conditions. Then the crude extract without any pretreatment was directly separated by high-speed countercurrent chromatography with different rotation mode using n-hexane/ethyl acetate/methanol/water (6:4:6:4, v/v/v/v) as the solvent system. In about 400 min, five major gingerols including 150 mg of [6]-gingerol, 50 mg of [8]-gingerol, 20 mg of [6]-shogaol, 43 mg of [6]-dehydrogingerdione, and 40 mg of [10]-gingerol were obtained from 1.2 g of crude extract in a single run with repeated injection. Their structures were identified by ¹H-NMR spectroscopy.     

Rheological signatures of ethylene methyl acrylate‐multiwalled carbon nanotube nanocomposites

The rheological behavior of nanocomposites based on multiwalled carbon nanotube (MWNT) with three commercial grades of ethylene methyl acrylate (EMA) copolymers containing 9, 24, and 30 wt% methyl acrylate (MA) was investigated under dynamic and steady shear flow (in a capillary) conditions. Storage modulus (in dynamic shear) value increases especially at higher frequency levels due to increased polymer‐filler interactions. Both the unfilled and filled composites exhibit rheological behavior of non‐Newtonian fluids. In both steady shear and capillary flow, the nanocomposites register a slightly higher viscosity than neat EMAs, with dependence on the MWNTs content. All systems with various loading of MWNTs represent an increase in elastic response with increasing frequency. The die swell decreases with the MWNTs loading. Dynamic and steady shear rheological properties register a good correlation in regard to the viscous versus elastic response of such systems inline with the Cox–Merz concept. Increased MA content leads to inferior dispersion of MWNTs in EMA matrix. Morphological studies exhibit that MWNTs become more aligned along longitudinal direction after extrusion leading to improved dispersion. Copyright © 2010 John Wiley & Sons, Ltd.