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Retention Mechanism (retention + mechanism)
Selected AbstractsMonomeric and polymeric anionic gemini surfactants and mixed surfactant systems in micellar electrokinetic chromatography.ELECTROPHORESIS, Issue 2 2005Part II: Characterization of chemical selectivity using two linear solvation energy relationship models Abstract Sodium di(undecenyl) tartarate monomer (SDUT), a vesicle-forming amphiphilic compound possessing two hydrophilic carboxylate headgroups and two hydrophobic undecenyl chains, was prepared and polymerized to form a polymeric vesicle (i.e., poly-SDUT). The anionic surfactants of SDUT and poly-SDUT (carboxylate head group) and sodium dodecyl sulfate, SDS (sulfate head groups) as well as mixed surfactant systems (SDS/SDUT, SDS/poly-SDUT, and SDUT/poly-SDUT) were applied as pseudostationary phases in micellar electrokinetic chromatography (MEKC). Two linear solvation energy relationship (LSER) models, i.e., solvatochromic and solvation parameter models, were successfully applied to investigate the effect of the type and composition of pseudostationary phases on the retention mechanism and selectivity in MEKC. The solvatochromic and solvation parameter models were used to help understand the fundamental nature of the solute-pseudostationary phase interactions and to characterize the properties of the pseudostationary phases (e.g., solute size and hydrogen bond-accepting ability for all pseudostationary phases). The solute types were found to have a significant effect on the LSER system coefficients and on the predicted retention factors. Although both LSER models provide the same information, the solvation parameter model is found to provide much better results both statistically and chemically than the solvatochromic model. [source] Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanismsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2004Yi Ren Abstract Identity ion-pair SN2 reactions LiX + CH3X , XCH3 + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [,H(inv)] are found to be much higher than the corresponding values for the gas phase anionic SN2 reactions, decreasing in the following order: F (263.6 kJ mol,1) > Cl (203.3 kJ mol,1) > Br (174.7 kJ mol,1) > I (150.7 kJ mol,1). The barrier gaps between the two mechanisms [,H (ret) , ,H (inv)] increase in the order F (,62.7 kJ mol,1) < Cl (4.4 kJ mol,1) < Br (24.9 kJ mol,1) < I (45.1 kJ mol,1). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic SN2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole,dipole complexes CH3X · · · LiX (,Hcomp) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol,1 for I up to 58.9 kJ mol,1 for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair SN2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair SN2 reactions in the solution, but the potential profile for reaction LiI + CH3I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [,H (inv)] with the geometric looseness of the inversion transition state %C,X,, the dissociation energies of the C,X bond (DC,X) and Li,X bond (DLi,X) are observed, respectively. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 461,467, 2004 [source] Preparation and characterization of polymethacrylate monolithic capillary columns with dual hydrophilic interaction reversed-phase retention mechanism for polar compoundsJOURNAL OF SEPARATION SCIENCE, JSS, Issue 15-16 2009í Urban Abstract Monolithic columns for capillary hydrophilic interaction liquid chromatography (HILIC) were prepared in fused-silica capillaries by radical co-polymerization of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide and ethylene dimethacrylate in various binary and ternary porogen solvent mixtures with azobisisobutyronitrile as the initiator of the polymerization reaction. Columns showed mixed separation modes: reversed-phase (RP) in water-rich mobile phases and HILIC at high concentrations of acetonitrile (>60,80%) in aqueous,organic mobile phases. A continuous change in retention was observed at increasing concentration of water in acetonitrile, giving rise to characteristic U-turn plots of retention factors versus the concentration of water in the mobile phase, with minima corresponding to the transition between the mechanisms controlling the retention. The selectivity of organic polymer monolithic columns for HILIC separations can be varied by adjusting the concentration of sulfobetaine monomer and the composition of the porogen solvent in the polymerization mixture. Under HILIC conditions, the monolithic capillary sulfobetaine columns show separation selectivities for polar phenolic acids similar to those of a commercial silica-based sulfobetaine ZIC-HILIC column, which, however, has limited selectivity in the RP mode due to lower retention. [source] The lipophilicity indices of flavonoids estimated by reversed-phase liquid chromatography using different computation methodsJOURNAL OF SEPARATION SCIENCE, JSS, Issue 12 2009Rodica Domnica Briciu Abstract The chromatographic behavior of some flavonoids was established by RP-HPLC on RP-18 (LiChroCART, LiChrosphere RP-18e), RP-8 (Zorbax, Eclipse XDB-C8), CN (Säulentechnik, LiChrosphere CN100) columns. The mobile phases were mixtures of methanol-water in different volume proportions from 70 to 80% v/v for RP-18 and RP-8, while for the CN column the proportions were between 66 and 70% v/v. The lipophilicity was expressed through different lipophilicity descriptors such as mean of k (mk), mean of log k (mlog k), log kW, S, f0, and scores of k and log k corresponding to the first principal component. The experimental lipophilicity indices are directly correlated with the computed values, via computer software and internet module, at a high analytical level. Furthermore, the results obtained applying principal component analysis to k and/or log k values allow the prediction and explanation of the interaction involved in the retention mechanism which takes place between the compounds and the employed stationary phases during the development. [source] Effect of stationary phase polarity on the retention of ionic liquid cations in reversed phase liquid chromatographyJOURNAL OF SEPARATION SCIENCE, JSS, Issue 17 2006Sylwia Kowalska Abstract Chromatographic analysis of ionic liquids on different types of packings offers interesting possibility to determine their retention mechanism. As a consequence, the major interactions between stationary phase ligands and analyzed chemical entities can be defined. The main aim of this work was to analyze cations of ionic liquids on chemically bonded stationary phases with specific structural properties. The attempt to predict the main interactions between positive ions of ionic liquids and stationary phase ligands was undertaken. For that purpose, butyl, octyl, octadecyl, phenyl, aryl, mixed, alkylamide, and cholesterolic packings were chosen and applied to the analysis of six most commonly used ionic liquids' cations. Obtained results indicate mainly dispersive and ,,, type of interaction part in the retention mechanism of analyzed compounds. [source] Optimization of the separation conditions of tetracyclines on a preselected reversed-phase column with embedded urea groupJOURNAL OF SEPARATION SCIENCE, JSS, Issue 7 2006Leila Kallel Abstract The use of a C12 stationary phase with embedded polar group has been investigated for the separation of seven tetracyclines. The influence of pH, organic modifier, buffer, and temperature on the peak shape and analyte separation was discussed. It appears that all the chromatographic conditions had a great effect on both the resolution and peak shape whereas the elution order was not affected. The baseline separation with symmetrical peaks of the seven tetracyclines can be obtained with a mobile phase containing either 5 mM phosphate buffer pH 2.5/ACN (84 : 16 v/v) or 5 mM perchlorate buffer pH 2.5/ACN (75 : 25 v/v) at a temperature not exceeding 20°C. This study reveals that the retention mechanism is ion-pairing. [source] A theoretical explanation for the retention mechanism of ion exclusion chromatographyJOURNAL OF SEPARATION SCIENCE, JSS, Issue 17 2003Bronis, aw K. G Abstract Ion Exclusion Chromatography is classically used for the separation of weak acid anions. Dilute strong acids (e.g. sulphuric or perchloric acid) or just water are used as eluents. To increase the exclusion effect, strong cation exchangers, characterized by high concentration of functional groups, are applied. The inner column volume of commercially available columns is increased by increasing their size in comparison to traditional ones (usually 300×7.8 mm ID). The description of the retention mechanism of this technique implicitly assumes that both mobile and stationary phases are typical aqueous solutions, and their dielectric constants are thus equal. This equality implies the equality of solute dissociation constants in both phases. Another implicit assumption is that the dead- and inner volumes of the column are constant, and independent of the mobile phase composition. The present paper shows that stationary and mobile phases are generally characterized by different physicochemical parameters. Thus, they cannot be considered as regular aqueous solutions. Additionally, we show that weak cation exchanger resins, which are characterized by a relatively small concentration of the functional groups, and weak acid based buffers can also be used in IEC. This would expand the possible applications of this method and enable, for example, the separation of strong acids (anions). The influence of ionic strength on the retention and dead- and inner column volumes is also discussed. Finally we also briefly describe the retention mechanism of Electrostatic Ion Chromatography. [source] Chromatographic classification and comparison of commercially available perfluorinated stationary phases for reversed-phase liquid chromatography using Principal Component AnalysisJOURNAL OF SEPARATION SCIENCE, JSS, Issue 3-4 2003Melvin R. Euerby Abstract A range of ten perfluorophenyl and perfluoroalkyl stationary phases has been evaluated using standard chromatographic tests and probes. Principal Component Analysis of the data has indicated that the phases can be divided into distinct groupings. Extending the dataset to include standard alkyl and phenyl phases provided further data interpretation to support the orthogonal selectivity claims made for perfluorinated phases. The analysis of a range of basic analytes showed an unusual extended retention of hydrophilic basic analytes with perfluorophases. Furthermore, a non-linear relationship between the amount of organic modifier and the logarithm of the retention factor was observed, for the hydrophilic bases, which could not be modelled with LC prediction softwares. This was in sharp contrast to the alkyl and phenyl phases examined. Basic analyte retention on perfluoroalkyl phases could be modelled adequately for the lipophilic bases. Exploration of the retention mechanism of these perfluoro phases indicated that silanol interactions were important in retention and selectivity. Using a rapid, isocratic, high organic modifier methodology, it was possible to analyse a mixture containing a lipophilic steroid, hydrophilic base and an internal standard in < 4 minutes with a perfluorophenyl phase. This had previously only been achievable with an alkyl phase under gradient elution conditions. [source] Preparation of Medium Cation Exchange Stationary Phase of Polymeric Matrix and Their Chromatographic PropertiesCHINESE JOURNAL OF CHEMISTRY, Issue 1 2007Gang Chen Abstract Based on the monodisperse poly(glycidyl methacrylate-co-ethylenedimethacrylate) beads (PGMA/EDMA) with macropore as a medium, a new hydrophilic medium cation exchange (MCX) stationary phase for HPLC was synthesized by a new chemically modified method. The stationary phase was evaluated with the property of ion exchange, separability, reproducibility, hydrophilicity, effect of salt concentration, salt types, column loading and pH on the separation and retention of proteins in detail. It was found that it follows ion exchange chromatographic (IEC) retention mechanism. The measured bioactivity recovery for lysozyme was (96±5)%. The dynamic protein loading capacity of the synthesized MCX packings was 21.8 mg/g. Five proteins were almost completely separated within 6.0 min at a flow rate of 4 mL/min using the synthesized MCX resin. The MCX resin was also used for the rapid separation and purification of lysozyme from egg white with only one step. The purity and specific bioactivity of the purified lysozyme was found more than 95% and 70345 U/mg, respectively. [source] Polar stationary phases for capillary electrochromatographyELECTROPHORESIS, Issue 23-24 2004Chuanhui Xie Abstract This review article summarizes the variety of polar stationary phases that have been employed for capillary electrochromatographic separations. Compared with reversed-phase stationary phases, the polar alternatives provide a completely different retention selectivity towards polar and charged analytes. Different types of polar stationary phases are reviewed, including the possible retention mechanisms. Electrochromatographic separations of polar solutes, peptides, and basic pharmaceuticals on polar stationary phases are presented. [source] Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanismsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2004Yi Ren Abstract Identity ion-pair SN2 reactions LiX + CH3X , XCH3 + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [,H(inv)] are found to be much higher than the corresponding values for the gas phase anionic SN2 reactions, decreasing in the following order: F (263.6 kJ mol,1) > Cl (203.3 kJ mol,1) > Br (174.7 kJ mol,1) > I (150.7 kJ mol,1). The barrier gaps between the two mechanisms [,H (ret) , ,H (inv)] increase in the order F (,62.7 kJ mol,1) < Cl (4.4 kJ mol,1) < Br (24.9 kJ mol,1) < I (45.1 kJ mol,1). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic SN2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole,dipole complexes CH3X · · · LiX (,Hcomp) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol,1 for I up to 58.9 kJ mol,1 for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair SN2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair SN2 reactions in the solution, but the potential profile for reaction LiI + CH3I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [,H (inv)] with the geometric looseness of the inversion transition state %C,X,, the dissociation energies of the C,X bond (DC,X) and Li,X bond (DLi,X) are observed, respectively. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 461,467, 2004 [source] Influence of Attachment Systems on Load Transfer of an Implant-Assisted Maxillary OverdentureJOURNAL OF PROSTHODONTICS, Issue 4 2004Mete I. Fanuscu DDS Purpose: This photoelastic study compared the load transfer characteristics of 2 retention mechanisms in an implant-assisted overdenture prosthesis. Materials and Methods: Four implants were incorporated into a photoelastic model of a moderately resorbed edentulous human maxilla. Two retention mechanisms were studied by changing components on the same model and the palateless overdenture. The retention mechanisms studied were bar splint with anterior clip and distal resilient attachments, and solitary ball/O-ring attachments. Loads, ranging from 1.4 to 14.4 kg, were applied to the palatal incline of central incisors and buccal incline of premolars with and without balancing contacts. Stresses developed around all the implants under each loading condition were photographed in the field of a circular polariscope. Results: With both retention mechanisms, protrusive and laterotrusive loads without balancing contacts caused instability of the overdenture, producing minimal stress around the implants in the supporting structure. High intensity stresses indicating intrusion of the posterior implants were noted when the bar/distal resilient attachment overdenture had balancing contacts for protrusive and laterotrusive loads. The posterior implants of ball/O-ring attachment overdenture exhibited high intensity stresses indicating not only intrusion, but also bending, when the occlusion was balanced. Conclusions: Balanced occlusion was required in both retention mechanisms for stability of the implant-assisted overdenture when clinically acceptable loads were applied. The protrusive and laterotrusive loads were not distributed equitably in either mechanism, since highest stresses occurred at the posterior implants. [source] Effect of coverage density and structure of chemically bonded silica stationary phases on the separation of compounds with various propertiesJOURNAL OF SEPARATION SCIENCE, JSS, Issue 6 2006Bogus, aw Buszewski Abstract The chemical character, geometry, and architecture of chemically formed surface layers determine interactions between stationary phase, analyte, and mobile phase, and therefore the retention mechanisms (partitioning, adsorption, ion exchange, steric exclusion) of separated analytes. These interactions also depend on the structure and chemical character of the solutes and the composition of the mobile phase. High-molecular-weight fullerenes (C60 and C70) and water-soluble selenium-containing peptides (833 and 2607 Da) were used for the evaluation of laboratory-prepared octadecyl stationary phases with high and low coverage density before and after end-capping. The aim of this work was to study differences in surface coverage density and homogeneity and conformational changes of chemically bonded moieties and the influence of these parameters on the separation of mixtures of selenopeptides and fullerenes with significantly different molecular masses. A topographical model of the chemically modified stationary surface is presented. [source] Assessment of Fed-Batch, Semicontinuous, and Continuous Epothilone D Production ProcessesBIOTECHNOLOGY PROGRESS, Issue 4 2005Scott A. Frykman Epothilone D is a member of a class of potent antineoplastic natural products produced by myxobacteria. Previously, we have described a fed-batch epothilone D production process in which an adsorber resin is incorporated into the bioreactor setup to capture and stabilize the product in situ, preventing its degradation within the bioreactor. The capture of epothilone D by these relatively large resin beads enables the development of continuous and semicontinuous culturing systems incorporating bead retention mechanisms to completely retain the product within the bioreactor, increasing the epothilone D product titer by almost 3-fold in both cases over a baseline fed-batch system. These product retention strategies, described here for production of the epothilones, are generally applicable to any system using adsorber resins as a method to capture product during a microbial cultivation. [source] | |||||||||||||