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At least 631 records · Page 35Linked to original sources
PubMed1989

Effects of mobile phase composition on the chromatographic and electrochemical behaviour of catecholamines and selected metabolites. Reversed-phase ion-paired high-performance liquid chromatography using multiple-electrode detection.

The effects of pH, ionic strength, organic modifier, heptanesulphonic acid and citric acid content of a high-performance liquid chromatography mobile phase on the chromatographic and electrochemical behaviour of norepinephrine, epinephrine, dopamine, 3,4-dihydroxybenzylamine, 3,4-dihydroxyphenylethylene glycol, 3,4-dihydroxyphenylalanine and 3,4-dihydroxyphenylacetic acid in a reversed-phase system have been systematically studied. Optimal mobile phase conditions have been derived allowing the separation and reductive-mode detection of these compounds, applicable to both alumina and ion-paired solvent extracts of plasma. It is demonstrated that mobile phase composition significantly affects the sensitivity of a triple-electrode electrochemical detection system, in reductive and oxidative modes, and that electrochemical pre-treatment of mobile phase is required to attain maximum detector sensitivity in the reductive mode.

Catecholamines↗
PubMed1998

Driving forces for phase separation and partitioning in aqueous two-phase systems.

A set of simple analytical equations, derived from the Flory-Huggins theory, are used to identify the dominant driving forces for phase separation and solute (e.g., protein) partitioning, in the absence and presence of added electrolyte, in every general class of aqueous two-phase systems. The resulting model appears to capture the basic nature of two-phase systems and all trends observed experimentally. Case studies are used to identify fundamental differences in and the magnitudes of enthalpic and entropic contributions to partitioning in polymer-polymer (e.g., PEG-dextran), polymer-salt, and thermoseparating polymer-water (e.g., UCON-water) two-phase systems. The model therefore provides practitioners with a better understanding of partition systems, and industry with a simple, fundamental tool for selecting an appropriate two-phase system for a particular separation.

Chemistry Techniques, Analytical↗
PubMed1998

Reversed-phase high-performance liquid chromatographic identification of lutein and zeaxanthin stereoisomers in bovine retina using a C30 bonded phase.

An efficient reversed-phase high-performance liquid chromatographic assay with ultraviolet detection at 450 nm for simultaneous determination of lutein and zeaxanthin stereoisomers in bovine retina is described. The procedure involves rapid and careful one-step hexane extraction of the carotenoids from the homogenized liquid and enrichment by on-line solid phase extraction on a polystyrene cartridge. The substances were eluted at a flow rate of 1 ml/min with acetone-water (85:15, v/v) on a C30 reversed-phase column. C30 phases exhibit superior shape selectivity for the separation of carotenoid stereoisomers compared with conventional C18 phases. For comparison a mixture of iodine-isomerized lutein and zeaxanthin standards was separated.

Animals↗
PubMed1993

Preparation and characterization of covalently bound tetraphenylporphyrin-silica gel stationary phases for reversed-phase and anion-exchange chromatography.

A tetraphenylporphyrin-based stationary phase for high-performance liquid chromatography is prepared by attaching a p-carboxyphenyl derivative of the porphyrin to aminopropyl silica gel through an amide bond. This stationary-phase support is used for the reversed-phase separation of several polycyclic aromatic hydrocarbons and is shown to exhibit novel shape selectivity toward planar aromatic solutes. Subsequent metalation of the immobilized tetraphenylporphyrin with either tin(IV) or indium(III) results in a stationary phase with anion-exchange characteristics. Both the tin(IV) and indium(III) metalloporphyrin-based stationary phases exhibit useful selectivity for the separation of aromatic sulfonates and aromatic carboxylates.

Chromatography, Ion Exchange↗
PubMed2003

Polar polymeric stationary phases for normal-phase HPLC based on monodisperse macroporous poly(2,3-dihydroxypropyl methacrylate-co-ethylene dimethacrylate) beads.

The effect of variables such as shape template size, porogen composition and percentage, content of cross-linking monomer, and polymerization temperature on the properties of uniformly sized 3-microm porous poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads prepared by the staged templated suspension polymerization technique has been studied. The porous properties of the beads including surface morphology, pore size distribution, and specific surface area have been optimized to obtain highly efficient stationary phases for normal-phase HPLC. A column packed with diol stationary phase obtained by hydrolysis of poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads affords an efficiency of 67,000 plates/m for toluene using THF as the mobile phase. The retention properties and selectivity of the diol beads are easily modulated by changes in the composition of the mobile phase. The performance of these beads is demonstrated with the separations of a variety of polar compounds including positional isomers, aniline derivatives, and basic tricyclic antidepressant drugs.

Aniline Compounds↗
PubMed2004

Retention of ionizable compounds in reversed-phase liquid chromatography. Effect of the ionic strength of the mobile phase and the nature of the salts used on the overloading behavior.

The retention mechanism of the protonated cation in propranolol chloride on C18-Xterra was investigated using mobile phases of various compositions. Accurate adsorption data were measured by frontal analysis, with a mixture of methanol and water (25% methanol), with no salt, as the mobile phase. The experimental isotherm has at least two inflection points, at concentrations of about 0.2 and 6.0 g/L, respectively. This precludes the modeling of these data with a simple convex-upward isotherm (e.g., Langmuir). The adsorption energy distribution or relationship between the number of sites on the adsorbent surface and the energy of adsorption on these sites was calculated by assuming Moreau isotherm behavior (S-shaped isotherm). This model has never been applied to describe the surface heterogeneity of any RPLC adsorbent. The calculation converged toward a bimodal energy distribution. Accordingly, the bi-Moreau model is the simplest theoretical model accounting for the adsorption data of propranolol from a mobile phase without salt. The complex-overloaded band profiles of propranolol measured in the presence of increasing concentrations of a supporting salt (KCl) in the mobile phase demonstrate that the same isotherm model applies also under these conditions, as was merely assumed in a previous work. The elution band profiles of propranolol calculated with the bi-Moreau isotherm model for solutions of salts of different natures (CaCl2, CsCl, Na2SO4) in the same mobile phase agree very well with the experimental band profiles.

Adsorption↗
PubMed2004

Comprehensive two-dimensional normal-phase (adsorption)-reversed-phase liquid chromatography.

A comprehensive two-dimensional HPLC system has been developed. It is based on the use of a microbore silica column operated in normal-phase (adsorption) mode (NP) in the first dimension and a monolithic type C18 column operated in reversed-phase (RP) mode in the second dimension. The interface was a 10-port, 2-position valve equipped with two storage loops. The first column was operated at a flow rate of 20 microL/min in isocratic mode, while the monolithic column flow rate was 4 mL/min and was operated in gradient mode. The sample loops had a volume of 20 microL each, and the analysis time in the second dimension was 1 min. In this way, every fraction from the first dimension was transferred on-line to the second dimension switching the automated valve every minute. A photodiode array detector has been used after the secondary column. The use of normal- and reversed-phase mode in the two dimensions can be helpful in the separation of complex mixtures of a natural origin that contain uncharged molecules of comparable dimension, different in polarity and hydrophobicity. The use of a microbore column in the first dimension permits the injection of a small volume in the secondary column, making the transfer of incompatible solvents from the first to the second dimension possible. Since the mobile phase in the NP separation is always stronger than the mobile phase at the head of the secondary column operated in RP mode, the initial eluent strength is important in order to obtain an effective focusing of the sample. The use of a monolithic type column in the second dimension permits the performance of very fast analysis operating at higher flow rates without loss of resolution, due to a higher permeability and increased mass-transfer properties in comparison to conventional particulate columns. Due to the brief reconditioning time necessary for monolithic columns, repetitive gradients can be carried out, extending the field of application to mixtures that contain components with different polarities. The utility of the system has been demonstrated in the analysis of the oxygen heterocyclic fraction of cold-pressed lemon oil, made up of coumarins and psoralens. These components may contain hydroxyl, methoxyl, isopentenyl, isopentenyloxyl, and geranyloxyl groups and oxygen-containing modification of the terpenoid side-chain groups, such as epoxides or vicinal diol groups. The relative location of the components in the 2D plane varied in relation to their chemical structure and allowed positive peak identification. The UV spectra recorded with the photodiode array detector supplied additional information that was used for the characterization of the studied sample.

Adsorption↗
PubMed2004

Colloid-imprinted carbons as stationary phases for reversed-phase liquid chromatography.

A novel colloid-imprinting method is employed for the preparation of carbonaceous stationary phases for reversed-phase liquid chromatography (RPLC). This colloid-imprinting method combined with oxidative stabilization treatment affords carbons with a porous shell/nonporous core structure. The particle morphology, pore size, pore shape, and Brunauer-Emmett-Teller surface area of these carbons can be finely tuned by selecting proper experimental conditions. Although their surface area and pore volume decrease noticeably after graphitization, their primary pore structure is maintained. In addition, the graphitization process eliminates the high-energy sites and substantially reduces structural heterogeneity, making colloid-imprinted carbons attractive stationary phases for reversed-phase liquid chromatography. The colloid-imprinted graphitic carbons with surface mesoporosity appeared to be attractive for chromatographic separations of alkylbenzenes under reversed-phase conditions.

Journal Article↗
PubMed1997

Monodisperse hydrolyzed poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads as a stationary phase for normal-phase HPLC.

The basic characteristics of a rugged, stable, and highly efficient polymeric stationary phase for normal-phase HPLC prepared by hydrolysis of porous monodisperse poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads have been studied and compared with those of bare Nucleosil silica and Nucleosil silica-diol. As a result of their overall chemical composition and the more than 10-fold larger surface coverage with hydroxyl groups, the polymer beads provide much higher retention of model solute molecules. In contrast to silica hydroxyls, all of the polymer surface diol groups are chemically equal and homogeneously distributed over all of the surface. In addition, they are less acidic than typical silanol functionalities. The exceptional selectivity of the polymeric column can be controlled by the composition of the mobile phase, as demonstrated with a broad spectrum of separations involving positional isomers of benzene derivatives, nonpolar isobutylene copolymers with minute composition differences, and hydrophilic poly(ethylene oxides) differing only in their chain length. Unlike silica phases, the properties of the polymeric stationary phrase are not affected by the presence of water in the mobile phase. As a result, repetitive gradient separations in eluents ranging in polarity from hexane through tetrahydrofuran to water can be easily accomplished.

Chromatography, High Pressure Liquid↗
PubMed2003

Solid-phase synthesis of alpha-Gal epitopes: on-resin analysis of solid-phase oligosaccharide synthesis with 19F NMR spectroscopy.

A route for solid-phase synthesis of the alpha-Gal epitopes Gal(alpha1-3)Gal(beta1-4)Glc and Gal(alpha1-3)Gal(beta1-4)GlcNAc is described. These trisaccharide antigens are responsible for hyperacute rejection in xenotransplantation of porcine organs. Optimization of the solid-phase synthesis relied on use of fluorinated protective groups for the carbohydrate building blocks and use of a fluorinated linker. This allowed convenient on-resin analysis of the reactions with gel-phase (19)F NMR spectroscopy. Conditions were established which allowed reductive ring-opening of 4,6-O-benzylidene acetals to be performed on the solid phase with high regioselectivity to furnish the corresponding 6-O-benzyl ethers. It was found that glycosylations could be conveniently carried out by using thioglycosides as donors with N-iodosuccinimide and trifluoromethanesulfonic acid as the promoter system. With use of these conditions a challenging alpha-glycosidic linkage was successfully installed with complete stereoselectivity in the final glycosylation. It was also established that fluorinated benzoates, benzyl ethers, and benzylidene acetals display almost identical chemical properties as their nonfluorinated counterparts, a finding that is essential for future use of fluorinated protective groups in solid-phase oligosaccharide synthesis.

Animals↗
PubMed2005

The microstructure of di-alkyl chain cationic/nonionic surfactant mixtures: observation of coexisting lamellar and micellar phases and depletion induced phase separation.

The evolution of the microstructure and composition occurring in the aqueous solutions of di-alkyl chain cationic/nonionic surfactant mixtures has been studied in detail using small angle neutron scattering, SANS. For all the systems studied we observe an evolution from a predominantly lamellar phase, for solutions rich in di-alkyl chain cationic surfactant, to mixed cationic/nonionic micelles, for solutions rich in the nonionic surfactant. At intermediate solution compositions there is a region of coexistence of lamellar and micellar phases, where the relative amounts change with solution composition. A number of different di-alkyl chain cationic surfactants, DHDAB, 2HT, DHTAC, DHTA methyl sulfate, and DISDA methyl sulfate, and nonionic surfactants, C12E12 and C12E23, are investigated. For these systems the differences in phase behavior is discussed, and for the mixture DHDAB/C12E12 a direct comparison with theoretical predictions of phase behavior is made. It is shown that the phase separation that can occur in these mixed systems is induced by a depletion force arising from the micellar component, and that the size and volume fraction of the micelles are critical factors.

Journal Article↗
PubMed2004

Photoinduced phase separation and miscibility in the condensed phase of a mixed Langmuir monolayer.

We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures ( approximately 5 mN/m). At higher surface pressures ( approximately 10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.

Azo Compounds↗
PubMed2004

Phase behavior and nano-emulsion formation by the phase inversion temperature method.

Formation of oil-in-water nano-emulsions has been studied in the water/C12E4/isohexadecane system by the phase inversion temperature emulsification method. Emulsification started at the corresponding hydrophilic-lipophilic balance temperature, and then the samples were quickly cooled to 25 degrees C. The influence of phase behavior on nano-emulsion droplet size and stability has been studied. Droplet size was determined by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the smallest droplet sizes were produced in samples where the emulsification started in a bicontinuous microemulsion (D) phase region or in a two-phase region consisting of a microemulsion (D) and a liquid crystalline phase (L(alpha)). Although the breakdown process of nano-emulsions could be attributed to the oil transference from the smaller to the bigger droplets, the increase in instability found with the increase in surfactant concentration may be related to the higher surfactant excess, favoring the oil micellar transport between the emulsion droplets.

Journal Article↗
PubMed2000

Chiral catalyst optimization using both solid-phase and liquid-phase methods in asymmetric aza Diels-Alder reactions

[formula: see text] In the presence of 1-5 mol % of a chiral zirconium catalyst, aza Diels-Alder reactions of aldimines with Danishefsky's dienes proceeded smoothly to afford the corresponding piperidine derivatives in high yields with high enantioselectivities. For the catalyst optimization, solid-phase and liquid-phase methods were successfully used. In the solid-phase approach, polymer-supported (R)-1,1'-binaphthols (BINOLs) have been synthesized and rapid optimization using the solid-phase reactions has been achieved. On the other hand, novel chiral zirconium cyanides were developed as excellent catalysts using the liquid-phase approach.

Journal Article↗
PubMedDate not supplied

Matrix metalloproteinase inhibitors (MMPIs): the beginning of phase I or the termination of phase III clinical trials.

The decade of the 1990s was ripe with enthusiasm for the use of MMPIs to treat cancer. Limitations to new cytotoxic chemotherapy approaches to treat solid cancers and a better understanding of tumor biology provided a strong impetus for alternative drug development. It is estimated that the pharmaceutical industry invested at least a billion dollars in this effort. Because MMPIs represent an entirely different therapeutic modality from proven anti-cancer agents, many of the therapeutic trials designed to test MMPIs in human patients with cancer bypassed traditional approaches to evaluate drug efficiency. The concept of systematic progression from small phase I (dose escalation to toxicity to examine drug safety), to phase II (drug treatment of patients with cancer types considered to be good candidates for the selected drug), to phase III (randomized trial of new drug versus best available therapy to determine drug efficacy) trials was modified. Much to the chagrin of everyone involved in these studies, the randomized trials of MMPIs in advanced cancer have, pretty much, flopped. This review article will attempt to dissect out aspects of previous human and animal studies that may be helpful in making decisions about the future of MMPI drug development for the treatment of cancer. The important questions to be addressed in this report are: What are the lessons that we have learned from preclinical (animal models) and clinical studies of MMPIs in cancer? Are we ready to abandon MMPIs as a therapeutic modality in cancer (termination of phase III trials) or do we need to have a better understanding of the myriad effects of MMPs in cancer before we proceed to develop different types of drugs that alter MMP activity in patients with cancer (beginning of new phase I trials)?

Animals↗
PubMed1989

Response of patients in phase II studies of chemotherapy in ovarian cancer: implications for patient treatment and the design of phase II trials.

Results using the same drug in phase II studies of treatment in ovarian cancer vary widely. An analysis of five phase II studies with a total of 93 patients was carried out to determine whether factors other than the efficacy of the drug affect response. The drugs for the phase II studies were chosen on the basis of in vitro activity or previous activity in humans. Univariate analysis showed that several factors were of significance in predicting response. The most significant was interval from the end of previous treatment to entry into a phase II study. Others were the original presenting stage of the patient, the second line treatment given and the best previous response to therapy. In multivariate analysis, however, only two factors were shown to be of importance which were interval and the FIGO stage of the patient. Using these two variables the discriminant analysis predicted 89% of those who did not respond and 75% of those who did, with an overall correct prediction of 85%. The importance of interval is emphasised by the observation that the response rate for those patients who progressed on treatment or who relapsed within 3-6 months of primary therapy had a response rate of less than 10%. Future phase II studies should probably exclude patients in this category, since the chance of their responding is very low.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed2001

Retention characteristics of porous graphitic carbon in reversed-phase liquid chromatography with methanol-water mobile phases.

The solvation parameter model is used to study the retention mechanism of neutral organic compounds on porous graphitic carbon with methanol-water mobile phases containing from 0-100% (v/v) methanol. The dominant contribution to retention is the cavity formation-dispersion interaction term, composed of favorable interactions in the mobile phase (hydrophobic effect) and additional contributions from adsorption on the graphite surface. Electron lone pair and dipole-type interactions in the adsorbed state result in increased retention. Hydrogen-bonding interactions are more favorable in the mobile phase resulting in lower retention. The changes in the system constants of the solvation parameter model for cavity formation-dispersion interactions and hydrogen-bond interactions are linearly related to the volume fraction of water in the mobile phase. The system constants for electron lone pair interactions and dipole-type interactions are non-linear and go through a maximum and minimum value, respectively, at a specific mobile phase composition. The solvation parameter model poorly predicts the retention properties of angular molecules. This is probably due to the failure of the characteristic volume to correctly model the contact surface area for the interaction of angular molecules with the planar graphite surface. General factors affecting the quality of model fits for adsorbents are discussed.

Journal Article↗
PubMed2004

Retention characteristics of protonated mobile phases injected into deuterated mobile phases in capillary liquid chromatography (LC) using on-line nuclear magnetic resonance (NMR) detection.

The behavior of protonated binary solvents injected into deuterated binary mobile phases in capillary LC is studied with NMR. Specifically, the solvent elution is followed on-flow with a capillary LC coupled to a 900 nL volume microcoil NMR probe. A range of identical composition 5% protonated (and 95% deuterated) solvents is injected into composition-matched deuterated mobile phases of CD(3)CN/D(2)O and CD(3)OD/D(2)O. The protonated components separate for all solvent combinations except at 80% CD(3)CN/20% D(2)O and similar to 72% CD(3)OD/28% D(2)O where only a single retention time is observed. The more hydrophilic protonated component, HOD, elutes first with higher percentages of hydrophilic solvent, D(2)O, in the mobile phase whereas retention is reversed with the higher percentage of the more hydrophobic solvent (CD(3)CN and CD(3)OD) in the mobile phase. The hydrophilic/hydrophobic nature of the chromatographic system as a function of mobile phase composition is characterized by following the retention times of protonated solvents.

Chromatography, Liquid↗