Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “phasing”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 469 records · Page 26Linked to original sources
PubMedDate not supplied

A comparative study of the reversed-phase HPLC retention behaviour of S-adenosyl-L-methionine and its related metabolites on Hypersil ODS and Supelcosil LC-ABZ stationary phases.

S-Adenosyl-L-methionine (SAM) and its metabolites S-adenosyl-L-homocysteine (SAH) and methyl-thioadenosine (MTA) are endogenous compounds that are heavily involved in a variety of biochemical processes, and have therefore been the target for several assays in body fluids and tissues. Reversed-phase chromatographic behaviour of SAM and its metabolites has been studied by using Supelcosil LC-ABZ column, specially designed for analysis of acidic, basic, zwitterionic and neutral compounds, and on a Hypersil ODS column as a function of mobile phase pH. The retentions of the compounds, expressed by the capacity ratio (k'), are measured on both column with mobile phases comprised of 10% acetonitrile and 10 mM ammonium formate buffer with pH values ranging from 2 to 9. Higher selectivity is observed on Supelcosil LC-ABZ within pH range 4-6. Different retention properties are observed at very low pH and seemed as if the Supelcosil LC-ABZ column reduced the effect of the mobile phase pH by about 1 pH unit. Whilst the Supelcosil column can be recommended for the routine analysis of SAM and its related metabolites in biological fluids by using mobile phase pH 5, the Hypersil ODS column may be suggested for use with mobile phase pH values of 3-4.

Adenosine↗
PubMed2004

Determination of the pH of binary mobile phases for reversed-phase liquid chromatography.

The measurement of pH in chromatographic mobile phases has been a constant subject of discussion during many years. The pH of the mobile phase is an important parameter that determines the chromatographic retention of many analytes with acid-base properties. In many instances a proper pH measurement is needed to assure the accuracy of retention-pH relationships or the reproducibility of chromatographic procedures. Three different methods are common in pH measurement of mobile phases: measurement of pH in the aqueous buffer before addition of the organic modifier, measurement of pH in the mobile phase prepared by mixing aqueous buffer and organic modifier after pH calibration with standard solutions prepared in the same mobile phase solvent, and measurement of pH in the mobile phase prepared by mixing aqueous buffer and organic modifier after pH calibration with aqueous standard solutions. This review discusses the different pH measurement and calibration procedures in terms of the theoretical and operational definitions of the different pH scales that can be applied to water-organic solvent mixtures. The advantages and disadvantages of each procedure are also presented through chromatographic examples. Finally, practical recommendations to select the most appropriate pH measurement procedure for particular chromatographic problems are given.

Buffers↗
PubMed2005

Alternative high-performance liquid chromatographic peptide separation and purification concept using a new mixed-mode reversed-phase/weak anion-exchange type stationary phase.

This article describes a new complementary peptide separation and purification concept that makes use of a novel mixed-mode reversed-phase/weak anion-exchange (RP/WAX) type stationary phase. The RP/WAX is based on N-(10-undecenoyl)-3-aminoquinuclidine selector, which is covalently immobilized on thiol-modified silica particles (5 microm, 100 A pore diameter) by radical addition reaction. Remaining thiol groups are capped by radical addition with 1-hexene. This newly developed separation material contains two distinct binding domains in a single chromatographic interactive ligand: a lipophilic alkyl chain for hydrophobic interactions with lipophilic moieties of the solute, such as in the reversed-phase chromatography, and a cationic site for anion-exchange chromatography with oppositely charged solutes, which also enables repulsive ionic interactions with positively charged functional groups, leading to ion-exclusion phenomena. The beneficial effect that may result from the combination of the two chromatographic modes is exemplified by the application of this new separation material for the chromatographic separation of the N- and C-terminally protected tetrapeptide N-acetyl-Ile-Glu-Gly-Arg-p-nitroanilide from its side products. Mobile phase variables have been thoroughly investigated to optimize the separation and to get a deeper insight into the retention and separation mechanism, which turned out to be more complex than any of the individual chromatography modes alone. A significant anion-exchange retention contribution at optimal pH of 4.5 was found only for acetate but not for formate as counter-ion. In loadability studies using acetate, peptide masses up to 200 mg could be injected onto an analytical 250 mm x 4 mm i.d. RP/WAX column (5 microm) still without touching bands of major impurity and target peptide peaks. The corresponding loadability tests with formate allowed the injection of only 25% of this amount. The analysis of the purified peptide by capillary high-performance liquid chromatography (HPLC)-UV and HPLC-ESI-MS employing RP-18 columns revealed that the known major impurities have all been removed by a single chromatographic step employing the RP/WAX stationary phase. The better selectivity and enhanced sample loading capacity in comparison to RP-HPLC resulted in an improved productivity of the new purification protocol. For example, the yield of pure peptide per chromatographic run on RP/WAX phase was by a factor of about 15 higher compared to the standard gradient elution RP-purification protocol.

Anion Exchange Resins↗
PubMed2005

On-line preconcentration of organic anions in capillary electrophoresis by solid-phase extraction using latex-coated monolithic stationary phases.

Quaternary ammonium functionalised polymeric latex particles were coated onto the wall of a fused-silica capillary or onto a methacrylate monolithic bed synthesised inside the capillary in order to create ion-exchange stationary phases of varying ion-exchange capacity. These capillaries were coupled in-line to a separation capillary and used for the solid-phase extraction (SPE), preconcentration and subsequent separation of organic anions by capillary electrophoresis. A transient isotachophoretic gradient was used for the elution of bound analytes from the SPE phase using two modes of separation. The first comprised a low capacity SPE column combined with a fluoride/octanesulfonate discontinuous electrolyte system in which peak compression occurred at the isotachophoretic gradient front. The compressed anions were separated electrophoretically after elution from the SPE preconcentration phase and resolution was achieved by altering the pH of the electrolyte in which the separation was performed. In the second approach, a latex-coated monolithic SPE preconcentration stationary phase was used in combination with a fluoride/perchlorate electrolyte system, which allowed capillary electrochromatographic separation to occur behind the isotachophoretic gradient front. This method permitted the removal of weakly bound anions from the SPE phase, thereby establishing the possibility of sample clean-up. The effect of the nature of the strong electrolyte forming the isotachophoretic gradient on the separation and also on the preconcentration step was investigated. Capillary electrochromatography of inorganic and organic species performed on the latex-coated monolithic methacrylate column highlighted the presence of mixed-mode interactions resulting from the incomplete coverage of latex particles onto the monolithic surface. Analyte preconcentration prior to separation resulted in compression of the analyte zone by a factor of 300. Improvement in the limit of detection of up to 10400 times could be achieved when performing the preconcentration step and the presented methods had limits of detection (S/N=3) ranging between 1.5 and 12 nM for the organic anions studied.

Anions↗
PubMed2006

Mobile phase effects on retention on a new butylimidazolium-based high-performance liquid chromatographic stationary phase.

A new HPLC stationary phase based on n-butylimidazolium bromide has been characterized by a linear solvation energy relationship (LSER) approach in the binary acetonitrile/water mobile phases. The retention properties of the stationary phase were systematically evaluated in terms of intermolecular interactions between 28 test solutes and the stationary phase. The results and further comparisons with conventional reversed phase system confirm that retention properties are similar to phenyl phases in acetonitrile/water mixtures. The results obtained with acetonitrile/water mixtures are also compared with results obtained using methanol/water mixtures.

Acetonitriles↗
PubMed2006

Solvation parameter models for retention on perfluorinated and fluorinated low temperature glassy carbon stationary phases in reversed-phase liquid chromatography.

The retention of solutes on two fluorinated low temperature glassy carbon (F-LTGC) stationary phases under reversed-phase liquid chromatographic conditions was studied by employing the solvation parameter model. The two fluorinated glassy carbon stationary phases were produced by slowly heating zirconia particles that were encapsulated with oligo[1,3-dibutadiyne-1,3-(tetrafluoro)phenylene] precursor polymer to two different final temperatures (200 and 400 degrees C). The resulting carbon particles had different amounts of fluorine after thermal processing. The solvation parameter models indicated that different intermolecular interactions are important in describing retention on the two stationary phases. The interactions that are important for describing retention on the 200 degrees C processed F-LTGC stationary phase are hydrogen bond basicity> or =dispersion>hydrogen bond acidity>dipolarity/polarizability. The interactions that describe the retention on the 400 degrees C processed F-LTGC are hydrogen bond basicity>dispersion>excess molar refraction> or =hydrogen bond acidity. The solvation parameter model for the 200 degrees C processed F-LTGC showed similar trends in the relative importance of intermolecular interactions as previously found for octadecyl-polysiloxane stationary phases, while the 400 degrees C processed F-LTGC had similar intermolecular interactions with solutes as found with porous glassy carbon in that pi-pi interactions with the carbon surface contribute more so to the retention.

Carbon↗
PubMed2003

Three-phase liquid-phase microextraction of weakly basic drugs from whole blood.

The behaviour of weak basic analytes in liquid-phase microextraction (LPME) and the optimisation of parameters in whole blood are described. Benzodiazepines and non-benzodiazepine drugs were chosen as model substances. Liquid-phase microextraction based on disposable polypropylene hollow fibres was used in the three-phase extraction of five weak bases from whole blood. The sample work up with the liquid-phase microextraction technique can be impeded by low recovery due to incomplete trapping in the acceptor phase of weakly basic drugs and the complexity of the whole blood matrix. Different parameters related to this problem were experimentally studied. Additionally the stability of the analytes was examined because of low pH in the acceptor phase. The investigation resulted in optimised LPME conditions for the extraction of weak bases from whole blood. The parameters limiting the recovery were evaluated.

Chromatography, High Pressure Liquid↗
PubMed2006

Multiphase flow and transport caused by spontaneous gas phase growth in the presence of dense non-aqueous phase liquid.

Disconnected bubbles or ganglia of trapped gas may occur below the top of the capillary fringe through a number of mechanisms. In the presence of dense non-aqueous phase liquid (DNAPL), the disconnected gas phase experiences mass transfer of dissolved gases, including volatile components from the DNAPL. The properties of the gas phase interface can also change. This work shows for the first time that when seed gas bubbles exist spontaneous gas phase growth can be expected to occur and can significantly affect water-gas-DNAPL distributions, fluid flow, and mass transfer. Source zone behaviour was observed in three different experiments performed in a 2-dimensional flow cell. In each case, a DNAPL pool was created in a zone of larger glass beads over smaller glass beads, which served as a capillary barrier. In one experiment effluent water samples were analyzed to determine the vertical concentration profile of the plume above the pool. The experiments effectively demonstrated a) a cycle of spontaneous gas phase expansion and vertical advective mobilization of gas bubbles and ganglia above the DNAPL source zone, b) DNAPL redistribution caused by gas phase growth and mobilization, and c) that these processes can significantly affect mass transport from a NAPL source zone.

Gases↗
PubMed2005

Photoacoustic phase study on the frequency dependence of phase angle and triplet state lifetime of Nd(III)-acetylacetonate.

The amplitude and phase of the photoacoustic (PA) signal generated in the rare earth complex powder of Nd(III)-acetylacetonate (Nd(AA)3) were examined. By using the reasonably simplified formula about phase angle at the wavelengths assigned to pi-pi* transition of ligand and f-f transitions of Nd3+, the instrumental contribution to phase angle at different chopping frequency was calculated. The frequency dependence of sample phase angle, the relative phase angle shift caused by instrumental factors and the calculation of triplet state lifetime of sample under study were also discussed. Being independent of sample, the instrumental contribution to phase angle was regarded as a parameter representing the situations of instruments and measurement and it became an approach to calculate the relaxation time of ligand in rare earth complex.

Acoustics↗
PubMed2005

Effect of vaccination with phase I and phase II Coxiella burnetii vaccines in pregnant goats.

Livestock is considered to be the major "source" of human Q fever. The efficacy of two currently available vaccines (Coxevac, phase I, CEVA Santé Animale and Chlamyvax FQ, phase II, MERIAL) against Coxiella excretion was investigated in terms of risks to human health. Two months before mating, 17 goats were vaccinated subcutaneously against Coxiella burnetii with an inactivated phase I vaccine and 16 goats were vaccinated with an inactivated phase II Coxiella mixed with Chlamydophila abortus vaccine. Fourteen goats were left unvaccinated. At 84 days of gestation, the goats were subcutaneously challenged with 10(4) bacteria of C. burnetii strain CbC1. Phase I vaccine was effective and dramatically reduced both abortion and excretion of bacteria in the milk, vaginal mucus and feces. In contrast, the phase II vaccine did not affect the course of the disease or excretion.

Animals↗
PubMed2001

The effect of ethanol on the phase transition temperature and the phase structure of monounsaturated phosphatidylcholines.

Previous studies from our laboratories have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines, C(X):C(Y)PC, and the effect of ethanol concentration, [EtOH], on the main phase transition temperature, T(m), and the phase structure of the lipid bilayer composed of C(X):C(Y)PC using differential scanning calorimetry and X-ray diffraction techniques [Huang and McIntosh, Biophys. J. 72 (1997) 2702--2709]. In the present work, we have extended these studies to characterize the effect of [EtOH] on the T(m) and the phase structure of the lipid bilayer composed of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines with various positions of the cis double bond. Specifically, five positional isomers of 1-eicosanoyl-2-eicosenoyl-sn-glycero-3-phosphocholines, C(20):C(20:1 Delta(n))PC with n=5, 8, 11, 13 and 17, were synthesized and studied. For C(20):C(20:1 Delta(n))PC with n=5 and 8, results from the calorimetric experiments showed that in response to various concentrations of ethanol, the change in T(m) of the lipid bilayer composed of monounsaturated lipids was characterized by a sigmoidal or biphasic profile in the plot of T(m) versus [EtOH]. In contrast, a continuous depression of the T(m) by ethanol was observed calorimetrically for C(20):C(20:1 Delta(n))PC with n> or =11. The X-ray diffraction experiments further demonstrated that C(20):C(20:1 Delta(5))PC and C(20):C(20:1 Delta(8))PC can undergo the ethanol-induced gel-to-fully interdigitated phase transition at T<T(m). Such a transition, however, was not observed for C(20):C(20:1 Delta(13))PC even at a very high ethanol concentration of 100 mg/ml. These distinct different effects of [EtOH] on the phase transition temperature and the phase structure can be attributed to various positions of the cis double bond in these monounsaturated phosphatidylcholines. And the different effects of ethanol can, in fact, be explained based on the molecular structures of these monounsaturated lipids packed in the gel-state bilayer as generated by molecular mechanics simulations. To the best of our knowledge, this is the first time that the ethanol-induced fully interdigitated bilayers are observed at T<T(m) for unsaturated phospholipids with well defined double bond positions in their sn-2 acyl chains.

Calorimetry, Differential Scanning↗
PubMed1991

Direct determination of crystallographic phases for diffraction data from lipid bilayers. I. Reliability and phase refinement.

Direct analysis of lipid lamellar packing based on the probabilistic estimate of sigma 1- and sigma 2-triplet phase invariants is evaluated here for a large variety of bilayer structures than examined in an original study of this problem (Dorset, D.L., 1990. Biophys. J. 58:1077-1087). Using x-ray crystal structures of five phospholipids, three glycerides and two cerebrosides, lamellar diffraction data were generated at the approximately 3 A resolution often found experimentally from oriented multilayers. For structures where no significant density occurs at the unit cell origin, the ab initio phase determination is successful for six of the ten structures. A seventh structure can be solved if a limited set of sigma 2-triples are used to determine the initial phase set based on the hierarchy of the A2 values. Bilayers, e.g., with solvent at the origin, can be analyzed if a modified criterion for accepting phase estimates for sigma 1-triples is used, as suggested by the distribution of normalized structure factors and the number of probable single-valued phase domains. In all cases, partial phase determinations can be refined effectively by density modification ("flattening") of the hydrocarbon region in real space. A figure of merit suggested by Luzzati et al. (Luzzati, V., A. Tardieu, and D. Taupin. 1972. J. Mol. Biol. 64:269-286) used to evaluate the success of such refinement can be supplemented by an evaluation of density smoothness, which can also detect the presence of near structure homomorphs not identified by the former test for density flatness.

Lipid Bilayers↗
PubMed2000

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. II. Optimization of the molecular parameter of the polymer.

Stationary phases obtained by coating side-chain liquid crystalline polymers (LCPs) with the mesogenic rod like units laterally attached to a polysiloxane backbone via a flexible spacer have been already reported. These phases show excellent planarity and shape recognition for polynuclear aromatic hydrocarbon (PAH) solutes in reversed-phase liquid chromatography. Optimization of these stationary phases in terms of molecular parameters of the polymer is here described. Fifteen stationary phases have been prepared varying different parameters such as the spacer length, the aliphatic tail length, and the proportion of laterally attached mesogenic units along the polymer chain. The results show that the combination of a long spacer and long terminal chains, which generates a smectic phase in the polymer bulk, leads to the best chromatographic performances towards planarity and shape recognition for PAH solutes.

Chromatography, High Pressure Liquid↗
PubMed2001

Reversed-phase liquid chromatographic separation of enantiomers on polysaccharide type chiral stationary phases.

The direct chiral separation by chiral stationary phases (CSPs) is one of the most important techniques to analyze enantiomeric purity as well as to get enantiomerically pure material quickly. Among various types of CSPs, polysaccharide type CSPs are well known by their versatility and durability. They are not only effective under normal-phase conditions, but also under reversed-phase conditions. In order to get a good separation under the reversed-phase conditions, it is the key to choose an appropriate mobile phase. For example, a simple mixture of water/acetonitrile or water/methanol are sufficient for a neutral analyte, while it is necessary to use an acidic solution instead of water for an acidic analyte and a solution of a chaotropic salt (or a basic solution) for a basic analyte, respectively. The paper also presents lists of more than 350 separation examples that include 22 validated methods for drug analyses from serum, plasma, or urine samples on polysaccharide type CSPs under reversed-phase conditions.

Chromatography, Liquid↗
PubMed1988

Rapid and simple dansylation of phenolic steroids using a two-phase system and phase transfer catalysis.

Dansylation of phenolic steroids was carried out in chloroform-water and hexane-water two-phase systems with a tetrabutylammonium salt as phase transfer catalyst. Derivatization was complete within a few minutes on shaking at room temperature. Direct injection of part of the organic phase into a normal-phase liquid chromatography system was possible. The calibration graph of ethinyl estradiol, dansylated in a chloroform-water two-phase system, was linear over three orders of magnitude with a correlation coefficient of 0.993 (n = 8). The detection limit of dansylated ethinyl estradiol was 100 pg (signal-to-noise ratio = 2). The reproducibility of the derivatization at an analyte concentration of 200 ng/ml in chloroform was 4.1% (relative standard deviation; n = 5). A mechanism is proposed for the phase transfer catalysed dansylation of phenolic compounds.

Catalysis↗
PubMed2001

Unique selectivity of perfluorinated stationary phases with 2,2,2-trifluoroethanol as organic mobile phase modifier.

The selectivity of Luna C18 Xterra C18 and Fluophase (perfluorinated C6) stationary phases has been investigated with aqueous acetonitrile, methanol and 2,2,2-trifluoroethanol mobile phases using linear solvation equations. The gradient retention times of a set of 60 compounds with known molecular descriptors have been determined. Linear solvation equations have been set up to describe the relationship between the gradient retention times and the molecular properties. The selectivity of the stationary phase/mobile phase systems was characterised by the regression coefficients of the molecular descriptors. The perfluorinated stationary phase showed very different selectivity using 2,2,2-trifluoroethanol (TFE) as co-solvent. Compounds with H-bond donor functionality were retained much less than in the other investigated high-performance liquid chromatography (HPLC) systems. This unique selectivity can be explained by the stronger adsorption of trifluoroethanol on the perfluorinated stationary phase surface, than on the hydrocarbon surface. It suggests the importance of the adsorbed organic modifiers in the separation mechanism during reversed-phase HPLC.

Chromatography, High Pressure Liquid↗
PubMed1986

Normal-phase high-performance liquid chromatographic separations of positional isomers of substituted benzoic acids with amine and beta-cyclodextrin bonded-phase columns.

The separation of positional isomers of several substituted benzoic acids was studied using two different columns, i.e. amine and beta-cyclodextrin bonded silicas, at appropriate normal-phase conditions. Although some other columns do separate substituted benzoic acids, particularly the octadecylsilica column in a reversed-phase mode, the present separation under simple normal-phase, isocratic conditions is unique. In particular, the retention order of these acids for the amino bonded-phase column can be roughly predicted using the pKa values of the analytes. On the other hand, due to the strong interaction between substituted benzoic acids and the beta-cyclodextrin bonded phase, a small amount of acetic acid has to be added into the mobile phase to overcome band broadening and tailing problems. Possible retention mechanisms are also discussed.

Amines↗
PubMed1990

(S)-2-hydroxypropyl-beta-cyclodextrin, a new chiral stationary phase for reversed-phase liquid chromatography.

(S)-2- and (R,S)-2-hydroxypropyl-beta-cyclodextrin have been bonded to silica gel and evaluated as stationary phases for reversed-phase liquid chromatography. Stationary phases also were prepared on two silicas having different pore sizes and surface areas. Dissimilarities were observed in enantiomeric selectivities between these columns and also between these and the native beta-cyclodextrin columns. With the exception of compounds 5 and 10, all other racemates reported here which have been successfully resolved on the new phases are enantiomers which have not been previously reported as separated on the beta-cyclodextrin stationary phase. In some cases, there were also differences in enantioselectivities observed between the (S)- and the (R,S)-hydroxypropyl-beta-cyclodextrin phases on the same silica. The results are discussed in terms of the retention mechanism and compared to results reported earlier for beta-cyclodextrin columns.

Chemical Phenomena↗