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 1,531 records · Page 85Linked to original sources
PubMed2004

Continuous beds (monoliths): stationary phases for liquid chromatography formed using the hydrophobic interaction-based phase separation mechanism.

The pioneering research work published by Hjertén et al. [J. Chromatogr. 473 (1989) 273] in 1989 dealing with development and application of the continuous bed (monolithic) technique as an attractive alternative for the classical packed columns in chromatography, stimulated further investigations in this direction. The research data published since that time on the development and application of the continuous beds formed using hydrophobic interaction-based phase separation mechanism are reviewed. Some innovative species of the beds, such as polyrotaxane beds or nonparticulate restricted-access materials for direct analysis of the biological fluids in the capillary format are also discussed. Characteristic features and practical details of the continuous bed technique are revealed. Due to many advantages, the continuous bed technique became a competitor with the traditional packings in capillary or chip-based microanalysis. The importance of the continuous bed morphology on the chromatographic characteristics is shown. The applicability of modern microscopic analysis to evaluate the morphology of the continuous beds is demonstrated.

Chromatography, Liquid↗
PubMed2004

The use of a monolithic column to improve the simultaneous determination of four cephalosporin antibiotics in pharmaceuticals and body fluids by HPLC after solid phase extraction--a comparison with a conventional reversed-phase silica-based column.

The use of a monolithic column (Chromolith, SpeedROD RP-18e, by Merck) was studied on the determination of cephalosporin antibiotics. Results were compared with those from a previously developed analytical method using conventional silica-based analytical column. A rapid, accurate and sensitive method has been developed and validated for the quantitative simultaneous determination of four cephalosporins: Cephalexine and Cephadroxil (first generation), Cefaclor (second generation) and Cefotaxim (third generation) in pharmaceuticals as well as in human blood serum and urine. Hydroflumethiazide (HFM) (3,4-dihydro-6(trifluoromethyl)-2H-1,2,4-benzothiadiazine-7-sulfonamide-1,1-dioxide) was used as an internal standard at a concentration of 1.5 ng/microL. A rectilinear relationship was observed up to 5 ng/microL for the four compounds. Analysis time was less than 4 min. The statistical evaluation of the method was examined by means of within-day repeatability (n=8) and day-to-day precision (n=8) and was found to be satisfactory with high accuracy and precision results. The method was applied to the determination of the cephalosporins in commercial pharmaceuticals and in biological fluids: human blood serum after solid phase extraction and urine simply after filtration and dilution. Recovery of analytes in spiked serum samples was in the range from 88.7 to 107.8%, while for urine samples recovery was from 98.0 to 105.6%. By comparing the figures of merit for the monolithic column and the silica-based one, regarding the determination of the four cephalosporins investigated in the present study, the outstanding efficiency of the monolithic column can be noticed.

Body Fluids↗
PubMed1992

ET-1 induced bronchoconstriction in the early phase but not late phase of anesthetized dogs is inhibited by indomethacin and ICI 198615.

Intratracheally injected or aerosolized ET-1 induced quick and long-lasting bronchoconstriction of anesthetized mongrel dogs, thus increasing respiratory resistance(Rrs) with concomitantly decreasing dynamic compliance(Cdyn). As collateral resistance(Rcs) was measured postexposure to aerosolized ET-1 using wedged bronchoscope technique, ET-1 increased Rcs in a dose and time dependent manner. The increase attained maximal in 2 min and then, gradually declined. When the dogs were pretreated with the intravenous injection of 0.1 micrograms/kg ICI 198615, an inhibitor of lipoxygenase, the constrictive response was slowed down. Essentially similar results were also observed with the intravenous injection of 5 mg/kg indomethacin. Our observations suggest that the early phase of the ET-1 induced bronchoconstriction is mediated by eicosanoid metabolites.

Anesthesia, Intravenous↗
PubMed1979

A theoretical model for lipid mixtures, phase transitions, and phase diagrams.

We present a new model for the thermodynamic properties of lipid bilayers. The model consists of a system of hard cylinders of varying radii that correspond to the different molecular radii of lipids having different numbers of gauche rotations in their chains. Scaled particle theory is used to provide an accurate estimate of the entropy of packing of the cylinders. To apply the model to bilayers we introduce a semiempirical attractive potential energy. Once the form of this potential is chosen, we adjust one parameter, the interaction strength, so that the model fits the transition temperatures and entropies for various phospholipids. The model then agrees quite well with other published data for these systems. We also directly generalize our model to lipid mixtures, and we obtain phase diagrams that we compare to existing data for these systems. We use the model to describe lipid protein interactions in bilayers as well.

Lipid Bilayers↗
PubMed2000

Purification of crude DNA oligonucleotides by solid-phase extraction and reversed-phase high-performance liquid chromatography.

Purification of target oligodeoxyribonucleotides from failure sequence by-products of synthesis is often required for polymerase chain reaction primers, DNA sequencing and other oligonucleotide applications. We have developed purification protocols based on a reversed-phase mechanism ("trityl on" purification) using a 96-well Oasis HLB extraction plate. The Oasis HLB sorbent combines excellent pH stability with a high loading capacity allowing for single-step purification of 0.2 microM scale synthesis. After sample loading and washing, the oligonucleotide trityl group is cleaved on the plate with 2% trifluoroacetic acid. Target DNA is eluted with acetonitrile-0.36 mM triethylamine acetate, pH 11.3 (10:90, v/v). Typical yield of purified product is 60-95%. Final purity, measured by capillary gel electrophoresis, was found to be 90% or greater. Alternatively, highly pure oligonucleotides can be obtained by a RP-HPLC "trityl off" method using an XTerra C18 column. The use of volatile triethylamine acetate buffer as an ion-pair for RP-HPLC eliminates the need for further desalting.

Chromatography, High Pressure Liquid↗
PubMed2001

Removal of metallic impurities from mobile phases in reversed-phase high-performance liquid chromatography by the use of an in-line chelation column.

An in-line stripper column packed with iminodiacetate chelation resin is placed between the pump and injection valve and shown to remove metallic impurities from an HPLC system. We used a test procedure based on assessing the relative peak asymmetries of 2,2'-bipyridyl, a chelating analyte and 4,4'-bipyridyl, a non-chelating analyte. Results from use of polyether ether ketone (PEEK) and stainless steel pumps are evaluated. Analytical columns with titanium/PEEK and stainless steel frits are used to assess the role of frits in metallic contamination. We demonstrated that although metal-free pumping systems contribute significantly lower metallic impurities than stainless steel systems, metal is nevertheless present in the mobile phase and the chelating stripper columns were found useful in protecting the analytical columns from contamination. The stainless steel frits were not found to be significant contributors to the contamination.

2,2'-Dipyridyl↗
PubMed1982

Correlation between the retention of cardiac glycosides in reversed-phase high-performance liquid chromatography with a diphenylsilyl stationary phase, the structure of their molecules and their biological activity.

The separation of mixtures of cardiac glycosides by reversed-phase high-performance liquid chromatography on silica gel with chemically grafted diphenylsilyl groups using water-ethanol as the eluent was carried out. It is shown that the configuration and conformation of the glycoside molecules, and the hydrophilic properties of their aglycones and glycones, influence the separation. The hydrophilic properties of the aglycones are more important than those of the glycones. The glycosides with more hydrophilic aglycones have higher biological activity. This is probably related to the easier transport of these glycosides to the receptor.

Animals↗
PubMed1988

Automated solid-phase catalyzed pre-column derivatization of fatty acids for reversed-phase high-performance liquid chromatographic analysis with fluorescence detection.

A method is described to automate the solid phase catalyzed reaction of a carbonyl group with labels containing a bromomethyl group. The acids to be determined must be soluble in an aprotic, non-aqueous reaction medium, such as acetonitrile. Special emphasis is placed on the solubility of the base, which serves as a catalyst. A suspension of potassium carbonate was prepared in acetonitrile. In this way, the effective surface area of the base is drastically enlarged resulting in faster reaction kinetics. The procedure is illustrated with the detection of fatty acids in serum and rat brain tissue with bromomethylmethoxycoumarin as label. The generality of our approach for HPLC determination of carbonyl compounds is discussed.

Animals↗
PubMed2001

Selectivity of solid-phase extraction phases in the determination of biodegradation products.

The extraction techniques connected with gas chromatography were used to describe quantitatively and qualitatively the biodegradation process. We investigated the biodegradation of hydrocarbons and non-ionic surfactants. Solid-phase extraction (SPE) and liquid-liquid extraction were used for the isolation of the non-degraded compounds and their degradation products. The selectivity of SPE has a significant influence on the isolation and preconcentration of organic compounds from water.

Biodegradation, Environmental↗
PubMed2001

Determination of phenylurea herbicides in natural waters at concentrations below 1 ng l(-1) using solid-phase extraction, derivatization, and solid-phase microextraction-gas chromatography-mass spectrometry.

A procedure is presented which allows the ultratrace level determination of phenylurea herbicides (PUHs) in natural waters. Samples were enriched by solid-phase extraction (SPE) on Carbopack B and alkylated with iodoethane and sodium hydride to yield thermostable products. After derivatization, the aqueous samples were extracted and injected by SPME. The use of iodoethane instead of iodomethane allowed the differentiation between parent compounds and the N-demethylated metabolites. Limits of detection were between 0.3 and 1.0 ng/l for the parent compounds. Standard deviations below 10% were achieved for samples containing more than 4 ng/l in very different matrices including Nanopure water, lake water, and waste water treatment plant (WWTP) effluent. Moreover, the para-hydroxylated metabolite of diuron could be quantified with the same procedure. The presence of further metabolites was assessed qualitatively. Chromatography was stable over a large number of measurements even with dirty samples from WWTP effluent. The precision and sensitivity of the developed analytical method allowed the investigation of the fate of PUHs in lakes, their degradation during drinking water treatment and their transport within the North Sea.

Calibration↗
PubMed1988

Ultrasound driven synthesis of reversed-phase stationary phases for liquid chromatography using 4-dimethyl-aminopyridine as acid-acceptor.

A reproducible new method for synthesizing high density monomeric reversed-phase chromatographic packings based on the use of ultrasonic waves as the driving force for the bonding reaction is presented. The use of ultrasound has two distinct advantages over traditional reflux methods. It provides a reaction driving force that is independent of temperature; in addition, the power of the ultrasonic driving force can be varied using a variable-power ultrasonic probe. The use of a novel acid-acceptor, 4-dimethylaminopyridine, is also recommended in the bonding reaction due to its ease of use and the resulting high bonding densities.

4-Aminopyridine↗
PubMed1987

Determination of alpha-alkyl-alpha-amino acids and alpha-amino alcohols by chiral-phase capillary gas chromatography and reverse-phase high-performance liquid chromatography.

The enantiomeric resolution by fused-silica capillary gas-liquid chromatography (GLC) of non-protein DL-alpha-alkyl-alpha-amino acids of the structure H2NCR1R2COOH (R1 = alkyl, R2 = alkyl, alkaryl) was investigated by using chiral [L-valine-tert.-butylamide, linked to a statistical polymer of dimethylsiloxane and (2-carboxypropyl)methylsiloxane, Chirasil-L-Val, and XE-60-S-Val-S-alpha-phenylethylamide] and non-chiral (methylphenylcyanopropylvinylpolysiloxane, CP-Sil-19 stationary phases. To evaluate the resolution coefficients, N-acylamino acid n-propyl esters (acyl = acetyl, propionyl, trifluoroacetyl, pentafluoropropionyl, heptafluorobutyryl) and diastereomeric esters with S(-)-2-methyl-1-butanol, S(+)-2-butanol and S(+)-2-octanol were used. Although alpha-alkyl-alpha-amino acids in general gave lower resolution coefficients than the enantiomers of protein amino acids, most alpha-alkyl-alpha-amino acids could be resolved by using suitable derivatization procedures and, preferably, isothermal conditions. In addition, a number of DL-alpha-alkyl-alpha-amino acids could be separated by ligand-exchange chromatography (L-hydroxyproline/Cu2+) by both thin-layer chromatography (Chiralplate) and high-performance liquid chromatography (HPLC) (Nucleosil Chiral-1). Further, a standard mixture composed of fifteen alpha-amino acids and eleven alpha-amino alcohols could be completely separated by C18 HPLC after derivatization with o-phthaldialdehyde-2-mercaptoethanol (OPA-2-ME). The time and temperature dependences of the relative fluorescence of the adducts were investigated kinetically.

Amino Acids↗
PubMed2002

Aroma compound analysis of Piper nigrum and Piper guineense essential oils from Cameroon using solid-phase microextraction-gas chromatography, solid-phase microextraction-gas chromatography-mass spectrometry and olfactometry.

The investigation of aroma compounds of the essential oils of dried fruits of black pepper (Piper nigrum) and black and white "Ashanti pepper" (Piper guineense) from Cameroon by means of solid-phase microextraction (SPME) was carried out for the first time to identify the odorous target components responsible for the characteristic odor of these valuable spices and food flavoring products. By means of GC-flame ionization detection (FID) and GC-MS (using different polar columns) the main compounds (concentration >3.0%, calculated as area of GC-FID analysis using a non-polar fused-silica open tubular RSL-200 column) of the SPME headspace samples of P. nigrum (black) and P. guineense (black and white) were found to be: P. nigrum (black)--germacrene D (11.01%), limonene (10.26%), beta-pinene (10.02%), alpha-phellandrene (8.56%), beta-caryophyllene (7.29%), alpha-pinene (6.40%) and cis-beta-ocimene (3.19%); P. guineense (black)--beta-caryophyllene (57.59%), beta-elemene (5.10%), bicyclogermacrene (5.05%) and alpha-humulene (4.86%); and P. guineense (white)--beta-caryophyllene (51.75%), cis-beta-ocimene (6.61%), limonene (5.88%), beta-pinene (4.56%), linalool (3.97%) and alpha-humulene (3.29%). The most intense odor impressions of the essential oils of the various dried pepper fruits were given byprofessional perfumers as follows: P nigrum (black)--fine, pleasant black pepper note; P. guineense (black)--black pepper top-note; and P. guineense (white)--pleasant white pepper note. These analytical results for the SPME headspace samples of three different pepper species from Cameroon are in accordance with the olfactoric data of the corresponding essential oils. A GC-sniffing technique was used to correlate the single odor impression of the identified SPME headspace volatiles of the three investigated pepper samples with the following results: themain compounds such as beta-caryophyllene, germacrene D, limonene, beta-pinene, alpha-phellandrene and alpha-humulene, as well as minor constituents such as delta-carene, beta-phellandrene, isoborneol, alpha-guaiene, sarisan, elemicin, calamenene, caryophyllene alcohol, isoelemicin, T-muurolol, cubenol and bulnesol, are of greatest importance for the characteristic pepper odor notes of these three Piper samples. Further aroma impressions can be attributed to mono- and sesquiterpenes, hexane, octane and nonane derivatives.

Chromatography, Gas↗
PubMed2003

Chromatographic evaluation of self-immobilized stationary phases for reversed-phase liquid chromatography.

The preparation of stationary phases for HPLC using polymers deposited on silica usually includes an immobilization step involving cross-linking by free radicals induced by ionizing radiation or by other radical initiators. The present paper reports changes which occur at ambient temperature in the character of poly(methyloctylsiloxane) deposited on porous silica particles as a function of the time interval between particle loading and column packing. Column performance and retention factors increase with time and these changes are attributed to rearrangement (self-assembly) which result in "self-immobilization" of the polymer molecules on the silica surface.

Chromatography, High Pressure Liquid↗
PubMed2003

Effect of the mobile phase composition on the isotherm parameters and the high concentration band profiles in reversed-phase liquid chromatography.

Single-component adsorption isotherm data were acquired by frontal analysis for phenol on a C18-Kromasil packed column, under reversed-phase liquid chromatography conditions, using various methanol-water solutions (30-60%, v/v, methanol). The isotherm model accounting best for these data was the biLangmuir model. With increasing methanol content, the two saturation capacities decrease, particularly that of the high-energy sites, the adsorption constant of the low-energy sites decreases significantly and that of the high-energy sites decreases strongly. These results allow a quantitative investigation of the properties of the high-energy sites (which are not necessarily the so-called active sites), a feature rarely discussed yet. The band profiles calculated with the numerical values of the isotherm model parameters derived by fitting the frontal analysis data to the model and using the equilibrium-dispersive model agree very well with the experimental band profiles in the whole concentration range.

Adsorption↗
PubMed2003

Fast fractionation of complex organic extracts by normal-phase chromatography on a solid-phase extraction polymeric sorbent. Optimization of a method to fractionate wine flavor extracts.

Some properties of LiChrolut-EN resins as normal-phase sorbent have been studied. Retention factors using pentane as solvent range from less than 2 (ethyl esters) to more than 56 (fatty acids and vanillin). All retention factors were smaller than 2 with dichloromethane. The efficiency of the bed was between 4 and 13 plates per cm. A method for the fast fractionation of wine flavor extracts has been further developed. Wine (75 ml) is extracted on a 0.5 g LiChrolut-EN bed. Volatile compounds are recovered in 5 ml of dichloromethane and the extract is further concentrated to 0.1 ml. Recoveries of the extraction procedure are above 85% for all compounds less polar than isoamyl alcohol. This extract is fractionated on a bed (5.0 cm height, 0.6 cm internal diameter) packed with 0.55 g of LiChrolut-EN resins. A first fraction is collected by the elution with 4 ml of pentane. A second one with 6 ml of a mixture pentane/dichloromethane (9:1) and a final fraction with 4 ml of dichloromethane. The first fraction is enriched in ethyl esters and some other non-polar compounds. The second fraction concentrates the alcohols and some volatile phenols, while the third is enriched in fatty acids, vanillin derivatives and some lactones. The recovery in the fractionation is complete. The profile obtained in the fractionation is very stable, and becomes distorted only when the column is loaded with an extract containing 80 mg of major volatiles (coming from more than 150 ml of wine). The fractionation of extracts from different wines showed that the performance of the process does not depend on the composition of the extract. Twenty-seven out of 32 studied compounds eluted reproducibly mainly in one fraction. The results suggest that the method can be applied as an aid for qualitative or quantitative analysis to any kind of organic extract as an alternative to liquid chromatography on silica-gel.

Chromatography, Liquid↗
PubMed1999

Semi-permeable surface analytical reversed-phase column for the improved trace analysis of acidic pesticides in water with coupled-column reversed-phase liquid chromatography with UV detection. Determination of bromoxynil and bentazone in surface water.

The coupled-column (LC-LC) configuration consisting of a 3 microm C18 column (50 x 4.6 mm I.D.) as the first column and a 5 microm C18 semi-permeable-surface (SPS) column (150 x 4.6 mm I.D.) as the second column appeared to be successful for the screening of acidic pesticides in surface water samples. In comparison to LC-LC employing two C18 columns, the combination of C18/SPS-C18 significantly decreased the baseline deviation caused by the hump of the co-extracted humic substances when using UV detection (217 nm). The developed LC-LC procedure allowed the simultaneous determination of the target analytes bentazone and bromoxynil in uncleaned extracts of surface water samples to a level of 0.05 microg/l in less than 15 min. In combination with a simple solid-phase extraction step (200 ml of water on a 500 mg C18-bonded silica) the analytical procedure provides a high sample throughput. During a period of about five months more than 200 ditch-water samples originating from agricultural locations were analyzed with the developed procedure. Validation of the method was performed by randomly analyzing recoveries of water samples spiked at levels of 0.1 microg/l (n=10), 0.5 microg/l (n=7) and 2.5 microg/l (n=4). Weighted regression of the recovery data showed that the method provides overall recoveries of 95 and 100% for bentazone and bromoxynil, respectively, with corresponding intra-laboratory reproducibilities of 10 and 11%, respectively. Confirmation of the analytes in part of the samples extracts was carried out with GC-negative ion chemical ionization MS involving a derivatization step with bis(trifluoromethyl)benzyl bromide. No false negatives or positives were observed.

Benzothiadiazines↗
PubMed2000

Effect of stationary phase hydrophobicity and mobile phase composition on the separation of carboxylic acids in ion chromatography.

In a previous work, we studied the retention behavior of monovalent and divalent carboxylic acids on a highly cross-linked polystryene-divinylbenzene anion-exchange column (IonPac AS4A-SC) using a carbonate-based buffer, and a retention model was applied to the chromatographic data obtained. In this work we characterized the retention of carboxylates (formic, acetic, propionic, lactic, pyruvic, oxalic, malonic, succinic, fumaric, maleic, tartaric, glutaric, adipic, malic, mucic, trans-beta-hydromuconic, trans,trans-muconic acids) on a column with higher hydrophilicity (IonPac AS11) according to analyte and stationary phase properties, using previously investigated eluent compositions and comparing the retention data obtained. Moreover, the effect of organic modifiers (CH3OH and CH3CN) in the eluent on the retention factors was also evaluated. The chromatographic data obtained on the IonPac AS11 column were fitted by the retention model and allowed one to obtain and to compare ion-specific selectivity constants (parameters of the model) with the ones obtained with the previous column.

Carboxylic Acids↗