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At least 1,279 records · Page 71Linked to original sources
PubMed1985

Silver-modified mobile phase for normal-phase liquid chromatographic determination of prostaglandins and their 5,6-trans isomers in prostaglandin bulk drugs and triacetin solutions.

A silver-modified, normal-phase, high-performance liquid chromatographic system has been developed for prostaglanding bulk drugs and triacetin solutions. Silver nitrate present in the mobile phase results in high selectivity for cis/trans isomers with conventional silica columns. Prostaglandins were esterified with alpha-bromo-2'-acetonaphthone prior to chromatography to provide high detectability at 254 nm. For dilute triacetin solutions, a sample preparation scheme based on gravity-flow chromatography with silica columns was developed to isolate the prostaglandin from triacetin prior to derivatization. The analytical technique was applied to triacetin solutions containing as little as 10 micrograms/ml arbaprostil [15-(R)-methyl-PGE2].

Chemical Phenomena↗
PubMed1983

Isolation of pituitary peptides by reversed-phase high-performance liquid chromatography. Expansion of the resolving power of reversed-phase columns by manipulating pH and the nature of the ion-pairing reagent.

The reversed-phase liquid chromatographic behaviour of pituitary peptides at various pH values was compared. The retention times of peptides at pH 1, 3 and 4.5 were essentially as expected for acidic solvent systems. However, at pH 5.5 or 7, radically different elution characteristics were observed. This is probably brought about by the increase in polarity that accompanies ionization of the peptide carboxyl groups at higher pH values. Basic hydrophobic ion-pairing reagents, such as tetrabutylammonium ion, can be used to maximize differences in polarity. We have previously shown that the use of acidic hydrophobic ion-pairing reagents at low pH (e.g. trifluoroacetic and heptafluorobutyric acids) enhances the basic polarity of peptides. The combination of both approaches facilitated the reversed-phase purification of posterior pituitary glycopeptide and alpha-N-acetyl-beta-endorphin1-27 from an extract of 200 rat neurointermediate pituitaries. The separation of these two peptides at low pH, which is particularly difficult, was achieved by the sequential use of solvent systems containing trifluoroacetic acid (0.1%), heptafluorobutyric acid (0.13%) and finally triethylamine acetate (0.01 M, pH 5.5).

Animals↗
PubMed1986

Optimization by isochronal analysis. II. Changes in mobile phase velocity and temperature, and in mobile phase composition and temperature.

The theory of resolution optimization by isochronal analysis is further extended to the cases involving simultaneous changes in mobile phase velocity and column temperature, and in mobile phase composition and temperature. In each case, the resolution equation, when written in terms of isochronal conditions, shows that improvements can be obtained without sacrificing analysis time. Several subcases, which relate to the dependence of the selectivity factors on the changed parameters, are described. For each system, the contributions of the various parameters to the resolution are examined. The results of these discussions are plotted as resolution surfaces, on which lines of constant analysis time are indicated. The experimental routes that chromatographers must take in order to apply isochronal analysis are described.

Chemical Phenomena↗
PubMed2002

Effect of the mobile phase composition on the separation and detection of intact proteins by reversed-phase liquid chromatography-electrospray mass spectrometry.

Various buffers (ammonium acetate, ammonium formate, and ammonium hydrogencarbonate), acids (formic acid, acetic acid, heptafluorobutyric acid, and trifluoroacetic acid), and bases (ammonium hydroxide and morpholine) covering the range from 2 to 11.5 have been investigated for their performance in the separation of proteins by reversed-phase liquid chromatography (RPLC) and in their detection by electrospray mass spectrometry (ESI-MS). These additives were first tested for the detection of standard proteins by ESI-MS by flow-injection analysis (FIA). Those additives yielding the highest signals were employed for the separation of standard proteins by using three different reversed-phase columns: two C18 columns (4.6 mm I.D. and 2.1 mm I.D.) and one perfusion column (2 mm I.D.). The sensitivity of the LC-MS system was evaluated with the column giving the best results and with those LC eluents enabling the LC separation of the proteins and also yielding the highest MS signals. For that purpose, calibration curves were compared for both LC-MS and FIA-MS. Formic acid was the additive yielding the highest responses in FIA-MS and trifluoroacetic acid (TFA) gave the best separation and recovery of the proteins. However, problems related to poor recovery of the proteins in the column when formic acid was used and the significant signal suppression observed in MS when TFA was employed, made neither of them suitable for the sensitive detection of the proteins in LC-MS.

Chromatography, Liquid↗
PubMed2002

Comparison of solid-phase extraction and solid-phase microextraction for carbofuran in water analyzed by high-performance liquid chromatography-photodiode-array detection.

In this study a direct solid-phase microextraction (SPME) procedure has been developed for the determination of carbofuran in water. Experimental parameters such as selection of SPME coating, effect of temperature, effect of salt addition and solvent desorption were studied and optimized. Analytical parameters such as linearity, precision, detection and quantitation limits, and matrix effects for solid-phase extraction (SPE) and SPME methods were evaluated for comparison purposes with the aim of selecting the most appropriate depending on the detection capabilities required. SPE and SPME were followed by high-performance liquid chromatography with diode-array detection, using a 50 x 4.6 mm I.D. guard column and a 150 x 4.6 mm I.D. analytical column, both packed with C18 silica. Both methods can be applied to real samples and give the same results, but SPE allows the detection of lower carbofuran concentrations (0.06 microg/L) as compared to

Carbofuran↗
PubMed2002

Characterisation of the surface Lewis acid-base properties of the components of pigmented, impact-modified, bisphenol A polycarbonate-poly(butylene terephthalate) blends by inverse gas chromatography-phase separation and phase preferences.

Inverse gas chromatography was used to study the surface Lewis acid-base properties of the major components of pigmented, impact-modified, polycarbonate-poly(butylene terephthalate) blends. An investigation of the Lewis acid-base interactions in these polymeric systems has been carried out, based on the values determined for the surface Lewis acidity constant (Ka), surface Lewis basicity constant (Kb) and on the chemical and physical structure of the materials involved. This analysis provided the rationale for an interpretation of the phase separation and the phase preference that exist in these polymer blends, and of the consequences to their physical and mechanical properties.

Acids↗
PubMed2002

Optimization of solid-phase extraction and solid-phase microextraction for the determination of alpha- and beta-endosulfan in water by gas chromatography-electron-capture detection.

Water contamination due to the wide variety of pesticides used in agriculture practices is a global environmental pollution problem. The 98/83/European Directive requires to measure residues of pesticides at a target concentration of 1.0 microg/l in surface water and 0.1 microg/l in drinking water. In order to reach the level of detection required, efficient extraction techniques are required. Although solid-phase extraction (SPE) is the most common technique for isolation and concentration of pesticides from water, solid-phase microextraction (SPME) is being increasingly applied for this purpose. In this study, a direct-SPME procedure has been developed for the determination of alpha-endosulfan and beta-endosulfan in waters; experimental parameters such as selection of SPME coating, effect of temperature, effect of salt addition, optimization of the sample volume, adsorption and desorption profiles and desorption temperature were studied and optimized. Analytical parameters such as linearity, precision, detection and quantitation limits, and matrix effects for SPE and SPME methods were evaluated for comparison purposes with the aim of selecting the most appropriate for a certain application. Both extraction techniques, SPE and SPME, were followed by gas chromatography with electron-capture detector.

Chromatography, Gas↗
PubMed2003

Preparation of a polybutadiene stationary phase immobilized by gamma radiation for reversed-phase high-performance liquid chromatography.

Polybutadiene (PBD) has been immobilized on HPLC silica by gamma radiation doses in the range from 5 to 180 kGy. Columns prepared from these reversed-phase materials, as well as from similar non-irradiated materials, were tested with standard sample mixtures and characterized by elemental analysis (% C) and infrared spectroscopy. A low dose of 5 kGy is sufficient to produce a layer of immobilized PBD which functions as an efficient and stable stationary phase. Higher doses give thicker immobilized layers having less favorable chromatographic properties.

Butadienes↗
PubMed2003

Application of single-drop microextraction and comparison with solid-phase microextraction and solid-phase extraction for the determination of alpha- and beta-endosulfan in water samples by gas chromatography-electron-capture detection.

Water contamination due to the wide variety of pesticides used in agriculture practices is a global environmental pollution problem. The 98/83 European Directive requires the measurement of pesticides residues at a target concentration of 1.0 microg/l in surface water and 0.1 microg/l in drinking water. In order to reach the level of detection required, efficient extraction techniques are necessary. The application of a new extraction technique: single-drop microextraction (SDME), followed by gas chromatography with electron-capture detection, was assessed for determining alpha-endosulfan and beta-endosulfan in water samples. Experimental parameters which control the performance of SDME, such as selection of microextraction solvent and internal standard, optimization of organic drop volume, effects of sample stirring, temperature and salt addition, and sorption time profiles were studied. Once SDME was optimized, analytical parameters such as linearity, precision, detection and quantitation limits, plus matrix effects were evaluated. The SDME method was compared with solid-phase microextraction and solid-phase extraction with the aim of selecting the most appropriate method for a certain application.

Chromatography, Gas↗
PubMed2003

Studies on the separation of hydronium ion using surfactant-modified reversed-phase stationary phases and eluents containing an acidified electrolyte.

A further investigation of a chromatographic system allowing determination of hydrogen ions is reported. For this purpose an octadecylsilica column dynamically modified with sodium dodecylsulfate (SDS) or lithium dodecylsulfate (LDS) was used as stationary phase and a slightly acidified electrolyte (usually KCl)-SDS solution was used as the eluent. The concentration of SDS, KCl and the acidity of the eluent affected the structure of aggregates formed by the molecules of dodecylsulfate at the surface of the stationary phase. These aggregates of dodecylsulfate were found to be responsible for the appearance of a chromatographic peak attributed to the presence of H3O+ ions in a sample. Other cations in the sample could be separated in the same manner, permitting the simultaneous separation of monovalent cations from H3O+. The detection limit for H3O+ ions was 2.25 x 10(-6) M using an eluent comprising 0.3 mM LDS, 50.0 mM KCl and 0.10 mM H2SO4. The proposed method is shown to be applicable for the determination of free H3O+ ions in aqueous solutions of strong acids.

Acids↗
PubMed1999

Peptide separation in normal-phase liquid chromatography. Study of selectivity and mobile phase effects on various columns.

An experimental procedure for peptide separation by normal-phase liquid chromatography (NPLC) was proposed in previous papers. In the present study, the chromatographic behavior of amino, cyano, amide, diol and silica columns, which have been used in non-aqueous NPLC, is investigated anew. The amino column was not appropriate for peptide separation because of poor recovery. The cyano column could not be used due to lack of retention. The amide, diol and silica columns were useful for peptide separation. The chromatograms on amide, diol and silica columns were a little different when the mobile phase composition was changed. The recovery of peptides was good: diol > amide > silica. Repeatability and reproducibility using amide, diol and silica columns was satisfactory.

Amides↗
PubMed1999

Separation of selected peptides by capillary electroendoosmotic chromatography using 3 microns reversed-phase bonded silica and mixed-mode phases.

The retention behaviour and selectivity of selected basic, neutral and acidic peptides have been studied by capillary electroendoosmotic chromatography (CEC) with Hypersil C8, C18, Hypersil mixed-mode, and Spherisorb C18/SCX columns, 250 (335) mm x 100 microns, packed with 3 microns particles, and eluted with mobile phases composed of acetonitrile-triethylamine-phosphoric acid (TEAP) at pH 3.0 using a Hewlett-Packard Model HP3DCE capillary electrophoresis system. The selected peptides were desmopressin (D), two analogues (A and B) of desmopressin, oxytocin (O) and carbetocin (C). The peptides eluted either before or after the electroendoosmotic flow (EOF) marker, depending on the concentration of acetonitrile used and the buffer ionic strength. The retention and selectivity of these peptides under CEC conditions were compared to their behaviour in free zone capillary electrophoresis (CZE), where the separation mode was based on the electrophoretic migration of the analytes due to their charge and Stokes radius properties. In addition, their retention behaviour in RP-HPLC was also examined. As a result, it can be concluded that the elution process of this group of synthetic peptides in CEC with a TEAP buffer at pH 3.0 is mediated by a combination of both electrophoretic migration processes and retention mechanisms involving hydrophobic as well as silanophilic interactions. This CEC method when operated with these 3 microns reversed-phase and mixed-mode sorbents with peptides is thus a hybrid of two well-known analytical methods, namely CZE and RP-HPLC. However, the retention behaviour and selectivity of the selected peptides differs significantly in the CEC mode compared to the RP-HPLC or CZE modes. Therefore this CEC method with these peptides represents an orthogonal analytical separation procedure that is complimentary to both of these alternative techniques.

Chromatography↗
PubMed2000

Computer simulation for the convenient optimization of isocratic reversed-phase liquid chromatographic separations by varying temperature and mobile phase strength.

Software is described which allows the rapid development of separations by means of isocratic reversed-phase liquid chromatography (RP-LC), based on the optimization of column temperature (T) and mobile phase strength (%B). For a given sample, four initial experiments are carried out at two different temperatures, using either isocratic or (better) gradient elution. If isocratic experiments are chosen for computer simulation, it is necessary to select appropriate values of %B for these initial runs. Literature data for solute retention as a function of T are reviewed, as a basis for estimating suitable values of %B at the two values of T selected. The use of optimized values of T and %B led to acceptable separations for three representative samples. The prediction of isocratic separation on the basis of initial gradient experiments is more convenient than the use of initial isocratic experiments, but less reliable. When gradient experiments are used, one additional isocratic experiment can improve the accuracy of such predictions by a "reflection" procedure. The latter approach was confirmed for predictions of both isocratic and gradient separation from initial gradient experiments.

Chromatography, Liquid↗
PubMed2000

Retention mechanism, isocratic and gradient-elution separation and characterization of (co)polymers in normal-phase and reversed-phase high-performance liquid chromatography.

Synthetic (co)polymers or (co)oligomers with two (or more) repeating groups show not only molar mass distribution, but also composition and sequence distribution of the individual repeat units. To characterize such two- (or more-) dimensional distribution, liquid chromatography under "critical conditions" has been suggested, where the separation according to one type of repeating units is suppressed by balancing the adsorption and the size-exclusion effects. In present work it is shown that by combination of adequately selected separation conditions in normal-phase and in reversed-phase systems, the two-dimensional distribution mode can be adjusted to result in the separation following the distribution of any of the two repeat units in ethylene oxide-propylene oxide block (co)oligomers. Based on the retention mechanism suggested, prediction and optimization of the conditions for isocratic and gradient-elution separations of (co)oligomers is possible. HPLC-MS with atmospheric-pressure chemical ionization is a valuable tool for unambiguous identification of the individual (co)oligomers and their tracking in course of method development. Gradient elution can be used for the separation and characterization of block (co)oligomers of ethylene oxide (EO) and propylene oxide (PO) according to the number of the units in one block, while the separation according to the distribution of the units in the other block is suppressed. The effects of the arrangement of the individual EO and PO blocks in the block (co)oligomers (the sequence distribution) affects significantly the retention behavior and the selection of the optimum separation conditions.

Chromatography, High Pressure Liquid↗
PubMed2000

Kinetics of solid-phase extraction and solid-phase microextraction in thin adsorbent layer with saturation sorption isotherm.

The effects of sorbent saturation in thin adsorbent layers have been much overlooked in earlier research and should be taken into account in both the theory and practice of solid-phase extraction (SPE) and solid-phase microextraction (SPME). The adsorption kinetics of a single analyte into a thin adsorptive layer was modeled for several cases of agitation conditions in the analyzed volume. The extraction process in the adsorbent layer was modeled using a Langmuir isotherm approximated by the linear isotherm at low concentrations and by a saturation plateau at concentrations exceeding the critical saturation concentration. Laplace transformations were used to estimate the equilibration time and adsorbed analyte concentration profile for no agitation, practical and perfect agitation in the analyzed volume. The equilibration time may be significantly reduced at high degrees of oversaturation and/or agitation in the analyzed volume. The resulting models indicated that the adsorbent layer becomes saturated at some critical value of the oversaturation degree parameter. The critical value of the oversaturation parameter is affected by both the concentration of the analyte in the analyzed volume and the sorbent characteristics. It was also shown that the adsorption process is carried out via the propagation of the saturation adsorption boundary toward the inner boundary of the adsorbent layer. These new adsorption models should serve as "stepping stones" for the development of competitive adsorption kinetic models for both SPE and SPME, particularly in cases where fast sampling is used.

Chromatography, Liquid↗
PubMed1985

High-performance liquid chromatographic assay for hydroxychloroquine and metabolites in blood and plasma, using a stationary phase of poly(styrene divinylbenzene) and a mobile phase at pH 11, with fluorimetric detection.

A sensitive rapid high-performance liquid chromatographic assay for hydroxychloroquine and three major metabolites, desethylhydroxychloroquine, desethylchloroquine and bisdesethylchloroquine, has been developed. An ion-suppression separation technique is used with a macroporous polymer, poly(styrene divinylbenzene), stationary phase and a mobile phase of methanol--water with triethylamine, pH 11. A single ether extraction from alkalinized plasma or whole blood, with chloroquine added as an internal standard, is used. Using fluorescence detection, with excitation at 337 nm, the limit of sensitivity is 1 ng/ml. Some anti-inflammatory drugs which may be used concurrently for rheumatoid disorders are shown not to interfere with the assay. Samples of whole blood and plasma obtained from rheumatoid patients contained hydroxychloroquine and all three metabolites.

Anti-Inflammatory Agents↗
PubMed1989

Trace-level determination of 3'-azido-3'-deoxythymidine in human plasma by preconcentration on a silver (I)-thiol stationary phase with on-line reversed-phase high-performance liquid chromatography.

A method was developed for the determination of 3'-azido-3'-deoxythymidine (AZT) in plasma. The method is based on the trace enrichment of AZT on a pre-column packed with a silver-loaded thiol stationary phase at pH 11.6. On-line desorption to the reversed-phase liquid chromatographic system is performed by injecting a plug of 50 microliters of 1 M perchloric acid on the silver (I)-thiol pre-column. Two different sample pretreatment methods - protein precipitation with perchloric acid and on-line clean-up via a polymeric PRP-1 pre-column - were applied for the determination of AZT in human plasma. The latter method allows the direct injection of plasma samples into the analytical system and can therefore easily be automated. With both methods detection limits in the order of 10(-8) M AZT were obtained after preconcentration of 1.0 ml of plasma, using UV detection at 267 nm.

Autoanalysis↗
PubMed1988

Analysis of the aqueous phase of human cervical mucus by reversed-phase high-performance liquid chromatography and capillary isotachophoresis.

The aqueous phase of human cervical mucus was analysed by reversed-phase high-performance liquid chromatography (HPLC) and capillary isotachophoresis (ITP). With HPLC, seventeen ultraviolet-absorbing and eight fluorescent components and with ITP five anionic and four cationic components could be determined. The sample pre-treatment consisted of a simple ultrafiltration. Ten samples from fertile women and eleven samples from infertile women were analysed. In six samples from the infertile group higher median concentrations of several components were found. This may be an indication of disturbances in the biochemical processes of the cervical mucus of woman with fertility problems.

Cervix Mucus↗