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PubMed1992

Rate-dependent accessory pathway conduction due to phase 3 and phase 4 block. Antegrade and retrograde conduction properties.

Six patients who had antegrade phase 3 and phase 4 block in the accessory pathway were examined. In each patient, antegrade conduction over the accessory pathway was absent both at the sinus rate and at slower heart rates. During premature atrial stimulation a "window" of accessory pathway conduction was identified in all patients. The outer limits of the window ranged from 480 ms to 670 ms. The inner limits ranged from 410 ms to 620 ms. The durations of the window ranged from 20 ms to 160 ms. Three patients with orthodromic atrioventricular reentrant tachycardia showed preserved retrograde accessory pathway conduction. The remaining three patients had impaired retrograde accessory pathway conduction. One of the patients showed retrograde phase 4 block in the accessory pathway. In two patients, retrograde concealed conduction in the accessory pathway induced by ventricular stimulation prolonged the outer limit of the window in the antegrade accessory pathway conduction. These findings suggest that the mechanism of antegrade phase 3 and phase 4 block in the accessory pathway may be spontaneous diastolic depolarization in the accessory pathway and conduction disturbance at the ventricular and/or atrial insertion of the accessory pathway.

Adult↗

PubMed1984

The effect of phase structures on spatial phase discrimination.

This study was concerned with the discrimination of multifrequency gratings which differed only in the relative phase of one sinewave component (the test frequency) relative to the other components (the background frequencies). The dependent variable was the contrast of the test frequency required to discriminate between the two gratings. This study found that increasing the test frequency's relative phase difference from 10 to 90 deg significantly increased an observer's contrast sensitivity. However, no overall change in contrast thresholds was measured for test gratings having a relative phase difference between 90 to 180 deg. Moreover, when the mean relative phase was changed from 0 to 45 deg, contrast thresholds did not change when discriminating between gratings having a 90 deg relative phase difference. The type of processing most likely to account for these results in discussed.

Discrimination, Psychological↗

PubMed1991

Reversed-phase high-performance liquid chromatography of amphoteric beta-lactam antibiotics: effects of columns, ion-pairing reagents and mobile phase pH on their retention times.

The separation of five amino beta-lactam antibiotics by reversed-phase high-performance liquid chromatography was studied as an insight into their retention behaviour. These five amphoteric compounds are cephradine, cephalexine, cefaclor, ampicillin and amoxicillin. Both octadecylsilane-bonded silica (C18) columns and phenyl-bonded silica (phenyl) columns were used, with mobile phase pH values between 2.5 and 7.4. In the absence of ion-pairing reagents the retention times for all the five compounds were the shortest at pH 4-6. The phenyl column was found to improve the separation between cephradine and ampicillin at pH values lower than 3, when these two compounds appeared as fused peaks on the C18 on C18 columns, with mobile phases both with and without ion-pairing reagents, were compared. The addition of 0.005 or 0.02 M tetraethylammonium acetate to the mobile phase did not result in significant ion-pair formation, except at pH values higher than 5.5. A strong ion-pairing effect was obtained at pH values higher than 6 with 0.005 or 0.02 M tetrabutylammonium phosphate, and the retention was decreased at pH values lower than 4. On the other hand, 0.005 M heptanesulphonic acid exhibited an ion-pair retention effect at pH values lower than 5. The molecular structures and pK(a) values were used to account for the retention behaviour of these antibiotics in the various mobile phases.

Amoxicillin↗

PubMed1991

Changes of nucleotide patterns in liver, muscle and blood during the growth of Ehrlich ascites cells: application of the reversed-phase and ion-pair reversed-phase high-performance liquid chromatography with radial compression column.

The pool of purine compounds was analysed in liver, skeletal muscle and blood of mice during the growth of Ehrlich ascites tumour cells. Three fast isocratic high-performance liquid chromatographic methods were used. (1) Determination of nucleotides by an isocratic ion-pair reversed-phase chromatography with a 10 mM ammonium phosphate buffer containing acetonitrile and tetrabutylammonium phosphate. (2) Separation of nucleosides and nucleobases in cell extracts by a reversed-phase system with methanol and 50 mM potassium phosphate buffer as eluent. (3) Nucleosides and nucleobases in body fluids were analysed by a reversed-phase system with 10 mM potassium phosphate containing methanol. These methods allow the rapid determination of purine compounds in small biological samples from various cell types and body fluids, with high accuracy and sensitivity. The pool of cellular nucleotides increased during the exponential phase of tumour growth. Adenosine accumulated significantly in all tissues in the stationary phase of tumour growth.

Animals↗

PubMed1995

Evaluation of 2-iminoimidazolidin-4-one and thymine as respective internal standards for normal-phase and reversed-phase high-performance liquid chromatographic determination of creatinine in human serum.

We evaluated two internal standards for HPLC determination of creatinine in human serum after ultrafiltration: 2-iminoimidazolidin-4-one for normal-phase HPLC on aluminium oxide, and thymine for C18 reversed-phase HPLC. Detection of 2-iminoimidazolidin-4-one was done at the same wavelength as that used for creatinine, i.e. 240 nm. For thymine, the wavelength was switched to 280 nm. The suitability of the selected compounds to serve as an internal standard in the described measurement procedures, including ultrafiltration of serum, was evaluated from the precision and accuracy obtained. The method based on normal-phase HPLC with 2-iminoimidazolidin-4-one showed an imprecision expressed as R.S.D. ranging from 0.8 to 3.4% (mean: 2.1%) and an inaccuracy, calculated from the deviations from target values determined by isotope-dilution gas chromatography-mass spectrometry, ranging from -1.3 to +1.8% (mean: +0.4%). For the reversed-phase HPLC procedure with thymine, the imprecision ranged from 0.3 to 1.3% (mean: 1.0%) and the inaccuracy from +0.1 to +3.9% (mean: +1.7%). The occasional observation of interferences with 2-iminoimidazolidin-4-one limited the application of the normal-phase method to a certain extent.

Chromatography, High Pressure Liquid↗

PubMed1989

Determination of enantiomeric drugs in physiological fluids using on-line solid phase derivatizations and reversed-phase liquid chromatography.

A new analytical approach has been developed for the determination of d,l-amphetamine in urine using on-line solid phase derivatizations in HPLC-UV/FL. Several other enantiomeric drugs were also investigated using the same method. The method was validated by several experiments, including: (1) kinetic studies for the reaction of each enantiomeric drug with the solid phase chiral reagent; (2) "single blind spiking" experiments; and (3) polarimetry for confirmation of the enantiomeric composition determined by the solid phase diastereomer formation-HPLC-UV/FL method. The resulting diastereomers were well resolved (Rs = 1.05-1.40) under typical reversed-phase HPLC conditions. Enantiomeric contamination at the 1.1% level could be detected. The lowest amount of d,l-amphetamine that could be simultaneously derivatized and detected was about 50 ng ml-1. The linearity of the overall measurement was 3-4 orders of magnitude. d,l-Amphetamine spiked into urine at different concentration levels and different d,l-ratios, only followed by filtration, were directly injected into the on-line solid phase derivatization-HPLC-UV/FL system for quantitation with relative standard deviations 1.8-6.4% and relative errors 0.6-9.8%.

Amphetamine↗

PubMed1994

Solid-phase extraction of small drugs on apolar and ion-exchanging silica bonded phases: towards the development of a general strategy.

In connection with the development of a general strategy for solid-phase extraction (SPE) of drugs, the use of the apolar octadecyl silica bonded phase and ion-exchanging phases with a benzene sulphonic acid or quaternary amine bonded functionality is investigated for the SPE of small polar drugs. This investigation was performed on a set of 15 drugs, belonging to varying pharmacological groups and with varying structures. For each analyte, its adsorption on the C18 and the ion-exchanging phase was controlled for an aqueous solution and for a spiked plasma sample. For those analytes retained on the sorbent, different elution solvents were compared. Although SPE methods could successfully be developed for some drugs, no general solution can be proposed and no solution was found for a few drugs. The main problems are that for these few drugs no sufficient retention is obtained on any SPE phase investigated or the selectivity is too low.

Amines↗

PubMed1996

An investigation of the properties of a "shielded" phase for the solid phase extraction of acidic and basic compounds.

The extraction properties of Hisep, a novel bonded phase for solid phase extraction, have been investigated using a number of acidic and basic analytes and compared with those of a conventional ODS bonded material. Marked differences in extraction properties for the test analytes propranolol, anisic acid and ICI 128436 from aqueous buffer were noted between the two materials. Thus, extraction onto the ODS material was by both ionic and reversed-phase mechanisms whilst retention on the Hisep phase appeared to be mediated predominantly by reversed-phase hydrophobic interactions. The presence of dog plasma had no effect on the extraction of propranolol but did reduce the extraction efficiencies observed for anisic acid and ICI 128436 in a pH dependent manner.

Adrenergic beta-Antagonists↗

PubMed2006

Circadian rhythm abnormalities in the acute phase of cerebral infarction correlate with poor prognosis in the chronic phase.

The aim of this study was to determine circadian rhythm abnormalities in patients with acute cerebral infarction and their relationship with prognosis in the chronic phase. The subjects were 50 patients who were admitted within 48 h of onset. Rectal temperature and wrist motor activity were recorded during the acute phase. Time-series data were assayed by MEM spectral analysis. Functional prognosis (degree of handicap) was assessed at 3 months after stroke onset by the modified Rankin Scale. Ambulatory patients tended to exhibit circadian rhythm in rectal temperature and/or wrist activity rhythms, whereas non-ambulatory patients did not (P < 0.001) in the acute phase of ischemic stroke. Our findings suggest the biological rhythms of rectal temperature and wrist activity during the acute stroke phase might be a possible prognostic indicator of their chronic phase.

Aged↗

PubMed2006

Lipoplexes formed from sugar-based gemini surfactants undergo a lamellar-to-micellar phase transition at acidic pH. Evidence for a non-inverted membrane-destabilizing hexagonal phase of lipoplexes.

The present study aims at a better understanding of the mechanism of transfection mediated by two sugar-based gemini surfactants GS1 and GS2. Previously, these gemini surfactants have been shown to be efficient gene vectors for transfection both in vitro and in vivo. Here, using Nile Red, a solvatochromic fluorescent probe, we investigated the phase behavior of these gemini surfactants in complexes with plasmid DNA, so-called lipoplexes. We found that these lipoplexes undergo a lamellar-to-non-inverted micellar phase transition upon decreasing the pH from neutral to mildly acidic. This normal (non-inverted) phase at acidic pH is confirmed by the colloidal stability of the lipoplexes as shown by turbidity measurements. We therefore propose a normal hexagonal phase, H(I), for the gemini surfactant lipoplexes at acidic endosomal pH. Thus, we suggest that besides an inverted hexagonal (H(II)) phase as reported for several transfection-potent cationic lipid systems, another type of non-inverted non-bilayer structure, different from H(II), may destabilize the endosomal membrane, necessary for cytosolic DNA delivery and ultimately, cellular transfection.

DNA↗

PubMed2004

Developments in the use of soluble ionic polymers as pseudo-stationary phases for electrokinetic chromatography and stationary phases for electrochromatography.

This article reviews the development, characterization and application of soluble ionic polymeric materials as pseudo-stationary phases for electrokinetic chromatography and as stationary phases for electrochromatography since 1997. Polymeric pseudo-stationary phases for electrokinetic chromatography, including cationic polymers, anionic siloxane and acrylamide polymers, polymerized surfactants (micelle polymers), and chiral polymers are reviewed. Also reviewed are suspended molecularly imprinted polymer micro-particles. Application of polymeric pseudo-stationary phases with electrospray ionization mass spectrometric detection is presented. Recent progress in the development and characterization of physically adsorbed stationary phases for electrochromatography using polymers of the same or similar chemistry is also reviewed.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed2004

Liquid chromatographic enantiomer separations of novel quinazolone derivatives on quinine carbamate based chiral stationary phases using hydro-organic mobile phases.

Quinine carbamate-type weak chiral anion-exchange selectors (SOs) and the respective chiral stationary phases (CSPs) have been used for the direct liquid chromatographic enantiomer separation of a wide range of chiral acids. In the present work, we demonstrate that these CSPs can also be extended to chiral discrimination of a set of neutral polar potential NMDA (N-methyl-D-aspartic acid) and/or AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) antagonist imidazo-quinazoline-dione derivatives (selectands, SAs) using acetonitrile and methanol containing hydro-organic and buffered mobile phases. The influence of mobile phase composition, column temperature and structure variation of the SAs and SOs on retention and enantioselectivity was systematically investigated to gain insight into the overall chiral recognition mechanism. As was expected for the reversed-phase mode, acetonitrile has a stronger eluotropic effect compared to methanol. Except for two analytes, the acetonitrile containing mobile phases provided baseline resolution (R(S)) of the enantiomers with R(S) values ranging between 1.68 and 2.76. Using methanol as the organic modifier enhanced the enantioselectivity. The enthalpic and entropic terms for the SO-SA association were calculated from the linear van't Hoff plots. Data reveal that the enantiomer separations are predominantly enthalpically driven.

Buffers↗

PubMed2005

Molecular simulation study of the bonded-phase structure in reversed-phase liquid chromatography with neat aqueous solvent.

The dramatic loss of retention in reversed-phase liquid chromatography when switching to 100% aqueous solvent and stopping flow (depressurizing) has long intrigued separation scientists. Recent experimental evidence suggests that the observed loss of retention is due to the loss of pore wetting with subsequent loss of solvent penetration in the porous matrix. One of the prevalent explanations of this phenomenon has been that the bonded phase chains, typically octadecyl silane bound to porous silica, would undergo significant conformational changes, viz. collapse, under pure aqueous conditions. As a definitive means toward elucidating the conformation of bonded-phase chains under pure aqueous conditions, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for a system of dimethyl octadecyl silane of intermediate coverage bound to the (111) face of beta-cristobalite and immersed in pure water. The results of two sets of simulations, which were started from two entirely different starting configurations as a validity check toward reaching the same equilibrium distribution of states, show that chains are neither clustering together nor laying on the surface but rather have a broad distribution of orientations and of conformational states. The interface between the bonded and solvent phases is rough on a molecular level, and clusters of water molecules are sometimes found to adsorb at the silica surface. This computational study lends further evidence that the driving force for the loss of retention when switching to pure aqueous conditions and depressurizing is not the collapse of bonded-phase chains.

Alkanes↗

PubMed2005

Temperature-responsive stationary phase utilizing a polymer of proline derivative for hydrophobic interaction chromatography using an aqueous mobile phase.

A new method of chromatography is proposed, utilizing a thermo-responsive polymer carrying an amino acid ester residue for the stationary phase of high-performance liquid chromatography (HPLC). We have been investigating the new concept of chromatography, a temperature-responsive chromatography, using temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm)-modified surface for HPLC with a constant aqueous media as the mobile phase. In this study, we designed and synthesized thermo-responsive poly(acryloyl-L-proline methyl ester) and its copolymer with N-isopropylacrylamide (NIPAAm). Homopolymers of acryloyl-L-proline methyl ester and copolymer were prepared by the reaction of radical telomerization. These polymers underwent a reversible phase transition from water-soluble forms into aggregates by changing the temperature, similar to PNIPAAm. The surface properties and functions of stationary phases modified with poly(acryloyl-L-proline methyl ester) were controlled by the external temperature. In the chromatographic system, we separated steroids and amino acids with a variety of hydrophobicities using a sole aqueous mobile phase. In contrast to a PNIPAAm-modified surface, a poly(acryloyl-L-proline methyl ester)-modified surface showed a greater affinity for hydrophobic amino acids.

Chromatography, Liquid↗

PubMed2005

Two-dimensional reversed-phase liquid chromatography using two monolithic silica C18 columns and different mobile phase modifiers in the two dimensions.

Comprehensive two-dimensional (2D) HPLC in the reversed-phase liquid chromatography (RPLC) mode using C18 silica monolith columns at first dimension (1st-D) (10 cm x 4.6mm I.D.) and second dimension (2nd-D) (5 cm x 4.6mm I.D.) was carried out successfully. A mixture of water and tetrahydrofuran (THF) was used as a mobile phase in the 1st-D separation, and a mixture of water and methanol (CH3OH) in the 2nd-D separation. Sample fractions from 1st-D column were directly loaded into an injection loop of the 2nd-D HPLC equipped with two injector valves for one column. The fractionation time at the 1st-D that was equal to the separation time at the 2nd-D was 45 or 60s. Total peak capacity up to 900 was obtained in about 60 min for the isocratic mode separation of aromatic compounds in this system. Gradient elution mode applied to both 1st-D and 2nd-D separations resulted in shorter separation time and better separation efficiencies than the isocratic mode. It was demonstrated that 2D-HPLC systems employing popular C18 stationary phases with different organic modifiers in mobile phases for each dimension could produce large peak capacity. The different selectivities were provided by the difference in polar interactions between a solute and the organic modifier existing in the stationary phase.

Chromatography, High Pressure Liquid↗

PubMed2006

Establishing the feasibility of coupled solid-phase extraction-solid-phase derivatization for acidic herbicides.

The significance of this research is that it improves analytical methodology used for organic chemicals in aqueous solutions by establishing the feasibility of heterogeneous chemical derivatization at the liquid-solid interface (i.e., solid-phase reaction or solid-phase derivatization). A solid-phase derivatization method for determining chlorinated herbicide acids was developed. Solid-phase extraction was used to concentrate and retain analytes on sorbents for subsequent solid-phase derivatization. Background interferences were removed from the chromatograms by electronically subtracting the responses of blank, nonfortified analyses from spiked samples. Two extraction sorbents (octadecyl bonded silica and polystyrene-divinylbenzene) and two derivatizing reagents (BF3-MeOH and trimethylsilyldiazomethane) were investigated. Recovery of 13 chlorinated herbicide acids--including pentachlorophenol, dinoseb, and bentazon (having a derivatizable functional group, -OH or -NH, bonded directly to a phenyl group); dicamba, picloram, acifluorfen, 3,5-dichlorobenzoic acid, and dacthal (having a derivatizable functional group, -COOH, bonded directly to a phenyl group); and 2,4-D, 2,4,5-T, dichlorprop, 2,4,5-TP, and 2,4-DB (having a derivatizable functional group, -COOH, bonded directly to a sp3 carbon atom)--was tested. The analytical method developed was proven successful for determining acidic herbicides, except for the dacthal diacid metabolite, in aqueous samples.

Acids↗

PubMed2004

Free energy balance for three fluid phases in a capillary of arbitrarily shaped cross-section: capillary entry pressures and layers of the intermediate-wetting phase.

In this work we derive rigorously the free energy balance for three fluid phases in a straight capillary of arbitrarily shaped cross-section. This balance is then used to derive the general equation for the capillary entry pressures of all possible two-phase and three-phase displacements. Moreover, the equation provides the criterion determining the existence of layers of the intermediate-wetting phase separating the wetting and non-wetting phases in the corners or cavities of a capillary, by also treating the spreading of such layers as a capillary displacement. For a number of combinations of interfacial tensions and contact angles, illustrating all the different relevant situations, we calculate the criteria for spreading of such a layer in the corner of a capillary with polygonal cross-section. In a capillary with a cross-section in the shape of an isosceles triangle of varying corner size, these criteria are used to determine the unique capillary entry pressures for piston-like displacement from alternative solutions of the general equation. These solutions relate to displacements in the presence or absence of layers in the various differently sized corners.

Journal Article↗

PubMed2005

Correspondence between spin-dynamic phases and pulse program phases of NMR spectrometers.

Spin state selective experiments have become very useful tools in solution NMR spectroscopy, particularly in the context of TROSY line narrowing. However, the practical implementation of such pulse sequences is frequently complicated by unexpected instrument behavior. Furthermore, a literal theoretical analysis of sequences published with specific phase settings can fail to rationalize such experiments and can seemingly contradict experimental findings. In this communication, we develop a practical approach to this ostensible paradox. Spin-dynamic design, rationalization, and simulation of NMR pulse sequences, as well as their confident and reliable implementation across current spectrometer hardware platforms, require precise understanding of the underlying nutation axis conventions. While currently often approached empirically, we demonstrate with a simple but general pulse program how to uncover these correspondences a priori in the general case. From this, we deduce a correspondence table between the spin-dynamic phases used in NMR theory and simulation on the one hand and pulse program phases of current commercial spectrometers on the other. As a practical application of these results, we analyze implementations of the original (1)H-(15)N TROSY experiment and illustrate how steady-state magnetization can be predictably, rather than empirically, added to a desired component. We show why and under which circumstances a literal adoption of phases from published sequences can lead to incorrect results. We suggest that pulse sequences should be consistently given with spin-dynamically correct (physical) phases, rather than in spectrometer-specific (software) syntax.

Carbon Isotopes↗