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At least 415 records · Page 23Linked to original sources

PubMed2006

Direct electron transfer between hemoglobin and pyrolytic graphite electrodes enhanced by Fe(3)O(4) nanoparticles in their layer-by-layer self-assembly films.

Alternate adsorption of negatively charged Fe(3)O(4) nanoparticles from their pH 8.0 aqueous dispersions and positively charged hemoglobin (Hb) from its pH 5.5 buffers on solid substrates resulted in the assembly of {Fe(3)O(4)/Hb}(n) layer-by-layer films. Quartz crystal microbalance (QCM), UV-vis spectroscopy, and cyclic voltammetry (CV) were used to monitor and confirm the film growth. A pair of well-defined, nearly reversible CV peaks for HbFe(III)/Fe(II) redox couples was observed for {Fe(3)O(4)/Hb}(n) films on pyrolytic graphite (PG) electrodes. Although the multilayered films grew linearly with the number of Fe(3)O(4)/Hb bilayers (n) and the amount of Hb adsorbed in each bilayer was generally the same, the electroactive Hb could only extend to 6 bilayers. This indicates that only those Hb molecules in the first few bilayers closest to the electrode surface are electroactive. The electrochemical parameters such as the apparent heterogeneous electron transfer rate constant (k(s)) were estimated by square wave voltammetry (SWV) and nonlinear regression. The Soret absorption band position of Hb in {Fe(3)O(4)/Hb}(6) films showed that Hb in the films retained its near native structure in the medium pH range. The {Fe(3)O(4)/Hb}(6) film electrodes also showed good biocatalytic activity toward reduction of oxygen, hydrogen peroxide, trichloroacetic acid, and nitrite. The electrochemical reduction overpotentials of these substrates were lowered significantly by {Fe(3)O(4)/Hb}(n) films.

Animals↗

PubMed2006

Adsorption of synthetic homo- and hetero-oligodeoxynucleotides onto highly oriented pyrolytic graphite: atomic force microscopy characterization.

DNA adsorption on electrode surfaces is of fundamental interest for the development of DNA-based biosensors. The free adsorption of 10-mer synthetic oligodeoxynucleotides (ODNs) onto highly oriented pyrolytic graphite (HOPG) surfaces was studied using Magnetic AC mode atomic force microscopy (MAC Mode AFM). The mechanism of interaction of nucleic acids with carbon electrode surfaces was elucidated, using 10-mer synthetic homo- and hetero-ODNs sequences of known base sequences, because they allow clear interpretation of the experimental data. AFM images in air revealed different adsorption patterns and degree of HOPG surface coverage for the ODNs, and correlation with the individual structure and base sequence of each ODN molecule will be presented. The results demonstrated that the hydrophobic interactions with the HOPG hydrophobic surface explain the main adsorption mechanism, although other effects such as electrostatic and Van der Waals interactions may contribute to the free adsorption process. The ODNs interacted differently with the HOPG surface, according to the ODN sequence hydrophobic characteristics, being directly depending on the molecular mass, the hydrophobic character of the individual bases and on the secondary structure of the molecule. The importance of the type of base existent at the ODN chain extremities on the adsorption process was investigated and different adsorption patterns were obtained with ODN sequences composed by the same group of bases aligned in a different order.

Adsorption↗

PubMed2004

Comparative analysis of enzymatically digested kappa-carrageenans, using liquid chromatography on ion-exchange and porous graphitic carbon columns coupled to an evaporative light scattering detector.

Enzymatically digested kappa (A-G4S)-carrageenans, apart from their biological activities in plants, could be used as 'model' molecules to elucidate potential problems in nuclear magnetic resonance spectroscopy of carrageenans. Thus, oligosaccharides obtained from kappa-carrageenan by enzymatic digestion using kappa-carrageenase have been separated on silica and polymeric based ion-exchange and porous graphitic carbon (PGC) columns, coupled to an evaporative light scattering detector. Oligomers were separated on ion-exchange columns using a gradient of ammonium acetate as a developing ion, while analysis on PGC column presented an additional adjacent peak next to each main one, using a gradient of ammonium acetate in water/acetonitrile as a mobile phase. The phenomenon can be attributed to different retention mechanisms that govern the PGC surface. Furthermore, it has been demonstrated that acetonitrile can regulate the selectivity between the peaks raising hopes for preparative chromatography.

Carrageenan↗

PubMed2004

Saccharose effects on surface association of phenol derivatives with porous graphitic carbon.

In this paper, the effect of saccharose on the association of phenol derivatives on both the porous graphitic carbon (PGC) surface and the C18 stationary phase and for two methanol fractions (v/v) in the mobile phase is described. A novel approach based on an extended Langmuir distribution isotherms was used. The results demonstrated that: (i) the saccharose can be adsorbed on the PGC surface; (ii) the phenol derivatives can be associated with saccharose adsorbed on the PGC surface; and (iii) the saccharose do not interact with the C18 stationary phase. This was confirmed by the thermodynamic data and the Wyman equation parameters.

Chromatography, High Pressure Liquid↗

PubMed2004

Retention characteristics of porous graphitic carbon in subcritical fluid chromatography with carbon dioxide-methanol mobile phases.

Numerous relationships usually used in high-performance liquid chromatography (HPLC) for describing the retention on porous graphitic carbon (PGC) have been applied in subcritical fluid chromatography, with CO2-methanol mobile phases. As reported in HPLC, octanol-water partition coefficient failed to fit the retention, whereas satisfactory results were obtained with the sum of partial negative charges. A better fit was reached by using the solvation parameter model, allowing a better understanding of the interactions developed between the solute, the stationary and the mobile phases. Results show that the dominant contribution to retention was given by the polarizability (E) and the volume (V), while the hydrogen-bond basicity (B) was not selected in the retention model, whatever the methanol content. The increase in methanol percentage favours the retention decrease, mainly through the volume for hydrophobic compounds, and through the hydrogen-bond acidity for polar compounds.

Carbon Dioxide↗

PubMed2005

Modelling of ceramide interactions with porous graphite carbon in non-aqueous liquid chromatography.

Interactions of solutes on porous graphitic carbon (PGC) with non-aqueous mobile phases are studied by the linear solvation energy relationship (LSER). Studies have been carried out with eight binary mixtures composed of a weak solvent (acetonitrile or methanol) and a strong solvent (tetrahydrofuran, n-butanol, CH2Cl2, 1,1,2-trichloro-2,2,1-trifluoroethane). The systematic analysis of a set of test compounds was performed for each solvent mixture in isocratic mode (50:50). The results were compared to those obtained on PGC with hydro-organic liquids and supercritical fluids. They were then correlated with the observed retention behaviour of lipid compounds, more particularly ceramides.

Ceramides↗

PubMed2005

Effects of modifiers in subcritical fluid chromatography on retention with porous graphitic carbon.

The effect of different modifiers in subcritical fluid chromatography (SubFC) on interactions between solute and porous graphitic carbon (PGC) and between solute and carbon dioxide-modifier mobile phases was studied by the use of linear solvation energy relationships (LSERs). This study was performed to allow efficient optimization of the composition of the carbon dioxide-modifier mobile phase in regard of the chemical nature of the solutes to be separated. With all modifiers tested (methanol, ethanol, n-propanol, isopropanol, acetonitrile, tetrahydrofuran and hexane), the solute/stationary phase interactions are greater than the solute/mobile phase ones. Dispersion interactions and charge transfer between electron donor solute and electron acceptor PGC mainly explain the retention on this surface, whatever the modifier. These interactions are quite constant over the range of modifier percentage studied (5-40%). For acidic compounds, the retention variation is mainly related to the change in the basic character of mobile and stationary phase due to the variation of modifier percentage. Changes in eluting strength are mostly related to adsorption of mobile phase onto the PGC with methanol and acetonitrile, and to the increase of dispersion interactions between the solute and the mobile phase for other modifiers. Relationships between varied selectivities and solvation parameter values have been studied and are discussed in this paper.

Chromatography, Liquid↗

PubMed2005

Determination and characterization of organic explosives using porous graphitic carbon and liquid chromatography-atmospheric pressure chemical ionization mass spectrometry.

A new LC-MS method for the determination and characterization of three groups of commonly used organic explosives (nitroaromatic compounds, cyclic nitroamines and nitrate esters) was developed using a porous graphitic carbon (PGC) (Hypercarb) column. Twenty-one different explosive-related compounds--including 2,4,6-trinitrotoluene, its by-products and its degradation products--were chromatographically separated in a single analysis. This efficient separation facilitates the identification of the manufacturer of the explosive using the identified analytes as a fingerprint. A final, conclusive identification of the analytes can be obtained using LC-MS equipped with an atmospheric pressure chemical ionization (APCI) interface. Solvent effects on chromatographic behaviour were investigated, as were the effects of solvent mixtures and mobile phase additives. The number and the relative positions of the nitro groups within analyte molecules influence their order of elution; these effects were investigated. The data thus generated can be interpreted to support a hypothesis concerning the retention mechanism of nitro-containing compounds when using PGC. Limits of detection ranged from 0.5 to 41.2 ng. The new methodology described herein improves the sensitivity and selectivity of explosive detection. The effectiveness of the method is demonstrated by the analysis of soil samples containing explosives residue from test fields in Sweden and Afghanistan.

Atmospheric Pressure↗

PubMed2005

Separation of substituted aromatic isomers with porous graphitic carbon in subcritical fluid chromatography.

The ability of porous graphitic carbon (PGC) to separate structural isomers has been reported in high-performance liquid chromatography (HPLC). This paper presents studies carried out in subcritical fluid chromatography (SubFC). Various polar and nonpolar modifiers were added to the carbon dioxide mobile phase, in proportions ranging from 5 to 40%. The effects of both the nature and the percentage of the modifier on aromatic isomer separations were studied. Two types of selectivity behaviour appear. The first one, related to steric recognition, is due to the number of contact points between the compounds and the flat surface of PGC. In this case, retention orders are often identical to that reported in HPLC. The second is related to the favourable interaction between the polar moieties of the solutes and the stationary phase. In this case, the retention and selectivity strongly depend on the mobile phase composition. Thus, the separations obtained are greatly enhanced, compared to those obtained in HPLC. The retention and selectivity variations observed when the composition of the mobile phase is changed are discussed based on linear solvation energy relationships (LSERs). Practical applications are presented, namely benzene, toluene, ethylbenzene and xylenes (BTEX) and flavour molecules separations.

Chromatography, Liquid↗

PubMed2006

Simultaneous quantification of reduced and oxidized glutathione in plasma using a two-dimensional chromatographic system with parallel porous graphitized carbon columns coupled with fluorescence and coulometric electrochemical detection.

A method for the simultaneous quantification of reduced and oxidized glutathione in human plasma employing a two-dimensional chromatographic system with parallel porous graphitized carbon (PGC) columns coupled with fluorescence (FLD) and coulometric electrochemical detection (ED) has been developed. Post-sampling oxidation of reduced glutathione (GSH) was prevented by derivatizing the -SH group with monobromobimane (MBB) and the glutathione-bimane adduct (GSMB) was detected by FLD. Oxidized glutathione (GSSG) was detected by ED optimized to give lowest possible limits of detection (LOD). The method is fully validated and is currently used for determination of GSH, GSSG and its redox potential in different clinical studies.

Anticoagulants↗

PubMed2006

Limitations of porous graphitic carbon as stationary phase material in the determination of catecholamines.

A fast and sensitive capillary liquid chromatography (cLC) column-switching method with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) detection for the simultaneous determination of dopamine (D), epinephrine (E), norepinephrine (NE) and serotonin (SE) was pursued. A sample volume of 100 microl was loaded with a mobile phase containing 0.1% pentafluoropropionic acid (PFPA) as ion-pairing agent on a 25 mm x 0.32 mm (i.d.) 5 microm Hypercarb column. A water-acetonitrile (AcN) gradient with 0.1% acetic acid (AcOH) backflushed the compounds onto a 34 mm x 0.32 mm (i.d.) 5 microm Hypercarb analytical column. However, during a series of analyses, oxidation of the catecholamines (CAs) was observed. This was suspected to be due to the loading mobile phase composition and precluded the usefulness of this method even though the achievable detection limit was in the range of 0.75-3.0 ng/ml. The combination of the porous graphitic carbon (PGC) material and the fluorinated strong acids which were required to get enough retention for preconcentration of large volumes cannot be used for easily oxidized compounds as the CAs.

Catecholamines↗

PubMed2006

Evaluation of reversed phase columns designed for polar compounds and porous graphitic carbon in "trapping" and separating neurotransmitters.

Quantification of neurotransmitters as biologically active analytes in neurological samples is of high interest for studying their effect on multiple targets. This work is part of a strategy involving two-dimensional liquid chromatography (2D LC) system with mass spectrometry (MS) detection. The concept of the on-line LC system is the coupling of reversed phase liquid chromatography (RPLC, the second separation dimension) to ion-exchange chromatography (IEC, the first dimension). Our objective in this study is to find the appropriate second dimension column, ensuring that samples of neurotransmitters are refocused and separated on it. Silica-based columns designed specifically to retain polar compounds were tested in LC conditions and compared with results obtained with a porous graphitic carbon (PGC, Hypercarb) column. These polar embedded, polar endcapped, and high-density alkyl chain columns successfully separated analytes in question using mobile phase systems with high percentage of water, or even pure water. Only Hypercarb column provided efficient retention of the most polar neurotransmitters and could be used for trapping and preconcentrating the compounds without rapid breakthrough.

Chromatography, Ion Exchange↗

PubMed2006

Mechanical and physical properties of carbon-graphite fiber-reinforced polymers intended for implant suprastructures.

OBJECTIVES: Mechanical properties and quality of fiber/matrix adhesion of poly(methyl methacrylate) (PMMA)-based materials, reinforced with carbon-graphite (CG) fibers that are able to remain in a plastic state until polymerization, were examined. METHODS: Tubes of cleaned braided CG fibers were treated with a sizing resin. Two resin mixtures, resin A and resin B, stable in the fluid state and containing different cross-linking agents, were reinforced with CG fiber loadings of 24, 36, and 47 wt% (20, 29, and 38 vol.%). In addition, resin B was reinforced with 58 wt% (47 vol.%). After heat-polymerization, flexural strength and modulus were evaluated, both dry and after water storage. Coefficient of thermal expansion, longitudinally and in the transverse direction of the specimens, was determined. Adhesion between fibers and matrix was evaluated with scanning electron microscopy (SEM). RESULTS: Flexural properties and linear coefficient of thermal expansion were similar for both fiber composites. With increased fiber loading, flexural properties increased. For 47 wt% fibers in polymer A the flexural strength was 547.7 (28.12) MPa and for polymer B 563.3 (89.24) MPa when water saturated. Linear coefficient of thermal expansion was for 47 wt% CG fiber-reinforced polymers; -2.5 x 10(-6) degrees C-1 longitudinally and 62.4 x 10(-6) degrees C-1 in the transverse direction of the specimens. SEM revealed good adhesion between fibers and matrix. More porosity was observed with fiber loading of 58 wt%. CONCLUSIONS: The fiber treatment and the developed resin matrices resulted in good adhesion between CG fibers and matrix. The properties observed indicate a potential for implant-retained prostheses.

Carbon↗

PubMed2004

Analysis of catecholamines and related substances using porous graphitic carbon as separation media in liquid chromatography-tandem mass spectrometry.

Capillary porous graphitic carbon (PGC) columns have been utilized for separation of several catecholamines and related compounds (i.e. L-tyrosine, L-DOPA, 3-O-methyl-DOPA, dopamine, 3,4-dihydroxy-phenyl-acetic acid (DOPAC), homovanillic acid, noradrenaline, vanillomandelic acid and adrenaline) on-line with electrospray ionization tandem mass spectrometry (ESI-MS/MS). The use of a mobile phase without ion-pairing agents and with high content of organic modifier facilitated the coupling to the selective and sensitive mass spectrometric detection. Minimum detectable sample concentration (MDC sample) for noradrenaline, dopamine and L-tyrosine in a standard solution was estimated to 3, 10 and 30 nM, respectively (3 S/N corresponds to MDQ for L-tyrosine of approximately 8 x 10(-14)mol). The developed strategy was applied for analysis of brain tissue, i.e. a substantia nigra (ns) sample.

3,4-Dihydroxyphenylacetic Acid↗

PubMed2005

Porous graphitic carbon chromatography-tandem mass spectrometry for the study of isoprostanes in human cerebrospinal fluid.

F2-isoprostanes are produced by the non-enzymatic peroxidation of arachidonic acid in membrane phospholipids. This paper describes a new method for the determination of all four classes of F2-isoprostanes in human cerebrospinal fluid (CSF) involving separation on a 1 mm x 150 mm porous graphitic carbon (PGC) column and detection by triple quadrupole mass spectrometry in negative-ion electrospray mode. The sample pre-treatment consisted of an ultrafiltration step, following which 300 microl of CSF sample could be injected directly onto a 1 mm x 10 mm PGC guard column functioning as a trap for the analytes. The loading solvent was Milli-Q water at 125 microl/min. After 3 min, the sample was switched into the separation column. The F2-isoprostanes were separated in 20 min using a linear solvent gradient comprising water, methanol, acetonitrile and ammonium hydroxide at a pH of 9.5 and a flow of 50 microl/min The limit of detection (calculated as 3S/N) was approximately 40 pM (14 pg/ml). The assay was linear within the examined range (18-450 pg/ml), using CSF spiked with iPF2alpha-III standard (r(2)>0.995). Repeatability data were calculated for CSF spiked to 90 pg/ml and the relative standard deviation (RSD) obtained was 3% (n=6).

Chromatography, High Pressure Liquid↗

PubMed2005

Influence of atmospheric plasma on physicochemical properties of vapor-grown graphite nanofibers.

Vapor-grown graphite nanofibers (GNFs) were modified by plasma treatments using low-pressure plasmas with different gases (Ar gas only and/or Ar/O2 gases), flow rates, pressures, and powers. Surface characterizations and morphologies of the GNFs after plasma treatment were investigated by X-ray photoelectron spectroscopy (XPS), contact angle, titration, and transmission electron microscopy (TEM) measurements. Also, the investigation of thermomechanical behavior and impact strengths of the GNFs/epoxy composites was performed by dynamic-mechanical thermal analysis (DMTA) and Izod impact testing, respectively. The plasma treatment of the fibers changed the surface morphologies by forming a layer with a thickness on the order of 1 nm, mainly consisting of oxygen functional groups such as hydroxyl, carbonyl, and carboxyl groups. After functionalization of the complete surfaces, further plasma treatment did not enhance the superficial oxygen content but slightly changed the portions of the functional groups. Also, the composites with plasma-treated GNFs showed an increase in T(g) and impact strength compared to the composites containing the same amount of plasma-untreated GNFs.

Atmosphere↗

PubMed2005

Use of porous graphitic carbon column for the separation of natural isomeric tropane alkaloids by capillary LC and mass spectrometry.

This study presents the outstanding chromatographic selectivity of a porous graphitic carbon support for the separation of four isomeric tropane alkaloids from the stem-bark of Schizanthus grahamii (Solanaceae). Capillary liquid chromatography coupled to mass spectrometry was studied after the appropriate selection of mobile phase composition, temperature, nature and concentration of the acidic modifier. Fragmentation behaviour by in-source collision-induced dissociation (CID) on a single quadrupole mass spectrometer, or MSn with an ion-trap, was investigated for structural identification. The ability to differentiate the isomers by in-source CID was demonstrated and a fragmentation pathway, based on MSn sequences together with accurate mass experiments, was proposed.

Alkaloids↗

PubMed2005

Determination of ruthenium in pharmaceutical compounds by graphite furnace atomic absorption spectroscopy.

A graphite furnace atomic absorption (GFAA) spectrometric method for the determination of ruthenium (Rh) in solid and liquid pharmaceutical compounds has been developed. Samples are dissolved or diluted in dimethyl sulfoxide (DMSO) without any other treatment before they were analyzed by GFAA with a carefully designed heating program to avoid pre-atomization signal loss and to achieve suitable sensitivity. Various inorganic and organic solvents were tested and compared and DMSO was found to be the most suitable. In addition, ruthenium was found to be stable in DMSO for at least 5 days. Spike recoveries ranged from 81 to 100% and the limit of quantitation (LOQ) was determined to be 0.5 microg g(-1) for solid samples or 0.005 microg ml(-1) for liquid samples based a 100-fold dilution. The same set of samples was also analyzed by ICP-MS with a different sample preparation method, and excellent agreement was achieved.

Chemistry, Pharmaceutical↗