SEARCH · PubMed Health

Results for “Graphite”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

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

At least 721 records · Page 40Linked to original sources

PubMed1991

Graphitized carbon black in quartz tubes for the sampling of indoor air nicotine and analysis by microwave thermal desorption-capillary gas chromatography.

Nicotine in a smoky indoor air environment can be determined using graphitized carbon black as a solid sorbent in quartz tubes. The temperature stability, high purity, and heat absorption characteristics of the sorbent, as well as the permeability of the quartz tubes to microwaves, enable the thermal desorption by means of microwaves after active sampling. Permeation and dynamic dilution procedures for the generation of nicotine in the vapor phase at low and high concentrations are used to evaluate the performances of the sampler. Tube preparation is described and the microwave desorption temperature is measured. Breakthrough volume is determined to allow sampling at 0.1-1 L/min for definite periods of time. The procedure is tested for the determination of gas and paticulate phase nicotine in sidestream smoke produced in an experimental chamber.

Air Pollutants↗

PubMed2000

Elution characteristics of natural cyclodextrins on porous graphitic carbon

The retention behavior of natural alpha-, beta-, and gamma-cyclodextrins on a porous graphitic carbon (PGC) stationary phase is investigated. Unusual retention properties for reversed-phase chromatographic conditions are observed with acetonitrile-methanol and water-methanol mixtures as mobile phases. It is assumed that the retention process is governed not only by the standard solvophobic effect but also by specific interactions described as "CD-PGC" effect. The retention factor versus the volumetric methanol fraction in the mobile phase show second-order curves expressing this double mechanism hypothesis. van't Hoff plots demonstrate the contribution of these two retention processes. The retention factor of each natural cyclodextrin is shown to depend on the mobile phase property to act as a proton acceptor, according to the solvent selectivity classification described by Snyder. The "CD-PGC" effect is interpreted as an equilibrium between different interactions: cyclodextrin-PGC stationary phase, London dispersion forces, and cyclodextrin-mobile phase hydrogen bonding. The balance of these interactions may monitor the orientation of the cyclodextrin molecule facing the carbon surface, which is therefore suspected to be the major parameter of this retention mechanism.

Journal Article↗

PubMedDate not supplied

Determination of tin in biological materials by atomic-absorption spectrometry with a graphite furnace.

The determination of tin in the blood, liver, kidney, and spleen by atomic-absorption measurement using a graphite furnace was investigated. The presence of 20 micrograms/mL Fe(III) reduced absorbance of 0.2 microgram/mL Sn to 50%. To remove the interference of Fe(III), it was reduced to Fe(II) by 10% ascorbic acid, and Sn(IV) was extracted by methylisobutylketone (MIBK) without a chelating agent. This technique can be applied when 100 micrograms/mL Fe coexists with 0.2 microgram/mL Sn. Four methods for determination of tin in the blood were examined: (a) standard addition, (b) low-temperature ashing/MIBK extraction, (c) wet ashing/MIBK extraction, and (d) direct determination. The results of (a), (b), and (c) showed a high correlation (r greater than 0.86, n = 12), while the values obtained by (d) were scattered widely and showed little correlation with those determined by the other three methods. In methods (b) and (c), the detection limit was 10 ng/mL (0.2 ng Sn) without expansion mode, 2 ng/mL (0.04 ng Sn) with X 5 expansion mode, the recovery more than 90%, and the coefficient of variation 6.6% (n = 8). Method (c) was recommended for large sample sizes, and was also suitable for determination of tin in liver, kidney, and spleen, using 0.5%, instead of 10%, ascorbic acid.

Animals↗

PubMedDate not supplied

Axial distribution of arsenic in individual human hairs by solid sampling graphite furnace AAS.

In cases of arsenic poisoning, the distribution of arsenic along the length of a hair can be used to distinguish between chronic and acute exposure. Individual 5-mm segments of a hair were analyzed for their arsenic content by solid sampling graphite furnace atomic absorption spectrophotometry with a mixed Pd-Mg(NO3)2 matrix modifier. Results obtained using arsenic calibration solutions and standard additions to uncontaminated hairs are analytically indistinguishable, allowing aqueous standards to be used for hair analysis. The characteristic mass is 24 pg, and the 3 sigma detection limit is approximately 10 pg. Results obtained for hairs from two victims of arsenic poisoning are in good agreement with those obtained using neutron activation analysis.

Arsenic↗

PubMed2005

Magnetic alignment experiment of fine graphite-crystals dispersed in He gas oriented to study alignment of crystalline-axes of nano-sized non-magnetic particles.

The ensemble of nano-sized crystals is expected to attain additional physical properties when preferential alignments of certain crystal-axes are achieved by a magnetic field. The reduction of temperature T may realize alignment even if the mole number of the particle N and the diamagnetic anisotropy per mole (Deltachi)(DIA) are considerably small for the nano-sized diamagnetic oxides, since alignment proceeds by the balance between the energy of rotational Brownian motion and field-induced anisotropy energy. Alignment of various basic inorganic oxides such as gypsum, quartz, forsterite, KDP or calcite, having a size of 20 nm diameter, is expected to occur by a field intensity of approximately 50 T at T = 10 K; this intensity is presently available at a high magnetic-field laboratory. It is expected that the magnetic alignment of nano-sized particles can be observed by dispersing the particles in He gas, as achieved recently for micron-sized graphite crystals; a cryogenic liquid cannot be used as a dispersing medium. Measured (Deltachi)(DIA) values accumulated for basic inorganic-oxides are explained quantitatively by assuming that individual bonding-orbital composing the material possesses a constant amount of diamagnetic anisotropy; hence the majority of diamagnetic nano-sized insulators are expected to show magnetic alignment at finite field intensity.

Journal Article↗

PubMed1983

Microanalysis of platinum in biological media by graphite furnace atomic absorption spectroscopy.

An accurate method for the analysis of platinum in biological matrix is described. The method requires minimal sample handling and pretreatment prior to injection into the graphite furnace. It is reproducible, with average recovery of 97.7% platinum in biological matrix. The data presented also demonstrate that Fe in particular (also NaCl and HNO3) significantly depresses the atomic absorption signal of platinum. Standard addition calibration curves are therefore essential for determination of platinum in biological samples.

Humans↗

PubMed1999

Surface-induced ordering of a molecular fluid of flat hexagonal structure in a narrow graphite slit.

We systematically investigate by Monte Carlo simulations the role of the wall structure on a fluid of flat hexagonal molecules confined between two graphite walls. Our simulations show that the centers of mass of the molecules in different layers undergo an order-disorder transition as the wall separation increases, irrespective of the details of the wall structure. The wall structure thus becomes insignificant for the intervening fluid even down to a surprisingly low wall separation.

Journal Article↗

PubMed2000

Disordered magnetism at the metal-insulator threshold in nano-graphite-based carbon materials

The magnetism of activated carbon fibers composed of a disorder network of nanographites was investigated, where each nanographite has about 1 edge-inherited localized spin. The susceptibility, for samples situated around the metal-insulator threshold, shows a cusp around 4-7 K in addition to the presence of a field-cooling effect. These behaviors are explained in terms of disordered magnetism caused by random strengths of inter-nano-graphite antiferromagnetic interactions mediated by pi-conduction carriers.

Journal Article↗

PubMed2000

Orientational ordering of N2O molecules adsorbed on graphite (0001): A novel commensurate pinwheel structure

The structure and orientational ordering of N2O molecules physisorbed on graphite (0001) is investigated applying x-ray, neutron, and low-energy electron diffraction techniques. Combining the results of the three techniques, we find that N2O forms a highly ordered, hexagonal, commensurate (sqrt[21]xsqrt[21])R10.89 degrees phase. The unit cell contains seven molecules which are arranged in a seven-sublattice pinwheel structure, unexpected for linear molecules on a hexagonal lattice. Potential energy calculations corroborate these results.

Journal Article↗

PubMed2000

Dynamic behavior of nickel atoms in graphitic networks

The dynamic behavior of nickel atoms in graphitic carbon onions, observed by in situ atomic-resolution electron microscopy, shows the formation of stable new C-Ni phases. Nickel is observed to take substitutional in-plane positions in graphene layers, forming a planar graphitelike C-Ni lattice. Evidence is furthermore seen for the formation of a cubic C-Ni phase, suggesting a possible phase transformation in C-Ni from a graphitelike to a diamondlike structure. The stability of the planar phases is shown by first-principles calculations which also indicate that the C-Ni planes are metallic.

Journal Article↗

PubMed2000

Low-temperature magnetization of submonolayer 3He adsorbed on HD preplated graphite

NMR studies of submonolayer 3He adsorbed on a bilayer of HD preplated graphite have been made down to 100 &mgr;K, which is more than 1 order of magnitude smaller than the exchange energy ( J). In the highly frustrated antiferromagnetic solid region, the magnetization obeys a Curie-Weiss law even at temperatures around J and then increases gradually down to 100 &mgr;K. Nevertheless, it does not show any anomalous behavior corresponding to a spin gap. The normalized magnetization versus the reduced temperature ( T/J) is independent of the density just after solidification. This is consistent with the result in the high-temperature region, that the main multiple-spin exchanges have a similar density dependence.

Journal Article↗

PubMed2001

Indication of superconductivity at 35 K in graphite-sulfur composites.

We report magnetization measurements performed on graphite-sulfur composites which demonstrate a clear superconducting behavior below the critical temperature T(c0) = 35 K. The Meissner-Ochsenfeld effect, screening supercurrents, and magnetization hysteresis loops characteristic of type-II superconductors were measured. The results indicate that the superconductivity occurs in a small sample fraction, possibly related to the sample surface.

Journal Article↗

PubMed2001

Magnetic-field-induced insulating behavior in highly oriented pyrolitic graphite.

We propose an explanation for the apparent semimetal-insulator transition observed in highly oriented pyrolitic graphite in the presence of magnetic field perpendicular to the layers. We show that the magnetic field opens an excitonic gap in the linear spectrum of the Coulomb interacting quasiparticles, in close analogy with the phenomenon of dynamical chiral symmetry breaking in the relativistic theories of the (2+1)-dimensional Dirac fermions. Our strong-coupling approach allows for a nonperturbative description of the corresponding critical behavior.

Journal Article↗

PubMed2001

Anomalous quasiparticle lifetime in graphite: band structure effects.

We report ab initio calculations of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW approximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.

Journal Article↗

PubMed2001

Ghost excitonic insulator transition in layered graphite.

Some unusual properties of layered graphite, including a linear energy dependence of the quasiparticle damping and weak ferromagnetism at low doping, are explained as a result of the proximity of a single graphene sheet to the excitonic insulator phase which can be further stabilized in a doped system of many layers stacked in the staggered ( ABAB...) configuration.

Journal Article↗

PubMed2002

Behavior of partial monolayers of argon adlayers deposited on graphite.

This calculation examines the properties of partial monolayers of argon deposited on graphite, over an interval 20< or =T< or =90 K. In particular, we focus upon the phase transitions. Two peaks in the specific heat are found. The first peak is very sharp and narrow. It has been identified as a rotation of the adlayer from an angle off the substrate symmetry axis, to that axis. The calculated orientational behavior is compared with predictions of the two dimensional Ising model. The broad peak around 49.5 K is identified as a melting transition which is characterized by a loss of local sixfold symmetry.

Journal Article↗

PubMed2003

Scaling relations for implantation of size-selected Au, Ag, and Si clusters into graphite.

The deposition of size-selected clusters represents a new route to the fabrication of truly nanometer-scale surface architectures, e.g., nanopores. We report a systematic experimental study, coupled with molecular dynamics simulations, of the implantation depths of size-selected Au7, Ag7, and Si7 clusters in the model graphite substrate. For impact energies between 1.0 and 5.5 keV, we find that the implantation depth scales linearly with the momentum of the clusters for all three types of cluster. This "universal" behavior is consistent with a (viscous) retarding force proportional to the velocity of the cluster, akin to Stokes's law.

Journal Article↗

PubMed2003

Multilayer adsorption and wetting of acetone on graphite.

Adsorption/desorption isotherms of acetone on highly oriented pyrolytic graphite have been measured by ellipsometry for temperatures above the bulk triple point. The behavior in the monolayer and submonolayer regime is conventional, with 2D gas-liquid and 2D liquid-solid coexistence regions. Further liquid monolayers grow on top of the completed monolayer. The growth is basically layer-by-layer. For temperatures between 190 K and the triple point a prewetting-type transition occurs with a thin-thick jump of the layer thickness on adsorption but a layer-wise removal of the film on desorption. In this temperature regime the first monolayer is solid and its molecules are oriented perpendicular to the substrate whereas the higher layers are orientationally disordered polar liquid.

Journal Article↗