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At least 343 records · Page 19Linked to original sources

PubMed2006

Metastable structures and recombination pathways for atomic hydrogen on the graphite (0001) surface.

We present scanning tunneling microscopy results which reveal the existence of two distinct hydrogen dimer states on graphite basal planes. Density functional theory calculations allow us to identify the atomic structure of these states and to determine their recombination and desorption pathways. Direct recombination is only possible from one of the two dimer states. This results in increased stability of one dimer species and explains the puzzling double peak structure observed in temperature programmed desorption spectra for hydrogen on graphite.

Journal Article↗

PubMed1998

Determination of trace lithium in human erythrocytes by electrothermal atomic-absorption spectrometry with pyrocoated graphite tubes and integrated platform.

Electrothermal graphite-furnace atomic-absorption spectroscopy with pyrocoated graphite tubes, integrated platform and matrix modification was used to determine submicromolar concentrations of trace lithium in human red blood cells. Matrix-matched samples were used to establish calibration curves for concentrations up to 0.58 microM (addition-calibration method) with satisfactory linearity (r2 > 0.99) and intra- and inter-day variability (CV < 11.4%). The median concentration of trace lithium in the cells of 40 healthy Caucasian volunteers devoid of medical or psychiatric history was 0.23 microM (inter-quartile range 0.20-0.30). The levels of trace lithium in the red blood cells correlated (r2 = 0.83) with plasma concentrations (median 0.13 microM, inter-quartile range 0.11-0.19) measured in the same blood sample. Dietary factors (e.g. consumption of lithium-containing mineral water) affected both levels. The red blood cell/plasma lithium ratio had a median value of 1.57 (inter-quartile range 1.16-2.07), implying that trace lithium is accumulated in erythrocytes. This contrasts with most reports of red blood cell/plasma ratio, measured during therapeutic treatment with lithium, for which the average value is 0.5-0.8, albeit for much higher concentrations of lithium (approx. 500-800 microM). The proposed analytical method has the required sensitivity and accuracy for determination of trace lithium in red blood cells and makes it possible to perform epidemiological studies to assess human exposure to environmental lithium in diet and beverages, and inter-individual variations in trans-membrane and renal lithium kinetics at the submicromolar level.

Bipolar Disorder↗

PubMed2004

Self-assembled hexa-peri-hexabenzocoronene graphitic nanotube.

An amphiphilic hexa-peri-hexabenzocoronene self-assembles to form a pi-electronic, discrete nanotubular object. The object is characterized by an aspect ratio greater than 1000 and has a uniform, 14-nanometer-wide, open-ended hollow space, which is an order of magnitude larger than those of carbon nanotubes. The wall is 3 nanometers thick and consists of helical arrays of the pi-stacked graphene molecule, whose exterior and interior surfaces are covered by hydrophilic triethylene glycol chains. The graphitic nanotube is redox active, and a single piece of the nanotube across 180-nanometer-gap electrodes shows, upon oxidation, an electrical resistance of 2.5 megohms at 285 kelvin [corrected]. This family of molecularly engineered graphite with a one-dimensional tubular shape and a chemically accessible surface constitutes an important step toward molecular electronics.

Journal Article↗

PubMed1998

In situ discovery of graphite with interstellar isotopic signatures in a chondrule-free clast in an L3 chondrite

Optical and scanning electron microscopy of a chondrule-free clast in the unequilibrated L3 chondrite Khohar revealed a spherical object consisting of an aggregate of small ( approximately 2- micrometer diameter), Ni-poor (0.5 to 2.89 weight percent) metal particles and fine-grained graphite (<1-micrometer diameter). The graphite has large D and 15N excesses (deltaD approximately 1500 per mil and delta15N approximately 1300 per mil) with two isotopically distinct signatures: N rich with a high D/H ratio and N poor with a high 15N/14N ratio. These excesses are the largest D and 15N excesses observed in situ in a well-characterized phase in a meteorite. The isotopic characteristics are suggestive of an interstellar origin, probably by ion-molecule reactions at low temperature in the interstellar molecular cloud from which the solar system formed. The structure and nonchondritic composition of the metal particles suggest they did not form under equilibrium conditions in the solar nebula.

Journal Article↗

PubMed2000

Graphite polyhedral crystals.

Polyhedral nano- and microstructures with shapes of faceted needles, rods, rings, barrels, and double-tipped pyramids, which we call graphite polyhedral crystals (GPCs), have been discovered. They were found in pores of glassy carbon. They have nanotube cores and graphite faces, and they can exhibit unusual sevenfold, ninefold, or more complex axial symmetry. Although some are giant radially extended nanotubes, Raman spectroscopy and transmission electron microscopy suggest GPCs have a degree of perfection higher than in multiwall nanotubes of similar size. The crystals are up to 1 micrometer in cross section and 5 micrometers in length, and they can probably be grown in much larger sizes. Preliminary results suggest a high electrical conductivity, strength, and chemical stability of GPC.

Journal Article↗

PubMed2003

Measurement and analysis of atomic and diatomic carbon spectra from laser ablation of graphite.

Spectra from plasma produced by laser-induced breakdown of graphite were recorded and analyzed to increase our understanding of the way in which carbon nanoparticles are created during Nd:YAG laser ablation of graphite. The effects of various buffer gases were studied. Electron density and temperature were determined from spectra of the first and second ions of atomic carbon. The C2 Swan spectrum was also prominent in most of the measured spectra. Temperature was inferred from each experimental Swan spectrum by determination of the temperature for which a synthetic Swan spectrum best fitted, in the least-squares sense, the measured spectrum.

Journal Article↗

PubMed2005

High efficiency, high quality x-ray optic based on ellipsoidally bent highly oriented pyrolytic graphite crystal for ultrafast x-ray diffraction experiments.

By the use of a thin highly oriented pyrolytic graphite crystal (HOPG) bent to a high-performance ellipsoidal shape it was possible to focus monochromatic x-rays of 4.5 keV photon energy with an efficiency of 0.0033, which is 30 times larger than for previously used bent crystals. Isotropic Ti K alpha radiation of a 150 microm source was focused onto a 450 microm spot. The size of the focal spot can be explained by broadening due to the mosaic crystal rocking curve. The rocking curve width (FWHM) of the thin graphite foil was determined to 0.11 degrees. The estimated temporal broadening of an ultrashort K alpha pulse by the crystal is not larger than 300 fs. These properties make the x-ray optic very attractive for ultrafast time-resolved x-ray measurements.

Journal Article↗

PubMed2003

Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite.

The properties of highly oriented pyrolytic graphite (HOPG) as a substrate for external reflection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physisorbed hydrocarbon multilayers, and chemisorbed monolayers of p-substituted aryl radicals on HOPG were used as samples, and the experimental spectra were compared and complemented with the results of spectral simulations. From reflectivity measurements of clean HOPG surfaces with polarized light as a function of the light incidence angle and the frequency, the anisotropic optical constants n (refractive index) and k (absorption index) were determined for in-plane and out-of-plane directions with respect to the graphite basal plane. These constants express the semimetallic properties of HOPG, indicated by an intermediate reflectivity between a typical metal and a dielectric substrate and by asymmetric, distorted peak shapes in adsorbate film spectra, which represent a transition state between symmetrical, positive absorptions on metals and inverted, negative peaks on dielectric substrates. Regarding spectral sensitivity and surface selection rules, HOPG behaves much like a metal and is therefore an equally suitable substrate for external reflection infrared (IR) measurements.

Journal Article↗

PubMed2001

Determination of traces of cadmium in natural water samples by flow injection on-line preconcentration-graphite furnace atomic absorption spectrometry.

A flow injection on-line preconcentration-graphite furnace atomic absorption spectrometric method was developed for the determination of traces of cadmium in natural water samples. Cadmium in samples was adsorbed on an iminodiacetate-type chelating resin, Muromac A-1 microcolumn (3 mm i.d. and 10 mm long), and then eluted with 2 mol l-1 HNO3. The eluate was introduced into the injection tip of an autosampler. The eluate zone with the highest analyte concentration was injected into the graphite furnace by cooperation of a peristaltic pump and a syringe pump of the autosampler, which were controlled by a programmable controller. The present system was successfully applied to the determination of cadmium in natural water samples. A detection limit of 0.2 ng l-1 was obtained with 12 ml sample loading. The recoveries were 99 and 108% for tap water (4 ml loading) and underground water (12 ml loading), respectively. Analytical results obtained for a river water reference material (JAC-0031, Japan Society for Analytical Chemistry) were close to the reference value.

Cadmium↗

PubMed1999

A case of an intraocular foreign body due to graphite pencil lead complicated by endophthalmitis.

We report a case of an 8-year-old boy who presented with an intraocular foreign body composed of graphite pencil lead. The patient had been accidentally poked in the right eye with a graphite pencil. Primary care consisted of corneal suturing and lens extraction. Two pieces of the pencil lead remained in the vitreous cavity following surgery, and 2 days later the patient developed endophthalmitis. Pars plana vitrectomy was performed immediately and the intraocular foreign bodies were removed through the scleral wound. Cultures of the vitreous fluid revealed no bacterial organisms. X-ray fluoroscopic analysis of the vitreous detected 1 ppm of aluminum (a constituent of the pencil lead). Although the clinical presentation indicated probable bacterial endophthalmitis, the detection of elemental aluminum within the vitreous cavity also suggested the possibility of further retinal toxicity due to some dissolving of the pencil lead.

Cataract↗

PubMed2002

[Atomization efficiency of graphite furnace in atomic absorption spectrometry].

In this paper, the advance of study of atomization efficiency for graphite furnace atomic absorption spectrometry was reviewed. The fundamental theory and the various calculated equations of atomization efficiency, as well the relationship among beta m, beta i(exp) and beta i(cal) were described. Problems requiring further improvement in the investigation of atomization efficiency of graphite furnace atomic absorption spectrometry were discussed. Peak area atomization efficiency is more practical than peak height. Because the mean residence time (tau R) of atoms in the furnace is larger than tau D (tau D = l2/8D, l is the tube length and D is the diffusion coefficient), calculated peak area atomization efficiency in the furnace is larger than experimental. Part of the atomization efficiency is affected by atomic loss mechanism. An important aspect of researching atomization efficiency is conducted mainly to improve the sensitivity and limit of detection of the analysis method.

Cadmium↗

PubMed2002

[Determination of trace barium in environmental samples by electric-heated AAS with lanthanum-coated graphite tube].

In this paper a new method was established for determination of trace barium in environmental samples by graphite furnace atomic absorption spectrometry. In the presence of matrix modifier magnesium nitrate, the matrix inference was eliminated efficiently. To avoid producing barium carbide, the graphite tubes were coated differently with lanthanum, zirconium, tungsten, molybdenum, and tantalum. Results showed that the tube with lanthanum was the best. The atomization temperature was diminished. The sensitivity was improved, and the tube with lanthanum gave the longest service life. So we used the tube to determine trace barium. The trace barium in Geodchemical Standard Reference Sample Soil-1 (GSS-1) was determined by the new method, the test results showed that the method was reliability and accurate. The method has been used for the determination of trace barium in environmental samples and in water of Yellow River. The detection limit for Ba was 2.1 x 10(-12) g and the relative standard deviation(RSD) was 5.4% for 15 ng.mL-1 Ba.

Barium↗

PubMed2000

[A rapid graphite furnace atomic absorption spectrometric method for the determination of trace copper and lead in surface water].

A rapid graphite furnace atomic absorption spectrometric method for the determination of trace copper and lead in surface water is investigated in this paper. The influences of the program for graphite furnace and matrix modifier are systematically researched. A rapid method without ashing step and chemical matrix modifier is established. The determination limits of copper and lead are 0.9 microgram.L and 1.2 micrograms.L, respectively. The relatively standard deviations (RSD) (n = 1) are 2.5% and 4.1%, respectively. The recoveries are 96.0%-97.2% and 90.0%-92.0%, respectively.

Copper↗

PubMed1992

Direct determination of zinc in serum by Zeeman atomic absorption spectrometry with a graphite furnace.

We developed a precise and accurate graphite furnace atomic absorption spectrometric method for the direct determination of zinc in serum. Serum samples are analyzed after 20-fold dilution with water of ultrapure analytical grade. No other reagent is used, from the moment of sampling until measurement. During atomization, the argon flow is kept at 150 mL/min instead of gas stop, to decrease the sensitivity and thus allow lower dilution ratios. Zinc concentrations are determined against a serum-matched calibration curve. Graphite tubes are uncoated and no L'vov platform is used. Between-run CVs were 5.9%, 3.5%, and 1.9% for serum zinc concentrations of 0.93, 1.15, and 1.43 mg/L, respectively. The characteristic mass was 9 pg, and the detection limit (meanblank + 3SDblank) was 0.060 mg/L.

Adult↗

PubMed2001

[Effect of graphite and phosphate investments on the biological characteristics of pure titanium castings].

OBJECTIVE: To investigate the effect of surface structural changes caused by graphite and phosphate investments on the biological characteristics of pure titanium castings. METHODS: Round casting pads of pure titanium were processed by graphite and phosphate investments respectively, then grouped into four. With the control of guta purca of the same size, Ultraviolet spectrometry was used to test the cytotoxicity of samples via the ISO, GB and Shanghai biomaterial research and testing center's standards. Mucous irritation test was performed on hamsters' pouch. RESULT: Titanium castings obtained from both investment materials have reached the national standard of cytotoxicity and mucous irritation test no matter they have been undergone surface treatment or not. CONCLUSION: Both investment materials cause no obvious changes on biological characteristics of pure titanium castings with the existing of surface structural changes.

English Abstract↗

PubMed1999

[Method for the determination of trace Li in serum of cancer patients by tungstate-coated graphite furnace atomic absorption spectrometry].

Using a tungstate coated graphite tube. The sensitivity of Li was significantly increased and trace Li in serum samples was determined by Zeeman graphite AAS with (NH4)2SO4 as a matrix modifier. The effects of different metal ions and biological matrix were eliminated. The characteristic concentration is 5.53 x 10(-3) microg x mL(-/1)%. The recovery is greater than 93.72%. The relative standard deviation is less than 8.93%.

Humans↗

PubMed1999

[Study on the determination of trace caesium in bittern by extraction separation graphite furnace atomic absorption spectrometry].

The atomizatiom behavior of Cs in bittern and the mechanism were studied by graphite furnace atomic absorption spectrometry. Interferences of inorganic acid from the bittern on absorption signal of Cs were examined. It is shown that there are gaseous interferences and molecular absorption interferences and ionization interferences with regard to the interference of chlorides on absorption signal of Cs. The effects of 3 crown ethers, 5 big-anion reagents and chlorides on extractability Cs have been compared. A extraction separation GFAAS method for determining Cs in bittern has been established by pyrolytic graphite tube, KNO3-NH4NO3 matrix modifier and 18-C-6, bromothymol blue, chloroform extraction separation. The method exhibits high sensitivity and selectivity, it has been used for the determination of trace Cs in bittern. The characteristic mass is 1.4 x 10(-10) g/0.004 4 A. The adding recovery is 89%-105% and the relative standard deviation is 4.13% (n = 13).

English Abstract↗

PubMed1998

[Determination of Sb in the resin and products of PET by graphite furnace atomic absorption spectrometry].

The method of determing Sb in resin and products of PET by graphite furnace atomic absorption spectrometry was studied. The sample was pretreated by 4% acetic acid and determined by graphite furnace atomic absorption spectrometry. The sensitivity of this method was 30.5 pg/1% absorption, the relative standard deviation was 3.12% and the rate of recovery was 91.68-97.74%. The results obtained by this method agreed with those of malachite green spectrophotometry.

English Abstract↗