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PubMed2005

Reflection absorption infrared spectroscopy and temperature programmed desorption investigations of the interaction of methanol with a graphite surface.

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the adsorption of methanol (CH(3)OH) on the highly oriented pyrolytic graphite (HOPG) surface. RAIRS shows that CH(3)OH is physisorbed at all exposures and that crystalline CH(3)OH can be formed, provided that the surface temperature and coverage are high enough. It is not possible to distinguish CH(3)OH that is closely associated with the HOPG surface from CH(3)OH adsorbed in multilayers using RAIRS. In contrast, TPD data show three peaks for the desorption of CH(3)OH. Initial adsorption leads to the observation of a peak assigned to the desorption of a monolayer. Subsequent adsorption leads to the formation of multilayers on the surface and two TPD peaks are observed which can be assigned to the desorption of multilayer CH(3)OH. The first of these shows a fractional order desorption, assigned to the presence of hydrogen bonding in the overlayer. The higher temperature multilayer desorption peak is only observed following very high exposures of CH(3)OH to the surface and can be assigned to the desorption of crystalline CH(3)OH.

Journal Article↗

PubMed2005

Hydrogen adsorption on graphite (0001) surface: a combined spectroscopy-density-functional-theory study.

The adsorption of H/D atoms on the graphite (0001) surface is investigated by means of both high-resolution electron-energy loss spectroscopy (HREELS) and periodic first-principle density-functional theory. The two methods converge towards two modes of adsorption: adsorption in clusters of about four hydrogen atoms and adsorption in pairs of atoms on contiguous carbon sites. The desorption energies estimated from the calculated dissociation energies range from 8 to 185 kJ mol(-1) leading to an estimated surface coverage at saturations of 30-44 at. %. These results are compared with previous thermal desorption spectroscopy results. New HREEL signal assignments are proposed based on quantum calculations.

Journal Article↗

PubMed2005

A molecular-beam study of the collision dynamics of methane and ethane upon a graphitic monolayer on Pt(111).

Utilizing a supersonic molecular-beam scattering technique, the angular intensity distributions of alkane molecules (CH4 and C2H6) have been measured, which are scattered from a chemically inert and highly oriented monolayer graphite (MG) on Pt(111). A MG which covers the Pt(111) surface with a full monolayer is found to induce a large energy loss of alkanes during collision with the surface by phonon creation due to the large mass ratio of an alkane molecule with respect to MG. Based on the classical cube model, only applicable to the molecules without internal mode excitation, the effective masses of MG of 76 (six atoms of carbon) and Pt(111) of 585 (three atoms of platinum) are determined from rare-gas atom scattering data. Despite the difference in the degree of freedom between CH4 and rare-gas atoms, CH4 scattering is found to be well described by the simple hard-cube model as a result of the high symmetry of the CH4 structure. With the recently developed ellipsoid-washboard model, an extension of the hard-cube model to include some internal mode excitation of impinging molecules in addition to the surface corrugation, it is found that unlike CH4 the cartwheel rotation mode of C2H6 is significantly excited during collision, while the helicopter mode excitation is negligible on a flat MG surface.

Journal Article↗

PubMed2006

Cn films (n=50, 52, 54, 56, and 58) on graphite: cage size dependent electronic properties.

Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected Cn+ (50< or =n<60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, Cn films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that Cn aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C60. The apparent activation energies for intact Cn sublimation range from 2.2 eV for C58 to 2.6 eV for C50 as derived from thermal desorption spectra. All Cn films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (EB approximately 2.5 eV). This feature has been assigned to Cn units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all Cn studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C60. C58 exhibited the lowest HOMO (6.5 eV). Band gaps of Cn films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C52) and 1.8 eV (C50) were observed, compared to 1.5 eV for solid C60.

Journal Article↗

PubMed2006

The rovibrational distribution of H2 and HD formed on a graphite surface at 15-50 K.

The rotational distributions of H2 and HD formed on a highly oriented pyrolitic graphite surface at temperatures of 15-50 K have been measured using laser spectroscopy. The population of the rovibrational levels nu=1, J=0-4 and nu=2, J=0-4 has been observed and the average rotational temperatures of the nascent H2 and HD molecules have been determined. We find that the average rotational temperature of the newly formed molecules is much higher than the surface temperature on which they have formed. We compare our results with other recent experimental data and theoretical calculations.

Journal Article↗

PubMed2006

Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. I. H-chemisorbed case.

Collision induced (CI) processes involving hydrogen atoms on a graphite surface are studied quantum mechanically within the rigid, flat surface approximation, using a time-dependent wave packet method. The Eley-Rideal (ER) reaction and collision induced desorption (CID) cross sections are obtained with the help of two propagations which use different sets of coordinates, a "product" and a "reagent" set. Several adsorbate-substrate initial states of the target H atom in the chemisorption well are considered, and CI processes are studied over a wide range of projectile energy. Results show that (i) the Eley-Rideal reaction is the major reactive outcome and (ii) CID cross sections do not exceed 4 A2 and present dynamic thresholds for low values of the target vibrational quantum number. ER cross sections show oscillations at high energies which cannot be reproduced by classical and quasiclassical trajectory calculations. They are related to the vibrational excitation of the reaction products, which is a rather steep decreasing function of the collision energy. This behavior causes a selective population of the low-lying vibrational states and allows the quantization of the product molecular states to manifest itself in a collisional observable. A peak structure in the CID cross section is also observed and is assigned to the selective population of metastable states of the transient molecular hydrogen.

Journal Article↗

PubMed2006

Scanning tunneling spectroscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite.

We report on a hybrid scheme to perform efficient and accurate simulations of scanning tunneling spectroscopy (STS) of molecules weakly bonded to surfaces. Calculations are based on a tight binding (TB) technique, including a self-consistent calculation of the electronic structure of the molecule, to predict STS conductance spectra. The use of a local basis makes our model easily applicable to systems with several hundreds of atoms. We performed first-principles density-functional calculations to extract the geometrical and electronic properties of the system. In this way, we can include, in the TB scheme, the effects of structural relaxation upon adsorption on the electronic structure of the molecule. This approach is applied to the study of regioregular poly(3-dodecylthiophene) polymer chains adsorbed on highly oriented pyrolytic graphite. Results of spectroscopic calculations are discussed and compared with recently obtained experimental data.

Journal Article↗

PubMed2006

Accelerated molecular dynamics simulation of the thermal desorption of n-alkanes from the basal plane of graphite.

We utilize accelerated molecular dynamics to simulate alkane desorption from the basal plane of graphite. Eight different molecules, ranging from n-pentane to n-hexadecane, are studied in the low coverage limit. Acceleration of the molecular dynamics simulations is achieved using two different methods: temperature acceleration and a compensating potential scheme. We find that the activation energy for desorption increases with increasing chain length. The desorption prefactor increases with chain length for molecules ranging from pentane to decane. This increase subsides and the value of the preexponential factor fluctuates about an apparently constant value for decane, dodecane, tetradecane, and hexadecane. These trends are consistent with data obtained in experimental temperature-programed desorption (TPD) studies. We explain the dependence of the preexponential factor on alkane chain length by examining conformational changes within the alkane molecules. For the shorter molecules, torsional motion is not activated over experimental temperature ranges. These molecules can be treated as rigid rods and their partial loss in translational and rotational entropies upon adsorption increases as chain length increases, leading to an increasing preexponential factor. At their typical TPD peak temperatures, torsions are activated in the longer adsorbed chain molecules to a significant extent which increases with increasing chain length, increasing the entropy of the adsorbed molecule. This increase counteracts the decrease in entropy due to a loss of translation and rotation, leading to a virtually constant prefactor.

Journal Article↗

PubMed2005

Ultra-high vacuum scanning tunneling microscopy and theoretical studies of 1-halohexane monolayers on graphite.

A simple model system for the 2D self-assembly of functionalized organic molecules on surfaces was examined in a concerted experimental and theoretical effort. Monolayers of 1-halohexanes were formed through vapor deposition onto graphite surfaces in ultrahigh vacuum. Low-temperature scanning tunneling microscopy allowed the molecular conformation, orientation, and monolayer crystallographic parameters to be determined. Essentially identical noncommensurate monolayer structures were found for all 1-halohexanes, with differences in image contrast ascribed mainly to electronic factors. Energy minimizations and molecular dynamics simulations reproduced structural parameters of 1-bromohexane monolayers quantitatively. An analysis of interactions driving the self-assembly process revealed the crucial role played by small but anisotropic electrostatic forces associated with the halogen substituent. While alkyl chain dispersion interactions drive the formation of a close-packed adsorbate monolayer, electrostatic headgroup forces are found to compete successfully in the control of both the angle between lamella and backbone axes and the angle between surface and backbone planes. This competition is consistent with energetic tradeoffs apparent in adsorption energies measured in earlier temperature-programmed desorption studies. In accordance with the higher degree of disorder observed in scanning tunneling microscopy images of 1-fluorohexane, theoretical simulations show that electrostatic forces associated with the fluorine substituent are sufficiently strong to upset the delicate balance of interactions required for the formation of an ordered monolayer. The detailed dissection of the driving forces for self-assembly of these simple model systems is expected to aid in the understanding of the more complex self-assembly processes taking place in the presence of solvent.

Journal Article↗

PubMed2005

Self-assembled graphitic nanotubes with one-handed helical arrays of a chiral amphiphilic molecular graphene.

Self-assembly of a Gemini-shaped, chiral amphiphilic hexa-peri-hexabenzocoronene having two chiral oxyalkylene side chains, along with two lipophilic side chains, yields graphitic nanotubes with one-handed helical chirality. The nanotubes are characterized by an extremely high aspect ratio of >1,000 and have a uniform diameter of 20 nm and a wall thickness of 3 nm. The nanotubes with right- and left-handed helical senses were obtained from the (S)- and (R)-enantiomers of the amphiphile, respectively, due to an efficient translation of point chirality into supramolecular helical chirality. The (S)- and (R)-enantiomers coassemble at varying mole ratios to give nanotubes, whose circular dichroism profiles are almost unchanged over a wide range of the enantiomeric excess of the amphiphile (100-20%). The high level of chirality amplification thus observed indicates a long-range cooperativity in the self-assembling process. In sharp contrast, a hexabenzocoronene amphiphile with chiral lipophilic side chains did not form nanotubular assemblies. The present work demonstrates the majority rule in noncovalent systems and also may provide a synthetic strategy toward realization of molecular solenoids.

Journal Article↗

PubMed2003

Analysis of lead in 55 brands of dietary calcium supplements by graphite furnace atomic absorption spectrometry after microwave digestion.

The lead (Pb) contents of calcium (Ca) supplements available in Korea were determined by graphite furnace atomic absorption spectrometry using Zeeman background correction and peak area mode. All samples were microwave-digested in concentrated HNO(3). Ammonium dihydrogen phosphate and magnesium nitrate were used as matrix modifiers. Fifty-five brands of Ca supplements were classified into seven categories based on the major composite: bone, milk, oyster/clam shell, egg shell, algae, shark cartilage and chelated. The mean Pb contents of Ca supplements were 1.1 microg g(-1) (coefficient of variation 5.7%), ranging from n.d. (not detected) to 6.7 microg g(-1). Ca supplements made of bone have the highest Pb contents (2.3 microg g(-1)) with a wide range (0.1-6.7 microg g(-1)). The results were similar to those reported in other countries. The mean daily intakes of Pb from the supplement could be about 5 microg (mean Pb concentration 1.1 microg g(-1) x mean daily intake 4.5 g) taking 2% of provisional tolerable daily intake that the FAO/WHO Joint Food Additive and Contaminants Committee has set to evaluate its safety. However, measures to prevent potentials of Pb toxicity from overtaking some Ca supplements should be considered.

Adolescent↗

PubMed2004

Determination of lead and cadmium in wines by graphite furnace atomic absorption spectrometry.

The lead (Pb) and cadmium (Cd) content of various wines on the Korean market were determined by graphite furnace atomic absorption spectrometry using Zeeman background correction and peak area mode. All wine samples were microwave-digested in concentrated HNO(3). Ammonium dihydrogen phosphate and magnesium nitrate were used as matrix modifiers for both Pb and Cd analyses. The mean Pb content of the wines was about 29 microg l(-1) ranging from 5 to 87 microg l(-1). Also, the means of Cd were about 0.5 microg l(-1) ranging from < 0.1 to 3.0 microg l(-1). The mean recoveries of Pb and Cd were 92.8 and 101.3% and their analytical detection limits were 1.0 and 0.1 microg l(-1), respectively. Sixty brands of wine were classified into red and white, but no statistically significant difference in Pb and Cd content was observed.

Cadmium↗

PubMedDate not supplied

Determination of cadmium and lead in foods by graphite furnace atomic absorption spectrometry with Zeeman background correction: test with certified reference materials.

Two methods for the determination of cadmium and lead based on graphite furnace atomic absorption spectrometry (GFAAS) with Zeeman background correction are described. The main difference between the methods is the method of sample destruction, bomb digestion versus dry ashing. The precision and accuracy of the methods has been tested by analysing 16 different reference materials, with cadmium concentrations varying from 2.9 micrograms/kg to 2.7 mg/kg and lead concentrations from 44 micrograms/kg to 13.5 mg/kg. The methods produced results which differ, with a few exceptions, less than 10% from the certified contents, with relative standard deviations of around 5%. The methods are not difficult to carry out, are not very laborious and without modifications are applicable to a large variety of products. The sample solutions obtained can also be used for the determination of other elements, e.g. Al, Cr, Cu, Hg, Mn, Ni and Zn. The detection limits, for test portions of 1 and 5 g (wet products) are 0.5-1 microgram/kg for cadmium and 5-20 micrograms/kg for lead.

Animals↗

PubMed1980

Determination of total tin in environmental biological and water samples by atomic absorption spectrometry with graphite furnace.

Analysis of traces of tin using several analytical techniques (X-ray fluorescence, neutron activation, polarographic techniques and atomic absorption) have been tested. Parameters such as simplicity, rapidity, sensitivity and interferences are compared in order to choose the most useful method for practical purpose. Finally, flameless atomic absorption was chosen for the determination of total tin concentration in different natural samples. Digestion of biological samples (plant, plankton, fish, etc.) was achieved by using Lumatom (a trade organic chemical). Thus, the digested sample is directly injected into the graphite furnace. This digestion technique is suitable and rapid with a minimum of error (contamination and losses). For tin analysis in water samples, a preconcentration of tin is carried out by coprecipitation with 1, 10-phenanthroline and tetraphenyl boron. The precipitate is separated and dissolved in alcohol or in Lumatom. The sensitivity of this method is 0.1 ng absolute tin.

Animals↗

PubMed1986

Determination of ultra trace amounts of cobalt in fish by graphite furnace Zeeman effect atomic absorption spectrometry.

A method is described for determining stable cobalt concentrations in fish flesh and bone using polarized Zeeman effect graphite furnace atomic absorption spectrometry (ZAAS). Cobalt analysis on freshwater fish flesh samples (10 g dry weight) required predigestion and wet-ashing at 70-80 degrees C. Cobalt is chelated with ammonium pyrrolidine dithiocarbamate (APDC) extracted with methyl isobutyl ketone (MIBK) and analysed by ZAAS. The mean cobalt content calculated from the standard additions method using three replicate fish flesh samples was 4.23 +/- 1.0 microgram Co. Kg-1 (dry weight). Analyses were also carried out on flesh and bone samples from similar sized fish, of the same species, taken from three area lakes.

Animals↗

PubMed1999

Rapid screening for organochlorine and organophosphorus pesticides in milk using C18 and graphitized carbon black solid phase extraction cleanup.

A rapid, multiresidue, solid phase extraction (SPE) technique for the isolation and gas chromatographic determination of organochlorine and moderately polar organophosphorus pesticide residues in milk is described. Milk is sonicated with an acetonitrile-acetone-methanol mixture and centrifuged. The supernatant is subjected to a cleanup using both C18 and graphitized carbon black SPE columns. The pesticide residues are determined by gas chromatography with electron capture and flame photometric detection. The method required minimal volumes of solvent and resulted in the production of minimal volumes of hazardous waste.

Animals↗

PubMed2004

The determination of brain magnesium and zinc levels by a dual-probe microdialysis and graphite furnace atomic absorption spectrometry.

OBJECTIVE: The aim of this study was to develop a microdialysis-graphite furnace atomic absorption spectroscopy (MD-GFAAS) for monitoring dynamic changes of extracellular magnesium (Mg) and zinc (Zn) in the cortex of gerbils subjected to focal cerebral ischemia, that had been produced in anesthetized gerbils by occlusion of the right middle cerebral artery. METHODS: Two microdialysis probes were inserted into both sides of the cortex to simultaneously collect dialysates during cerebral ischemia. Dynamic changes in these analytes, on ipsilateral and contralateral sides of the brain, were assayed by MD-GFAAS. Optimal conditions and analytical precision of GFAAS were studied in the present assay. RESULTS: The present study demonstrated significant decreases in Mg (65% of baseline) and zinc (74% of baseline) maintained their levels within 3 h on the ipsilateral side of cortex during cerebral ischemia. Slight changes of Mg and Zn on the contralateral sides were also observed. CONCLUSION: The derangement of extracellular Mg and Zn could be important in the progression of cell injury and may be associated with cerebral ischemia insult.

Animals↗

PubMed2004

On-line microdialysis-graphite furnace atomic absorption spectrometry in the determination of brain magnesium levels in gerbils subjected to cerebral ischemia/reperfusion.

OBJECTIVES: Description of use of equipment for on-line microdialysis (MD) coupled with graphite furnace atomic absorption spectrometry (GFAAS) system, for dynamic monitoring of extracellular Mg in gerbils subjected to transient focal cerebral ischemia. METHODS: Gerbils' right middle cerebral artery (MCA) and common carotid artery (CCA) were occluded for 60 minutes, and then reperfused for 60 minutes with Ringer's solution, after which extracellular fluid samples were collected via a microdialysis probe inserted into the right cortex before, during and after inducing ischemia. Reperfusion was at a rate of 2 microL/min through the microdialysis probe, on-line diluted with measured water injected onto the GFAAS via an autosampler for Mg analysis. RESULTS: The detection limit of the Mg concentrations has ranged from 0.50 to 3.00 microg/L; our detection limit was 0.03 microg/L. We applied this on-line system to monitor extracellular Mg levels in the cortex during focal cerebral ischemia. Mg concentrations significantly decreased to 41% of baseline during cerebral ischemia and gradually returned to 67% of baseline after 60 minutes of reperfusion. CONCLUSIONS: We presume that derangement of Mg homeostasis could be important in brain cell injury and is closely associated with cerebral ischemia event. The described analytic system permits autosampling in the brain and allows for continuous determination of Mg and trace minerals in minute sample volumes in a living system.

Animals↗