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At least 685 records · Page 38Linked to original sources
PubMed1989

Direct determination of cadmium in urine using graphite furnace atomic absorption spectrometry with Zeeman-effect background correction.

A procedure is described for the direct determination of cadmium in human urine using graphite furnace atomic absorption spectrometry with Zeeman-effect background correction. Except for a straightforward 1 + 1 V/V dilution of samples with 1.5% nitric acid, no matrix modifier or sample pre-treatment was necessary, thus reducing the risk of contamination. The concentration of cadmium in urine was evaluated directly from a calibration graph prepared using a metal-spiked human urine pool. In this way the time-consuming method of standard additions was avoided, permitting an increased sample throughput (120-150 samples per day; 90 s per analysis) with minimal attention of the analyst. In routine use, the precision (both within day and day to day) and limit of detection were of the order of less than 10% and 0.05 micrograms l-1 of Cd, respectively. The method is suitable for the biological monitoring of cadmium in the general population or in occupationally exposed persons.

Cadmium↗
PubMed1989

Determination of molybdenum in plasma using graphite furnace atomic absorption spectrometry.

A sensitive method is described for the determination of Mo in plasma or serum by graphite furnace atomic absorption spectrometry. The method involves extraction of the metal as the 8-hydroxyquinoline complex and is free of the interference effects that prevent the direct analysis of plasma for Mo. Recoveries of internal standards were excellent and results from the analysis of a National Institute of Standards and Technology Standard Reference Material were in good agreement with certified values. The sensitivity of the method, based on the analysis of 1 ml of plasma, is ca. 3 ng ml-1.

Animals↗
PubMed1990

Evaluation of biological sample mineralisation methods for the determination of fluorine by graphite furnace molecular absorption spectrometry.

Various mineralisation methods were evaluated as means of treating different liquid and solid biological samples for the determination of fluorine by the formation of aluminium monofluoride in an electrothermal graphite furnace and molecular absorption spectrometry (AIF-MAS). Simple sample dilution and the use of 0.01 M Al3+ + 0.01 M Sr2+ solution as a matrix modifier are sufficient to determine the fluorine content in most liquid samples, although some require the addition of 0.3 M ammonium nitrate to the matrix modifier solution in order to diminish background absorbance. In solid samples, treatment methods routinely used with fluoride ion-selective electrodes such as microdiffusion, furnace ashing - microdiffusion and oxygen flask combustion, were tested for compatibility with AIF-MAS. The results were compared with those obtained with a fluoride ion-selective electrode. The proposed mineralisation methods were checked for applicability to different plants, foodstuffs and other biological materials. Some of the methods gave an over-all precision of better than 10%, which is often acceptable, and all methods gave recoveries above 80%. Differences between labile + ionic fluoride and total fluorine can be established by sample treatment.

Evaluation Studies as Topic↗
PubMed1990

Determination of trace amount of cobalt in feed grains and forages by solvent extraction and graphite furnace atomic absorption spectrometry.

A method is described for the determination of trace amounts of cobalt in feed grains and forages with a detection limit of 1 ng g-1. Samples are ashed in a muffle furnace and complexed with 2-nitroso-1-naphthol. Following solvent extraction, cobalt is determined using graphite furnace atomic absorption spectrometry. The assay can be carried out in a normal analytical laboratory without the need for special "clean" rooms. Reagents have been selected to keep reagent blank values at low levels, and heptan-2-one is used as extracting solvent to avoid problems with evaporation. The assay has been used for diagnostic purposes and to formulate special low cobalt diets for sheep for experimental purposes.

Animal Feed↗
PubMed1993

Electrocatalytic reduction of hydrogen peroxide at a stationary pyrolytic graphite electrode surface in the presence of cytochrome c peroxidase: a description based on a microelectrode array model for adsorbed enzyme molecules.

Electrochemical reduction of H2O2 at pyrolytic graphite disc electrodes of radius 2.5 mm occurs at readily accessible potentials (600 mV versus the standard hydrogen electrode) in the presence of yeast cytochrome c peroxidase. Introduction of the enzyme into the electrolyte solution initiates large changes in the ellipsometric angles measured for the electrode-solution interface, consistent with time-dependent enzyme adsorption. This process may be correlated with changes in electrochemical activity. Over the same time course, linear-sweep voltammograms are characterized by a transition from a sigmoidal to a peak-type waveform. It is proposed that the time-dependent behaviour may be rationalized by use of a microscopic model for substrate mass transport, in which the two-electron reduction of peroxide occurs at electrocatalytic sites consisting of adsorbed enzyme molecules. A voltammetric theory based on treating the adsorbed redox enzymes as an expanding array of microelectrodes is in excellent agreement with experiment.

Adsorption↗
PubMed2001

Optimization of the simultaneous batch determinations of Bi(III), Hg(II) and Cu(II) at an epoxy-graphite electrode bulk modified with 2-mercaptobenzothiazole.

An epoxy-graphite tube composite electrode bulk modified with 2-mercaptobenzothiazole was investigated for the simultaneous, batch determinations of Bi(III), Hg(II) and Cu(II) by differential-pulse anodic stripping voltammetry after open circuit preconcentration of the three metal ions. Systematic optimization was studied with respect to resolution and sensitivities of the differential-pulse peaks. The resolution between peaks was quantified and employed in a window diagram approach to choose the best conditions. The developed method gave detection limits of 4.2 x 10(-9), 3.6 x 10(-9) and 9.5 x 10(-8) M for Hg(II), Bi(III) and Cu(II), respectively (S/N = 3; 3 min preconcentration). The conditions whereby simultaneous determinations could be performed in the absence of cross-interferences of the analytes were also examined. Additional selected substances in the sample background were also studied for possible interferences. The developed method was applied to the simultaneous, batch determinations of Bi(III), Hg(II) and Cu(II) in a standard sample and a human hair sample.

Journal Article↗
PubMed2001

Speciation of nickel in airborne particulate matter by means of sequential extraction in a micro flow system and determination by graphite furnace atomic absorption spectrometry and inductively coupled plasma mass spectrometry.

A four-stage sequential extraction procedure for the speciation of nickel has been applied to ambient aerosol samples. The determination of the soluble, sulfidic, metallic and oxidic Ni fractions in particulate matter was carried out by graphite furnace (electrothermal) atomic absorption spectrometry (ETAAS) and inductively coupled plasma mass spectrometry (ICP-MS). An EDTA solution, a mixture of diammonium citrate and hydrogen peroxide, and a KCuCl3 solution were used as leaching agents for the determination of the soluble, sulfidic and metallic species, respectively, and nitric acid was used for the determination of oxidic compounds after microwave digestion of particulate matter sampled on filters. A new micro scale filter holder placed in a closed flow injection analysis (FIA) system for use in nickel speciation by means of sequential extraction, and the results of the optimisation of the extraction conditions are described. The temperature program for ETAAS was optimised for all extraction solutions with the aid of temperature curves. Pyrolysis temperatures of 900. 600 and 1,000 degrees C were found to be optimum for EDTA, hydrogen peroxide plus ammonium citrate and KCuCl3-containing solutions, respectively. Airborne dust was sampled on lilters at two locations near to a metallurgical plant in Dortmund, Germany. Concentrations in the low ng m(-3) range down to the detections limits (0.1-0.3 ng m(-3)) and various nickel species were found to be present in the collected dust. The mean fractions of total nickel (sampling period of one month) were found to contain 36+20% of soluble, 6 +/- 4% of sulfidic, 11 +/- 15% of metallic and 48 +/- 18% of oxidic nickel.

Air Pollutants↗
PubMed2002

A novel nanoporous graphitic composite.

A novel nanoporous composite containing micrographitic carbon layers is synthesized by preliminarily expanding the interlayer of an oxidized product of graphite using surfactant, followed by Si bridging/pillaring, and carbonization.

Journal Article↗
PubMed2004

Abrasive immobilization of carbon nanotubes on a basal plane pyrolytic graphite electrode: application to the detection of epinephrine.

The performance of a basal plane pyrolytic graphite (bppg) electrode modified with carbon nanotubes is described. Abrasive immobilization of multiwall carbon nanotubes on a bppg electrode was achieved by gently rubbing the electrode surface on a filter paper supporting carbon nanotubes. The resulting electrode showed excellent mediation of epinephrine oxidation: a decrease in the overvoltage of the epinephrine electro-oxidation (200-500 mV) was observed as well as a dramatic increase in the peak current (4 times) compared to that seen at a bare bppg electrode. The oxidation peaks of epinephrine and ascorbic acid which overlap on bare bppg electrode were separated successfully (by ca. 220 mV) at the surface of the modified bppg electrode. The modified electrode showed good stability in comparison to most modified carbon nanotubes electrodes prepared by alternative methods.

Ascorbic Acid↗
PubMed2004

Evaluation of the surface charge properties of porous graphitic carbon stationary phases treated with redox agents.

The effect of treatment of porous graphitic carbon (PGC) stationary phases with hydrogen peroxide and with sodium sulfite on the retention behavior of analyte compounds has been investigated using benzene, aromatic sulfonate ions, and benzyltrialkylammonium ions as model compounds. It is shown that the retention times of the cationic analytes are increased by treating the PGC column with the reducing agent, while decreased by treating it with the oxidizing agent. On the other hand, the retention times of the anionic analytes are decreased by treating the column with the reducing agent, while increased by treating it with the oxidizing agent. The effect of the redox treatment on the retention of benzene is negligibly small. The investigation of the ion-exchange property of the PGC packings have shown that PGC has anion-exchange property and the anion-exchange capacity is decreased by treating PGC with the reducing agent, whereas it is increased by treatment with the oxidizing agent. This means that the modification of the retention selectivity of the PGC stationary phases with redox treatment can be interpreted in terms of the change of the surface charge. The mechanism of chemical modification of the PGC stationary phase with redox treatment is discussed on the basis of the experimental results obtained on the ion-exchange capacity and the redox activity.

Journal Article↗
PubMed2005

Ion transfer processes at 4-(3-phenylpropyl)-pyridine/aqueous electrolyte/electrode triple phase boundary systems supported by graphite and by mesoporous TiO2.

Biphasic electrode systems allow electrochemical reactions to be driven in a microphase of organic liquid (typically 1-100 nL), which is coupled via ion transfer processes to the surrounding aqueous electrolyte medium. Microdroplet deposits on basal plane pyrolytic graphite as well as thin film deposits of the organic phase within a mesoporous titanium oxide host film are investigated. Cobalt tetraphenylporphyrin (CoTPP) is dissolved in the organic liquid 4-(3-phenylpropyl)-pyridine (PPP) and deposited in the form of microphases at suitable electrode surfaces. The electrode is immersed in aqueous electrolyte environments. It is shown that two stable and highly reversible one-electron metal-centred redox processes occur consistent with Co(III/II)TPP and Co(II/I)TPP in the presence of axial pyridine ligands. The electrochemical characteristics for both processes are strongly affected by the liquid/liquid ion exchange accompanying the redox processes. The potential for both the Co(III/II)TPP and the Co(II/I)TPP redox processes can be adjusted independently by the choice of the nature and concentration of the aqueous electrolyte. The reversible potential observed for the CoTPP metal complex is dominated by the Gibbs energy of transfer for the 'spectator ions'. Conditions can be chosen to eliminate ion transfer effects on the potential scale for biphasic oxidation and reduction processes.

Journal Article↗
PubMed2004

Highly stable carbon-protected cobalt nanoparticles and graphite shells.

The fabrication of carbon-shell protected cobalt nanoparticles and hollow graphitic shells has been achieved via a pyrolysis process by using monodispersed cobalt nanoparticles as a template. These materials are mesoporous and highly stable under strong acidic and basic conditions.

Journal Article↗
PubMed2005

Capillary electrophoresis coupled to mass spectrometry from a polymer modified poly(dimethylsiloxane) microchip with an integrated graphite electrospray tip.

Hybrid capillary-poly(dimethysiloxane)(PDMS) microchips with integrated electrospray ionization (ESI) tips were directly fabricated by casting PDMS in a mould. The shapes of the emitter tips were drilled into the mould, which produced highly reproducible three-dimensional tips. Due to the fabrication method of the microfluidic devices, no sealing was necessary and it was possible to produce a perfect channel modified by PolyE-323, an aliphatic polyamine coating agent. A variety of different coating procedures were also evaluated for the outside of the emitter tip. Dusting graphite on a thin unpolymerised PDMS layer followed by polymerisation was proven to be the most suitable procedure. The emitter tips showed excellent electrochemical properties and durabilities. The coating of the emitter was eventually passivated, but not lost, and could be regenerated by electrochemical means. The excellent electrochemical stability was further confirmed in long term electrospray experiments, in which the emitter sprayed continuously for more than 180 h. The PolyE-323 was found suitable for systems that integrate rigid fused silica and soft PDMS technology, since it simply could be applied successfully to both materials. The spray stability was confirmed from the recording of a total ion chromatogram in which the electrospray current exhibited a relative standard deviation of 3.9% for a 30 min run. CE-ESI-MS separations of peptides were carried out within 2 min using the hybrid PDMS chip resulting in similar efficiencies as for fused silica capillaries of the same length and thus with no measurable band broadening effects, originating from the PDMS emitter.

Electrophoresis, Capillary↗
PubMed2004

Ab initio study of small graphitic cones with triangle, square, and pentagon apex.

Accurate geometries of carbon nanocones of different sizes with a triangle, square or pentagon at the apex have been determined for the first time using a quantum chemical optimization method. The structure close to the apex is distorted from an ideal conical surface. The charging effect of the central defect is quite different from that predicted by tight-binding calculations. The symmetry behavior of the frontier orbitals and the size of the highest occupied molecular orbital-lowest unoccupied molecular orbital gap versus cone type and size is explained. The density of states quickly converges towards that of graphite when the size of the cone increases. In comparison to previous results in the literature it is found that the local densities of states of cones, that are locally different but belong to the same topo-combinatoric class, share common features.

Journal Article↗