石墨炉原子吸收法中热解涂层石墨管在无标准分析法测定环境样品中铬的应用

本文探讨了热解涂层石墨管测定铬的最佳实验条件,研究了各种酸和干扰物质的基体效应,提出用浓氨水为基体改进剂克服高氯酸的干扰。比较了各原子化温度时的理论和实验特征量。使热解涂层石墨管有可能用于无标准分析法测定各种环境样品中的铬。     

Edge Plane Pyrolytic Graphite Electrodes for Halide Detection in Aqueous Solutions

The behavior of chloride, bromide and iodide at edge plane pyrolytic graphite electrodes has been explored in aqueous acid solutions. The voltammetric response in each case has been compared with that of basal plane pyrolytic graphite, glassy carbon and boron‐doped diamond. The electrochemical oxidation of chloride is found to only occur on boron‐doped diamond while the electrochemical reversibility for the oxidation of bromide on edge plane pyrolytic graphite is similar to that seen at glassy carbon whilst being superior to basal plane pyrolytic graphite and boron‐doped diamond. In the case of iodide oxidation, edge plane and basal plane pyrolytic graphite and glassy carbon display similar electrode kinetics but are all superior to boron‐doped diamond. The analytical possibilities were examined using the edge plane pyrolytic graphite electrode for both iodide and bromine where is was found that, based on cyclic voltammetry, detection limits in the order of 10^?6 M are possible.     

Determination of beryllium,barium, vanadium and some other elements in water by atomic absorption spectrometry with electrothermal atomization

The determination of beryllium, barium and vanadium by atomic absorption spectrometry in an uncoated graphite furnace poses several problems, e.g. bad reproducibility, memory effects, etc. These difficulties can be avoided by using tubes coated with pyrolytic graphite and carbide. The optimal temperature for the pyrolytic graphite coating and the quantity of lanthanum that should be introduced for the carbide coating are discussed. Beryllium, barium and vanadium in surface water and tap water can be determined without memory effects and with detection limits of 0.01, 1 and 1μg l^-1, respectively. Good agreement was found with the results obtained by activation analysis and flame or flameless (with uncoated tubes) atomic absorption spectrometry after preconcentration. The lifetime of the coated tubes was increased, and improved results were also found for the determination of other carbide-forming and/or high-melting elements such as molybdenum, cobalt, nickel, copper and chromium.     

Gas sensing using edge-plane pyrolytic-graphite electrodes: electrochemical reduction of chlorine

The voltammetric responses of chlorine in aqueous acid solutions have been explored using different carbon-based electrodes. Edge-plane pyrolytic graphite has more electrochemical reversibility than glassy carbon, basal-plane pyrolytic graphite, or boron-doped diamond electrodes. A significant reduction in the overpotential is observed on the edge-plane pyrolytic-graphite electrode in contrast with the other carbon-based electrode substrates. These results suggest that edge-plane pyrolytic graphite can be optimally used as the working electrodes in Clark-cell devices for low-potential amperometric gas sensing of Cl₂.     

石墨炉原子吸收法测定钒的研究

本文比较了在标准石墨管,热解涂层和全热解石墨管中钒的吸收信号形状。用全热解石墨管和EDTA铵盐作基体改进剂,直接测定水系沉积物中痕量钒。方法的特征量为61pg/0.0044A。     

Exploring the electrocatalytic sites of carbon nanotubes for NADH detection: an edge plane pyrolytic graphite electrode study

The electrocatalytic properties of multi-walled carbon nanotube modified electrodes toward the oxidation of NADH are critically evaluated. Carbon nanotube modified electrodes are examined and compared with boron-doped diamond and glassy carbon electrodes, and most importantly, edge plane and basal pyrolytic graphite electrodes. It is found that CNT modified electrodes are no more reactive than edge plane pyrolytic graphite electrodes with the comparison with edge plane and basal plane pyrolytic graphite electrodes allowing the electroactive sites for the electrochemical oxidation of NADH to be unambiguously determined as due to edge plane sites. Using these highly reactive edge plane sites, edge plane pyrolytic graphite electrodes are examined with cyclic voltammetry and amperometry for the electroanalytical determination of NADH. It is demonstrated that a detection limit of 5 microM is possible with cyclic voltammetry or 0.3 microM using amperometry suggesting that edge plane pyrolytic graphite electrodes can conveniently replace carbon nanotube modified glassy carbon electrodes for biosensing applications with the relative advantages of reactivity, cost and simplicity of preparation. We advocate the routine use of edge plane and basal plane pyrolytic graphite electrodes in studies utilising carbon nanotubes particularly if 'electrocatalytic' properties are claimed for the latter.     

Exploration of gas sensing possibilities with edge plane pyrolytic graphite electrodes: nitrogen dioxide detection

The voltammetric response of nitrogen dioxide in aqueous sulfuric acid using an edge plane pyrolytic graphite electrode has been explored and contrasted with that from basal plane pyrolytic graphite, glassy carbon or boron-doped diamond electrodes. Edge plane graphite electrode is found to produce an excellent voltammetric signal in comparison with other carbon-based electrodes exhibiting a well-defined analytically useful voltammetric redox couple in 2.5 M sulfuric acid which is absent on the alternative electrodes.     

Study on the Atomization Mechanism of Hydrides in Graphite Furnace Atomizers

The influence of the surface state of the graphite furnace atomizer on the atomization of hydrides has been studied by means of surface film coating and quantum chemistry CN-DO/2 calculations. The results of the study prove that the atomization of AsH3, SbH3 and BiH3 in the graphite furnace atomizer is not a simple gas phase pyrolytic process, but a surface catalysis pyrolytic process.     

CVD graphene vs. highly ordered pyrolytic graphite for use in electroanalytical sensing

We explore and contrast the electroanalytical performance of a commercially available CVD grown graphene electrode with that of edge- and basal-plane pyrolytic graphite electrodes constructed from highly ordered pyrolytic graphite for the sensing of biologically important analytes, namely β-nicotinamide adenine dinucleotide (NADH) and uric acid (UA). We demonstrate that for the analytes studied here, in the best case, the electroanalytical performance of the CVD-graphene mimics that of edge plane pyrolytic graphite, suggesting no significant advantage of utilising CVD-graphene in this context.     

Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry: II. Total pyrolytic graphite platforms in pyrographite coated polycrystalline electrographite tubes

Morphological studies on graphite surfaces by scanning electron microscopy are presented for platforms made from total pyrolytic graphite, and for polycrystalline electrographite tubes with pyrographite coating in which the pyrolytic graphite platforms are inserted. Platforms made of pyrolytic graphite have a very long lifetime and can actually survive several tubes. But also the lifetime of polycrystalline electrographite tubes with pyrographite coating is substantially longer when the sample is deposited on a platform and not on the tube wall. The analytically useful lifetime of platform and tube comes close to 1000 determinations at an atomization temperature of 2650°C, and even in the presence of corrosive matrices 500 determinations can be performed without significant loss in sensitivity. Intercalation appears to be no problem with pyrolytic graphite platforms. Substantial deposition of graphite was found on the platform under different analytical conditions, leading to a secondary coating of the surface. The sensitivity of carbide-forming elements depends very much upon the quality of the pyrographite coating of the polycrystalline electrographite tubes in which the platforms are mounted.     

Electroanalytical Exploitation of Nitroso Phenyl Modified Carbon‐Thiol Interactions: Application to the Low Voltage Determination of Thiols

The electrochemical generation of nitrosophenyl groups covalently attached to graphite powder (nitrosophenylcarbon) from carbon powder chemically modified with nitrophenyl groups and their subsequent reaction with thiols (glutathione, cysteine and homocysteine) has been investigated as a method by which the later can be quantified. The modified carbon powder was immobilized onto a basal plane pyrolytic graphite electrode and characterized by cyclic voltammetry by scanning between 1.0 V and ?1.0 V vs. SCE in phosphate buffer (pH 7). Square wave voltammetry (SWV) was used for the determination of thiols and the SWV parameters were optimized. The nitrosophenylcarbon is electrogenerated from nitrophenylcarbon and can chemically oxidize thiols to disulfides. Subsequent reduction of nitrosophenylcarbon to phenylhydroxylaminecarbon during the square wave voltammetric process leads to a decrease in the reductive current. This can be correlated to the concentration of thiol present within the medium. The cyclic voltammetric responses of basal plane pyrolytic graphite electrode, edge plane pyrolytic graphite electrode, glassy carbon electrode and boron‐doped diamond electrode in the direct oxidation of thiols were also investigated and all were found to have a significantly higher overpotential compared to the described method using nitrosophenylcarbon.     

Surface and subsurface examination of graphite tubes after electrodeposition of noble metals for electrothermal atomic absorption spectrometry

The modification phenomena of noble metals (Pd, Ir, Rh) electrodeposited onto the inner surface of pyrolytic graphite (PG) coated furnaces were investigated mainly by electron microprobe analysis with energy dispersive X-ray emission detection. The conditions of electrodeposition were optimized in order to achieve the best analytical performance of atomic absorption measurements. Investigations concerning the distribution of noble metals on the tube surface and in-depth were performed at different stages of the tube history. It was found that the noble metals used for the modification do not form a compact layer on the surface but penetrate into the pyrolytic graphite structure already at the deposition step. When two metals were deposited together, both penetrated into the graphite structure. The degree of penetration of the pyrolytic graphite at high temperature differs for various metals. It was also demonstrated that electrodeposited noble metals remain in sub-surface domains of the graphite for hundreds of atomization cycles, which means that they can be used as permanent modifiers.     

Fundamental studies on improvement of precision and accuracy in flameless atomic-absorption spectroscopy using the graphite-tube atomizer Lead in whole blood

Some fundamental aspects concerning the formation of ash on a pyrolytic graphite surface and the resulting effects on precision and accuracy in flameless AAS, have been examined for the determination of lead in whole blood. The nature and composition of the ash from whole blood and the state of the pyrolytic graphite surface have been observed with the aid of a scanning electron microscope and non-dispersive X-ray microprobe analyser. The use of nitric acid to separate the analytical peak from the matrix peak, an examination of high-temperature clearing of the matrix, and removal of ash remnants between each analysis are reported. A procedure to improve precision and accuracy in flameless AAS using the graphite-tube atomizer is reported.     

Application of Nanocrystalline Graphite‐like Pyrolytic Carbon Film Electrode in the Electroanalytical Determination of Famotidine

Direct electro‐oxidation of famotidine at different graphitic carbon‐based electrode materials was evaluated. These materials included conventional electrodes of edge‐plane pyrolytic graphite, basal‐plane pyrolytic graphite, carbon paste, and glassy carbon as well as nano‐structured carbon‐based materials such as pyrolytic carbon film, carbon nanotube, and nano‐graphene. Raman spectroscopy and scanning electron microscopy were employed to analyze their structural and morphological features. It was found that the pyrolytic carbon film electrode, after a simple and fast anodic activation, shows superior electroanalytical performance. The method was successfully applied for the electroanalytical determination of famotidine in tablet dosage forms and urine samples.     

Analyte stabilization by electrodeposited palladium modifier for electrothermal atomic absorption spectrometry: characterization by scanning electron microscopy and anodic stripping voltammetry

In electrothermal atomic absorption spectroscopy (ETAAS) effective stabilization of analytes can be achieved by an initial in situ electrodeposition of 0.2 mug of Pd. This amount of Pd is on average a factor of 50 lower than that typically used for conventional chemical modification. The surface features of this modifier have been characterized by scanning electron microscopy (SEM) measurements on sectioned pyrolytic graphite furnaces and contrasted with those for modifier produced from thermally reduced Pd salts. Electrodeposition produces a uniform array of Pd domains stretching approximately 2 mm from the centrally positioned Pt/Ir anode (which doubles as the autosampler sample delivery tube). In contrast, thermally reduced Pd salts produce a modifier which concentrates in domains near the drying edge of the modifier solution. Anodic stripping voltammetry (ASV) established that Pb electrodeposited onto Pd-modified pyrolytic graphite affixes to the Pd rather than the graphite. ASV measurements using a basal plane pyrolytic graphite working electrode also established that (i) the stripping potentials for monolayer and multilayer Pb are shifted anodically by 0.16 and 0.18 V, respectively by binding to Pd rather than graphite and (ii) deposition of Pb from dilute acidic medium (1% HNO(3)) leads only to monolayer Pb, in contrast to deposition from acetate buffer (pH 4.0-4.4) which produces predominantly multilayer Pb.     

Factors Affecting Selenium Atomization Efficiency in Graphite Furnace Atomic Absorption

Abstract

The effect of graphite furnace surface treatment, and the addition of “matrix modifiers” such as nickel or lanthanum on the observed selenium atomization siqnal in electrotherval atomic absorption analysis has been investiqated. The results indicate that the removal of signal depression caused by the addition of metal solution to analyte solution is not simply a devolatilization of the selenium. The effect appears to be a modification of the graphite surface which leads to more efficient atom formation. The role of the surface was investigated monitoring the atomic absorption signal generated from graphite furnaces which were untreated, pyrolytic graphite-coated, zirconium-or tantalum-coated, and metal-coated followed by pyrolytic graphite coating. The dependence of the analyte signal on the concentration of added metal was investigated for these surfaces. The optimum results obtained were the metal-coated/pyrolytic graphite-coated cuvettes. These cuvettes showed reduced effect of “matrix modifier,” suggesting that the surface treatment can replace the “matrix modifer,” Surface chemistry consistent with the atomic absorption observations and surface analysis data is presented.     

Application of anodized edge-plane pyrolytic graphite electrode for analysis of clindamycin in pharmaceutical formulations and human urine samples

Different graphitic carbon-based electrode materials were evaluated for direct electro-oxidation of clindamycin and electroanalytical parameters such as sensitivity, residual background current, and signal-tobackground current ratio were compared to select the best one for the clindamycin electroanalysis. Such electrode materials include glassy carbon, carbon paste, pyrolytic graphite (edge-plane and basal-plane), carbon nanotube, reduced graphene oxide, and carbon black. The edge-plane pyrolytic graphite electrode after a simple and fast electrochemical pretreatment showed superior performance compared with the other carbon electrodes. Raman and Fourier transform infrared spectroscopy were employed to analyze the surface microstructure and chemical bonding of the carbon materials and scanning electron microscopy was used to study their surface morphologic features. The applicability of the electrochemically activated edge-plane pyrolytic graphite electrode for the determination of clindamycin in pharmaceutical formulations and human urine samples was evaluated.     

Edge Plane Pyrolytic Graphite Electrodes for Stripping Voltammetry: a Comparison with Other Carbon Based Electrodes

The first examples of using edge plane pyrolytic graphite electrodes for anodic and cathodic stripping voltammetry (ASV and CSV) are presented, notably the ASV of silver and the CSV of manganese. In the former example, detection limits for silver (based on 3σ) of 8.1 nM and 0.185 nM for 120 s and 300 s accumulation time, respectively, were achievable using the edge plane electrode, which were superior to those observed on glassy carbon, basal plane pyrolytic graphite and boron‐doped diamond electrodes. In the second example, a detection limit for manganese of 0.3 μM was possible which was comparable with that achievable with a boron‐doped diamond electrode but with an increased sensitivity. Comparison of the edge plane pyrolytic graphite electrode with boron‐doped diamond electrodes reveals that the edge plane electrode has comparable detection limits and sensitivities whilst exhibiting a lower signal‐to‐noise ratio and large potential window for use in trace analysis suggesting boron‐doped diamond can be conveniently replaced by edge plane pyrolytic graphite as an electrode material in many applications.     

Determination of Ni and Cr in soils by slurry graphite furnace atomic absorption spectrometry

The analytical conditions for chromium and nickel determination in soils by slurry sampling graphite furnace atomic absorption spectrometry (GFAAS) are presented. Stability tests for slurries have been carried out. The ratio of the amount of the analyte found in the liquid phase to the total amount is investigated. The influence of the soil matrix on the background is described. Less sensitive resonance lines of chromium rather than an internal gas flow through the graphite furnace are recommended for some soil samples with high contents of chromium.     

Electrochemical Response of Cobalt(II) in the Presence of Ammonia

The electrochemical oxidation of cobalt(II) at gold, boron‐doped diamond, basal and edge plane pyrolytic graphite, and highly oriented pyrolytic graphite electrodes in aqueous solutions containing NH₃ has been studied using cyclic voltammetry, with subsequent chemical and electrochemical processes explained in detail. Furthermore, the electro‐reduction of [Co(NH₃)₆]³⁺ in the presence of different electrolytes has also been studied to obtain a better understanding of the oxidation pathway of the Co(II)‐ammine complexes. In aqueous solution the mechanism can be described by the following scheme: