1,720,973 research outputs found
Study of the nickel-based chemically modified electrode obtained by electrochemical deposition of an NiII-tetramethyl-dibenzo-tetraaza [14] annulene complex. Redox catalysis of carbohydrates in alkaline solutions. II
No full textThe electrochemical treatment of an electropolymerized (NiIIL)n thin film (where L represents 5,7,12,14-tetramethyl-dibenzo[b,i]-1,4,8,11-tetraaza [14] annulene) gives rise to a nickel-based catalytic deposit on conducting substrates [glassy carbon (GC), platinum, or gold electrodes], which shows strong similarities to the nickel hydroxide electrode. The effect of potential cycling on surface composition, chemical status, and morphology of this nickel-based chemically modified electrode (Ni-CME) has been investigated by cyclic voltammetry (CV) [1], X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. XPS analysis was very useful for characterizing the chemical changes in the nickel coordination caused by electrochemical treatments in alkaline solution. The featureless surface of the as-prepared GC/(NiII)Ln, film compared with the fractal nature of the GC/Ni-CME was revealed by electron micrographs. The Ni- CME exhibits high electrocatalytic activity towards the oxidation of carbohydrates in alkaline solution. The electrooxidation capability of the Ni-CME is highlighted by the amperometric detection of sugars after anion-exchange liquid chromatographic separations with alkaline mobile phases
On the characterization of carbon fibre surfaces by X-ray excited Auger electron spectroscopy
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
XPS investigation of ultra-high-vacuum storage effects on carbon fibre surfaces
No full textProlonged storage in ultra-high vacuum (UHV) can give rise to chemical modifications of carbon fibre surfaces. In the case of fibres contaminated before insertion into the UHV chamber, surface contaminants can be desorbed during storage. When uncontaminated carbon fibres are inserted into the chamber, layers of hydrocarbons can be progressively built up, probably coming from the UHV system. The effects can be conveniently monitored in situ, in the analysis chamber of an XPS spectrometer. A careful analysis of the Cls regions provided evidence that the higher the level of contaminants and/or the content of oxidized species on the fibres' surface, the more evident the influence of the UHV storage time on XPS spectra. These findings suggest that preliminary tests on time-dependent modifications of the surface status of carbon fibres under UHV conditions can provide a more reliable XPS analysis
Anion-exchange chromatography with electrochemical detection of alditols and sugars at a Cu2O-carbon composite electrode
No full textAn anion-exchange column coupled with an amperometric sensor was used for the quantitative analysis of alditols and simple sugars. The sensing electrode is composed of cuprous oxide dispersed in a graphite powder–polyethylene composite matrix. The resulting Cu2O–carbon composite electrode is stable in alkaline media and possesses good sensitivity, wide linear dynamic ranges and low detection limits for alditols, mono- and disaccharides. Alditols and carbohydrates are weakly ionizable compounds, so an anion-exchange column was employed for their chromatographic separation with an alkaline eluent. The separation problems due to the presence of low but uncontrolled amounts of carbonate in the alkaline mobile phase have been largely solved by the addition of Ca2+ or Ba2+ at a millimolar level and the consequent formation of carbonate insoluble salts. Using this strategy, the alkaline eluent provides improved separations without compromising the column's lifetime, electrode performance and chromatographic system. Under the optimal operating conditions, the detection limits of d-sorbitol, d-mannitol and d-glucose were 50, 40 and 80 pmol, respectively, with a linear concentration range up to 5 mM. Examples of applications, which include the separation and detection of d-sorbitol, d-mannitol and common sugars present in food samples, are illustrated
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Non-linear least squares refinement with constraints. Evaluation through curve fitting on emulated XPS-like spectra and application to the analysis of carbon fibres
No full textA non-linear least squares iterative refinement has been implemented which shows high performance on a multiple-peak spectrum including baseline or background. Constraints as well as links within a range are introduced to drive the mathematical optimization: each peak parameter (i.e. height, position, Gaussian/Lorentzian mixing ratio and HWHM on both left and right sides) has assigned to it an allowed range of variation and can be strained to be correlated with other parameters belonging either to the same peak (symmetrical peaks) or to other peaks (doublets, triplets, etc.). Peak shapes typical of XP spectra are used and applications in the field of XPS are discussed. Through emulated curves with Poisson distributed noise, the accuracy and precision of back-calculated (refined) parameters have been estimated. Moreover, a confidence level calculated from X2 and degrees of freedom has been suggested to check the overall fitting of experimental curves where the signal-to-noise ratio is a priori unknown. An application to real C ls XP spectra is described as an example and a list of suggestions is given to match operator requirements. Finally, features of NLLSRC are discussed with respect to other approaches
Study of a cobalt-based surface modified glassy carbon electrode: Electrocatalytic oxidation of sugars and alditols
No full textSugars and alditols undergo an electrocatalytic oxidation in alkaline solution at cobalt-oxide-based glassy carbon (Co/GC) electrodes. The catalytic deposit was investigated by cyclic voltammetry and scanning electron microscopy (SEM). It is pointed out that, at relatively low coverage of cobalt, two main redox processes, CoII → CoIII and CoIII → CoIV, are observed in 0.2 M NaOH. The redox behavior of the Co/GC electrode is strongly dependent on hydroxide ion concentration, and no electrocatalytic activity is observed at pH lower than 11.5. Under voltammetric conditions the modified electrode exhibits a non-Nernstian potential/pH shift (−88 ± 3 mV per pH unit). A reasonable explanation of the E/pH behavior in alkaline solution is given and an oxidation mechanism of CoII to CoIII is proposed in terms of hydrous cobalt oxides. No detrimental effects on the electrocatalytic oxidation of polyhydric compounds are observed in the presence of high acetonitrile concentrations (up to 40%). Inspection by SEM of the cobalt-based deposit evidenced the presence of grain-like microcrystallites on the glassy carbon surface
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