1,721,074 research outputs found
Characterization of gas phase aggregates of bis(2-ethylhexyl)-sulfosuccinate (AOT) and divalent metal ions: elimination of radical species in the decomposition pathways of even-electron [AOTMIICl2]– anions
No full textStructure and properties of even-electron anionic species formed by bis(2-ethylhexyl)sulfosuccinate (AOT) and divalent metal ions (MII) with stoichiometry [AOTMIICl2] have been investigated by using electrospray ionization and different mass spectrometry techniques, such as high resolution, accurate mass measurements, collision-induced dissociation (CID) multiple-stage mass spectrometry. Owing to CID, eliminations of neutrals, mainly consisting in hydrochloric acid, 2-ethyl-1-hexene and 2-ethylhexanol, and an unexpected loss of an alkyl radical have been observed. The radical anions [C4HO6SMIICl] so produced have been characterized by MS3 experiments. Density functional theory calculations have been carried out for investigating structure and stability of the ionic species formed in the decomposition pathways. Copyright (c) 2012 John Wiley & Sons, Ltd
Recent Approaches for Chemical Speciation and Analysis by Electrospray Ionization (ESI) Mass Spectrometry
Full text linkIn recent years, the chemical speciation of several species has been increasingly monitored and investigated, employing electrospray ionization mass spectrometry (ESI-MS). This soft ionization technique gently desolvates weak metal–ligand complexes, taking them in the high vacuum sectors of mass spectrometric instrumentation. It is, thus, possible to collect information on their structure, energetics, and fragmentation pathways. For this reason, this technique is frequently chosen in a synergistic approach to investigate competitive ligand exchange-adsorption otherwise analyzed by cathodic stripping voltammetry (CLE-ACSV). ESI-MS analyses require a careful experimental design as measurement may face instrumental artifacts such as ESI adduct formation, fragmentation, and sometimes reduction reactions. Furthermore, ESI source differences of ionization efficiencies among the detected species can be misleading. In this mini-review are collected and critically reported the most recent approaches adopted to mitigate or eliminate these limitations and to show the potential of this analytical technique
Surfactant Self-Assembling in the Gas Phase: Bis(2-ethylhexyl)sulfosuccinate-Divalent Metal Ion Anionic Aggregates
No full textRATIONALE Investigation of fundamental aspects driving surfactant self-assembling and of the capability of including guest molecules or ions in their micellar aggregates is an exciting research field for theoretical and technological reasons. In this light, assembling and chelating properties of sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) towards divalent metal ion chlorides have been investigated in the gas phase by electrospray ionization mass spectrometry in negative ion mode, tandem mass spectrometry and energy-resolved mass spectrometry. METHODS Water/methanol solutions of AOTNa and chloride salts of nickel, magnesium, calcium and manganese, with different AOTNa/metal salt ratios, were infused into the electrospray source of a LCQ DECA ion trap mass spectrometer, operating in negative ion mode, at a flow rate of 5 mu L/min. Low energy collision-induced dissociations were carried out by using helium with collision energy in the range 15?eV. RESULTS A variety of negatively singly charged monometallated and mixed metal aggregates have been observed, some of which were able to incorporate the metal counter ion of the inorganic salt used. The stability of these aggregates was evaluated by energy-resolved mass spectrometry which showed, for the anions [AOTMIICl2], a stability order Ca?>?Mn?>?Mg?>?Ni. Their decomposition pathways show the unusual formation of the radical anions [C4HO6SMIICl]. CONCLUSIONS This study shed some light on the assembling and chelating properties of AOT towards divalent metal ions to form negatively charged assemblies, some of them incorporating the metal counter ion of the inorganic salt used. Differently from what was observed with positively charged AOT-MII aggregates, solvated species were not detectable. An exception to the even-electron rule was observed in the decomposition pathway of [AOTMIICl2]. Copyright (c) 2012 John Wiley & Sons, Ltd
Atractyligenin chemistry. Part 4: Synthesis of the 15β-ol epimer of atractyligenin
No full textThe oxidation with PCC of atractyligenin 3, the aglicone of the toxic glycoside 1 isolated from Atractylis gummifera L., led to the 15-oxo-derivative 6, together with a small amount of 2,15-dioxoderivative 5, with a selective oxidation of the allylic 15-OH group of the molecule. Further reduction with NaBH4 and KOH hydrolysis gave stereoselectively the 15β-ol epimer of atractiligenin 9
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
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
Do electrospray mass spectra of surfactants mirror their aggregation state in solution?
No full textOne important feature in the gas phase chemistry of surfactants is to ascertain whether their aggregates produced by electrospray
ionization reflect those formed in the starting solution. With this aim, we have performed ESI-MS, ESI-MS/MS and
ER-MS spectra of bis(2-ethylhexyl)sulfosuccinate (AOTNa) solutions in different solvents, i.e. water, water/methanol,
methanol and n-hexane. The results clearly indicate that, notwithstanding the strongly different aggregation state in solution
(direct micelles in water and in water/methanol, molecular dispersion in methanol and reverse micelles in n-hexane) and
marked effects of the solvent polarity on the total ionic current, the surfactant aggregates in gas phase show identical
structural features. Analogous conclusions can be drawn analyzing the infrared multiple photon dissociation (IRMPD) spectra
of AOTNa solutions in water/methanol and n-hexane. Moreover, according to the idea that gas phase can be considered an
apolar environment par excellence, data consistently suggest a reverse micelle-like aggregation. Some peculiarities of the
mechanisms leading to aggregate formation through electrospray ionization of surfactant solutions in solvent media with
different polarity have been also discussed
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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