1,720,982 research outputs found
Perspectives of Gas Phase Ion Chemistry: Spectroscopy and Modeling
Get PDFThe study of ions in the gas phase has a long history and has involved both chemists and physicists. The interplay of their competences with the use of very sophisticated commercial and/or homemade instrumentations and theoretical models has improved the knowledge of thermodynamics and kinetics of many chemical reactions, even if still many stages of these processes need to be fully understood. The new technologies and the novel free-electron laser facilities based on plasma acceleration open new opportunities to investigate the chemical reactions in some unrevealed fundamental aspects. The synchrotron light source can be put beside the FELs, and by mass spectrometric techniques and spectroscopies coupled with versatile ion sources it is possible to really change the state of the art of the ion chemistry in different areas such as atmospheric and astro chemistry, plasma chemistry, biophysics, and interstellar medium (ISM). In this manuscript we review the works performed by a joint combination of the experimental studies of ion–molecule reactions with synchrotron radiation and theoretical models adapted and developed to the experimental evidence. The review concludes with the perspectives of ion–molecule reactions by using FEL instrumentations as well as pump probe measurements and the initial attempt in the development of more realistic theoretical models for the prospective improvement of our predictive capability
Formation of H3O+ and OH by CO2 and N2O trace gases in the atmospheric environment
Get PDFThe impact of cosmic rays’ energetic subatomic particles on climate and global warming is still controversial and under debate. Cosmic rays produce ions that can trigger fast reactions affecting chemical networks in the troposphere and stratosphere especially when a large amount of relevant trace gases such as carbon dioxide, methane, sulfur dioxide and water are injected by volcanic eruptions. This work focuses on synchrotron experiments and an ab initio theoretical study of the ion chemistry of carbon dioxide and nitrous oxide radical cations reacting with water. These molecules catalyze a fast exothermic formation of hydronium ions H3O+ and the hydroxyl radical OH, the main oxidant in the atmosphere. Moreover, theoretical calculations demonstrate that at the end of the catalytic cycle, CO2 and N2O are produced vibrationally excited and subsequently they quench in the microsecond time scale by collision with the surrounding atmospheric molecules at the pressure and temperature of the upper-troposphere/stratosphere. The chemistry involved in these reactions has a strong impact on the oxidant capacity of the atmosphere, on the sulfate aerosol production, on the cloud formation and eventually on the chemical networks controlling climate and global warming models
Novel microsampling approach using fabric-phase sorptive extraction (FPSE) for cannabinoid analysis in blood
Get PDFThe most important bioactive specialised metabolites of Cannabis sp. include cannabinoids, namely tetrahydrocannabinol (THC), cannabidiol (CBD) and their analogues. They are promising compounds with many biological activities, although THC is also known as an illicit compound in several countries. Due to their widespread occurrence and biological activity, cannabinoids are frequently investigated in terms of detection and quantification in various biological matrices. The increasing introduction of products based on plant derivatives and their compounds in the recreational, industrial, pharmaceutical, and illicit markets demands the constant development and refinement of rapid and reliable methods for the determination of these analytes in biological samples. Blood microsampling has emerged as a pivotal innovation in bioanalytical science, offering minimally invasive collection, reduced solvent consumption, and broad reliability and applicability across modern workflows. In this study, an innovative dried microsampling technology based on fabric phase sorptive extraction (FPSE) has been exploited for whole blood sampling, using polymeric fabric patches coated with three different sorbent chemistries: polyethylene glycol (CW20M), polydimethylsiloxane (PDMS) and polytetrahydrofuran (PTHF), for the analysis of two main active cannabinoids and five of their metabolites. An efficient HPLC-MS/MS method was developed, obtaining r2 ≥ 0.9991, LOD ≤ 0.5 ng/mL and LOQ ≤ 1.5 ng/mL for all analytes. Just 20 μL of whole blood was needed to generate a satisfactory dried FPSE spot. The sorbent activation procedures were optimised for the different chemistries, ensuring compatibility with the analytes and high extraction efficiency. Dried sample pretreatment steps by solvent extraction were optimised before HPLC-MS/MS analysis. Analytical results were satisfactory for all analytes, with recovery always higher than 85 %, blood matrix effect lower than 14 % and FPSE matrix effect lower than 9 %. For intraday precision, the RSD value was consistently below 9.3 %, while it was lower than 9.8 % for interday precision. This is the first study where FPSE has been applied to the determination of THC, CBD and their main endogenous metabolites as a novel blood microsampling-based approach
An experimental and theoretical investigation of XPS and NEXAFS of 5-halouracils
No full textThe C, N and O 1s excitation and ionization processes of 5X-uracil (X = F, Cl, Br, and I) were investigated using near edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission (XPS) spectroscopies. This study aims at the fine assessment of the effects of the functionalization of uracil molecules by halogen atoms having different electronegativity and bound to the same molecular site. Two DFT-based approaches, which rely on different paradigms, have been used to simulate the experimental spectra and to assign the corresponding features. The analysis of the screening of the core holes of the different atoms via electronic charge density plots has turned out to be a useful tool to illustrate the competition between the partially aromatic and partially conjugate properties of this class of molecules. © 2018 the Owner Societies
Ultrafast Hydrogen Migration in Photoionized Glycine
Get PDFHydrogen migration in the glycine cation has been investigated using a combination of a short train of attosecond extreme ultraviolet pulses with few-optical-cycle near-infrared pulses. The yield of the photofragments produced has been measured as a function of pump–probe delay. These time-dependent measurements reveal the presence of a hydrogen migration process occurring in 48 fs. Previous mass spectrometric experiments and theoretical calculations have allowed us to identify the conformations and cation states involved in the process induced by the broad band extreme ultraviolet radiation
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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