1,721,175 research outputs found
METODOLOGIE CROMATOGRAFICHE NELLO STUDIO DEI PROCESSI DI ADSORBIMENTO
No full textIn analytical chromatography, the sample we analyze is usually rather dilute and allows
the development of a rather straightforward method. Due to the minute concentrations we
deal with in analytical chromatography, we face a linear behavior. The retention time of
the analytes and the selectivity of a given separation can be forecast by simple rules that
tremendously help us to develop efficient and fast separations. However, when we increase
the sample size and a finite amount of sample is introduced in a chromatographic column,
we leave the shelter of linear chromatography and have to cope with more complex peak
shapes and phenomena.
When the amount of the sample is comparable to the adsorption capacity of the
zone of the column the migrating molecules occupy, the analyte molecules compete for
adsorption on the surface of the stationary phase. The molecules disturb the adsorption
of other molecules, and that phenomenon is normally taken into account by nonlinear
adsorption isotherms. The nonlinear adsorption isotherm arises from the fact that the
equilibrium concentrations of the solute molecules in the stationary and the mobile phases
are not directly proportional. The stationary phase has a finite adsorption capacity;
lateral interactions may arise between molecules in the adsorbed layer, and those lead
to nonlinear isotherms. If we work in the concentration range where the isotherms are
nonlinear, we arrive to the field of nonlinear chromatography where thermodynamics
controls the peak shapes. The retention time, selectivity, plate number, peak width,
and peak shape are no longer constant but depend on the sample size and several other
factors. In addition to be a fundamental piece of information to modeling and optimizing
preparative separations, adsorption isotherm determination is the key to study analytestationary
phase interactions. Besides they allow for the characterization in terms of
adsorption energy distribution of the surface heterogeneity. If biomolecules (peptides,
proteins, etc.) or molecules with biological activity (such as drugs, etc.) were chemically
bound to the stationary phase one speaks in terms of bioaffinity chromatography. In
these cases, adsorption isotherm measurements are a powerful tool to study molecular
recognition processes between the adsorbed biomolecules and any analyte injected into
the chromatographic column.
During this PhD thesis, different aspects of fundamentals of adsorption processes at
the liquid-solid interface have been considered. In parallel, we focused on the setup of
instrumental techniques for the automatic determination of adsorption isotherms.
For that which concerns the study of fundamentals of adsorption, stationary phases
traditionally employed in liquid chromatography (C18) have been characterized by investigating
the adsorption from binary mixtures of compounds with different chemico-physical
properties. This kind of study was essentially realized by measuring excess isotherms.
Through them, the preferential adsorption of one component with respect to the other
constituting the mixture can be determined. These studies are important to understand
the role of so-called organic modifiers in the chromatographic separation process. In fact,
in reversed phase chromatography the organic modifier is usually considered as a mean
to increase mobile phase analyte solubility, while its active role in the adsorption process
is often underestimated.
We then focused on bioaffiniy recognition studies by applying nonlinear concepts to
the characterization of adsorption of peptides and dipetides on a macrocyclic antibiotic
(Teicoplanin) chemically bounded to silica gel. It was demonstrated that nonlinear
chromatography can be considered a valid alternative to other techniques in use for the
determination of binding constants (such as ELISA, Surface Plasmon Resonance, etc.).
The fundamental advantage of nonlinear studies is that they permit to distinguish between
selective and non-selective interactions in the molecular recognition process, which
is not achievable by other investigation techniques.
Adsorption equilibria of insulin (a relatively small protein, about 5800 Da) in different
experimental conditions were studied with the purpose of understanding the mechanisms
responsible for the chromatographic behavior of this protein. In fact, insulin retention
is characterized by a very large sensitivity to the mobile phase composition. Nonlinear
adsorption data were coupled with circular dichroism and mass spectrometry measurements.
Our purpose was to understand if tertiary structure modifications or agglomerate
formation (dimers, trimers, etc.) could be at the origin of the observed phenomena.
Besides thermodynamic aspects, kinetic phenomena are fundamental to characterize
the chromatographic process. In chromatography, in particular, we refer to mass transfer
kinetics, that is the ensemble of the processes involved in the transfer of solute molecules
from the mobile to the stationary phase. In chromatography, these include axial dispersion
(molecular and eddy diffusion), external and internal mass transfer (pore diffusion
and surface diffusion), and adsorption-desorption kinetics. In this work, mass transfer
phenomena on a new kind of monolithic columns, obtained through gamma-ray polymerization,
were studied. The combined use of Van Deemter and kinetic plots allowed to
correlate column efficiency to the length of cross-linkers used in polymerization.
In addition to these fundamental studies, the other part of this work was about the
set-up of instrumentation for different kinds of purposes. A pseudo-bidimensional system
that allows for the deconvolution and online sampling of chromatographic peaks measured
under nonlinear and gradient conditions was developed. The importance of this system is
that it permits to achieve the information necessary for the determination of adsorption
isotherms in an optimized and automatic manner by exploiting numerical procedures,
known in literature as inverse methods. A second application was in the field of pharmaceutics.
By using a system which combines size exclusion and polar reversed-phase
columns, a method for the simultaneous purification and determination of iomeprol, a
radiopharmaceutical analyte, in human plasma was developed and validated
Extraction and determination of aflatoxins in rich-lipidic matrices using deep eutectic solvent as the extraction medium and UHPLC-FLD analysis
No full textSince their discovery in the 1960s, aflatoxins have presented a significant public health concern and posed a serious challenge to food quality control. Their carcinogenic potential and widespread occurrence in various food matrices, including nuts, maize, rice, and dairy products, have prompted regulatory authorities to establish maximum allowable levels for these contaminants in these commodities. Official methods traditionally rely on methanol/water or acetonitrile/water solutions as extraction media due to the slightly polar nature of aflatoxins. Pistachio samples, in particular, present a complex matrix rich in lipids, requiring a critical sample preparation step before analysis. Usually, to reduce the presence of lipidic interferents from the matrix, a de-fatting step using hexane is mandatory, in the present study it was not necessary. We introduce a novel method utilizing deep eutectic solvents (DES) as the extraction medium. DES, a new class of green solvents formed by combining a hydrogen bonding donor and acceptor in an appropriate molar ratio, yields a clear solution with a melting point significantly lower than that of its constituents. Previous studies have demonstrated the non-toxic, biodegradable nature of most DESs, along with their efficient extraction capabilities for various target compounds. Given the complex nature of extracts obtained from aflatoxin-contaminated pistachios, a subsequent clean-up and concentration step was required. The DES extracts were diluted with water and passed through a Supelco Discovery® DSC-18 solid-phase extraction (SPE) cartridge. Trapped aflatoxins were then eluted with a minimal quantity of methanol and injected into a high-performance liquid chromatography with fluorescence detection (HPLC-FLD) system equipped with a partially porous Supelco Ascentis Express® C18 column. The mobile phase, composed of methanol and water, utilized gradient elution conditions ranging from 30% to 60% MeOH over 7.5 minutes. The flow rate was maintained at 0.45 mL/min, the injection volume was 2 μL, and the column temperature was held at 35°C. This innovative approach shows the feasibility of utilizing DES as an extraction medium for the challenging lipidic matrix of aflatoxin-contaminated pistachios, thereby reducing the reliance on conventional solvents and minimizing process waste
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
The versatility of MS-hyphenated GC forms for targeted and non-targeted analysis
No full textClassical targeted analysis aims at quantifying one of some well-known analytes in the sample, considering the remaining chemical domains as interference and superfluous. In doing this, the entire analytical workflow, from the sample preparation to the data elaboration, is selectively optimized towards the defined analyte.
However, advances in multidimensional gas chromatography, mass spectrometry and high-throughput data processing capabilities have instigated a paradigm shift in molecular analysis over the past decades [1-2].
Their combo makes up a final hyphenated high-resolution technique, allowing the separation and monitoring of many more analytes and chemical domains in the samples. This idea of non-targeted analysis is the driving approach of the modern omics sciences for the discovery and understanding of complex systems.
The presentation will showcase examples of targeted and non-targeted analysis exploiting gas chromatography and its multidimensional form (GC×GC) coupled to mass spectrometry over a wide range of applications [3-6].
References
[1] Peter Q. Tranchida, Flavio A. Franchina, Paola Dugo, Luigi Mondello. Comprehensive two-dimensional gas chromatography-mass spectrometry: Recent evolution and current trends. Mass Spectrometry Reviews 35, 4 (2016) 524-534. https://doi.org/10.1002/mas.21443
[2] Delphine Zanella, Monica Romagnoli, Sofia Malcangi, Marco Beccaria, Tatiana Chenet, Chiara De Luca, Fabio Testoni, Luisa Pasti, Ugo Visentini, Giampiero Morini, Alberto Cavazzini, Flavio A. Franchina. The contribution of high-resolution GC separations in plastic recycling research. Analitycal and Bioanalitycal Chemistry, 415 (2023) 2343–2355. https://doi.org/10.1007/s00216-023-04519-8
[3] Flavio A. Franchina, Delphine Zanella, Lena M. Dubois, Jean-François Focant. The role of sample preparation in multidimensional gas chromatographic separations for non-targeted analysis with the focus on recent biomedical, food, and plant applications. Journal of Separation Science 44, 1 (2021) 188-210 https://doi.org/10.1002/jssc.202000855
[4] Delphine Zanella, Jean-François Focant, Flavio A. Franchina. 30 th Anniversary of comprehensive two- dimensional gas chromatography: Latest advances. Analytical Science Advances, 2, 3-4 (2021) 213-224 https://doi.org/10.1002/ansa.202000142
[5] Peter Q. Tranchida, Flavio A. Franchina, Luigi Mondello. Analysis of essential oils through comprehensive two-dimensional gas chromatography: General utility. Flavour and Fragrance Journal, 32, 4 (2017) 218-227 https://doi.org/10.1002/ffj.3383
[6] Sofia Malcangi, Monica Romagnoli, Marco Beccaria, Martina Catani, Tatiana Chenet, Chiara De Luca, Simona Felletti, Luisa Pasti, Alberto Cavazzini, Flavio A. Franchina. Modern sample preparation approaches for small metabolite elucidation to support biomedical research Advances in Sample Preparation, 2 (2022)
100017 https://doi.org/10.1016/j.sampre.2022.10001
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
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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