1,721,004 research outputs found
A Visible‐Light‐Powered Polymerization Method for the Immobilization of Enantioselective Organocatalysts into Microreactors
No full textA versatile one-step photopolymerization approach for the immobilization of enantioselective organocatalysts is presented. Chiral organocatalyst-containing monoliths based on polystyrene divinylbenzene copolymer were generated inside channels of microfluidic chips. Exemplary performance tests were performed for the monolithic Hayashi–Jørgensen catalyst in continuous flow, which showed good results for the Michael addition of aldehydes to nitroalkenes in terms of stereoselectivity and catalyst stability with minimal consumption of reagents and solvents
PDMS free-flow electrophoresis chips with integrated partitioning bars for bubble segregation
No full textIn this work, a microfluidic free-flow electrophoresis device with a novel approach for preventing gas bubbles from entering the separation area is presented. This is achieved by integrating partitioning bars to reduce the channel depth between electrode channels and separation chamber in order to obtain electrical contact and simultaneously prevent bubbles from entering the separation area. The three-layer sandwich chip features a reusable carrier plate with integrated ports for fluidic connection combined with a softlithographically cast microfluidic PDMS layer and a sealing glass slide. This design allows for a straightforward and rapid chip prototyping process. The performance of the device is demonstrated by free-flow zone electrophoretic separations of fluorescent dye mixtures as well as by the separation of labeled amines and amino acids with separation voltages up to 297 V.German Federal Ministry of Education and Research [01RI0643B
Micro free-flow electrophoresis with injection molded chips
No full textIn this work we present an approach towards economic free-flow electrophoresis chips fabricated by injection molding as mass replication process. This is achieved by the development of a free-flow electrophoresis chip design suitable for one step molding fabrication. Integrated partitioning bars are incorporated as key elements for bubble segregation. A defined open gap of 20 mu m in height and 500 mu m in width was integrated between the separation chamber and the electrode channels, acting as a barrier for gas bubbles while maintaining electrical contact by the fluidic junction. Additionally, we present an approach to avoid internal electrodes in FFE microchips for a facile mass production process. The injection molded thermoplastic mu FFE chips are ready to use without subsequent labor-intensive implementation of membranes or comparable structures serving as salt bridges
A General Strategy for Performing Temperature Programming in High Performance Liquid Chromatography
No full textThe use of elevated temperature or temperature programming in liquid chromatography provides several advantages such as fast analysis, increased efficiency, a change of selectivity and an increase of the elution strength of the mobile phase. Method development in high- temperature liquid chromatography is usually governed by trial and error although a systematic approach is preferred. Therefore, it was investigated whether the empirical linear elution strength (LES) retention model can be adapted from temperature-programmed gas chromatography (GC) to temperature-programmed liquid chromatography (LC). It was found that by means of the LES model, retention times of selected steroids and polycyclic aromatic hydrocarbons can be precisely predicted depending on a simple linear temperature gradient in LC. An average relative error of less than 2% of predicted retention times was observed. Moreover, the influences of column chemistry, inner column diameter and composition of an isocratic mobile phase were studied. Because of these findings, the LES model was further extended in order to predict more complex segmented temperature gradients. For these gradients, the retention times of sulfonamides could be predicted precisely with an average relative error of 2.2%. The LES model in GC permits isothermal retention time predictions on the basis of temperature-gradient measurements. This approach was also employed in liquid chromatography and it is shown that this assumption cannot be transferred to temperature-programmed LC. Because of the need to predict isothermal retention times, predictions based on a plot of the natural logarithm of the retention factor were tested for temperature dependency. It was found that a plot of the natural logarithm of the retention factor versus temperature yields reliable isothermal retention time predictions. In order to improve the accuracy of retention time predictions based on temperature gradients even further, a second compound specific model parameter was also calculated temperature dependent. Using this approach, the relative error of retention time predictions of multi-step temperature gradients can be decreased to around 1.5%. Concurrently, a new experimental design was introduced which permits isothermal predictions on the basis of only four temperature-gradient input measurements. Moreover, a set of recommendations to assist the practitioner during method development in HT-HPLC was established. Finally, the linear solvent strength and the linear elution strength retention model were combined in order to predict simultaneous solvent and temperature gradients in LC. An average relative error of 0.6% of predicted retention times was observed. On the basis of the present work, temperature gradients can now be incorporated in systematic method development in liquid chromatography.In der Flüssigchromatographie (LC) bietet die Anwendung höherer Temperaturen oder der Temperaturprogrammierung verschiedene Vorteile. Trennungen können beschleunigt, die Effizienz kann erhöht und die Selektivität sowie die Elutionsstärke der mobilen Phase kann beeinflusst werden. Dennoch wird die Methodenentwicklung in der Hochtemperatur-LC nicht systematisch durchgeführt. Im Rahmen dieser Arbeit ist untersucht worden, ob das empirische linear elution strength (LES) Retentionsmodell aus der temperaturprogrammierten Gaschromatographie (GC) auf die temperaturprogrammierte LC übertragen werden kann. Dazu wurde das LES Modell verwendet, um Retentionszeiten von ausgewählten Steroiden und polyzyklischen aromatischen Kohlenwasserstoffen in Abhängigkeit eines Temperaturgradienten zu simulieren. Die Retentionszeiten der Analyten konnten mit einem mittleren relativen Fehler von weniger als 2% präzise vorhergesagt werden. Gleichzeitig wurden die Einflüsse der Säulenchemie, des Säuleninnendurchmessers und die Zusammensetzung isokratischer mobiler Phasen untersucht. Durch die anschließende Erweiterung des LES Modells konnten auch komplexe mehrstufige Temperaturgradienten präzise simuliert werden. Die Retentionszeiten von Sulfonamiden konnten mit einem mittleren relativen Fehler von 2,2% vorhergesagt werden. In der GC kann das LES Modell auch zur Simulation von isothermen Trennungen auf Basis von Temperaturgradienten verwendet werden. Dieser Ansatz konnte jedoch nicht auf die LC übertragen werden. Da die Simulation von isothermen Retentionszeiten erforderlich ist, wurden verschiedene Auftragungen des Logarithmus des Retentionsfaktors in Abhängigkeit von der Temperatur untersucht. Die Auftragung des Logarithmus des Retentionsfaktors gegen die Temperatur führt zu vertrauenswürdigen Vorhersagen. Um die Genauigkeit der Simulationen weiter zu verbessern, wurde ein zusätzlicher analytabhängiger Modellparameter temperaturabhängig berechnet. Dadurch konnte der relative Fehler der Vorhersage von mehrstufigen Temperaturgradienten um 1,5% gesenkt werden. Gleichzeitig wurde eine neue Kombination von Basismessungen vorgestellt mit der es möglich ist, isotherme Trennungen auf Basis von vier Temperaturgradienten vorherzusagen. Weiterhin wurden Empfehlungen formuliert, um den Anwender während der Methodenentwicklung in der Hochtemperatur-LC zu unterstützen. Abschließend wurden das linear solvent strength und das linear elution strength Retentionsmodell kombiniert, um simultane Lösungsmittel- und Temperaturgradienten zu simulieren. Der mittlere relative Fehler dieser Vorhersagen betrug 0,6%. Auf Grundlage dieser Arbeit ist es nun möglich, Temperaturgradienten als Parameter einer systematischen Methodenentwicklung in der Hochtemperatur-Flüssigchromatographie zu berücksichtigen
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
Unveiling Organocatalysts Action – Investigating Immobilized Catalysts at Steady‐State Operation via Lab‐on‐a‐Chip Technology
No full textThis paper reports a new method for studying stereoselective catalyzed reactions at a small scale. For this purpose, a micro-sized chemical reactor and a chiral chromatographic column were integrated on a single microfluidic chip hyphenated to mass spectrometry. By running the integrated reactor in truly continuous-flow operation with an automated injection strategy, catalytic processes and their stereoselectivity can be observed over an extended range of time with significantly small consumption of samples, solvents, and catalytic material
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
An integrated chip-approach to study enentioselective heterogeneous catalyst at the microscale
No full textIn this work, we present novel approaches to study stereoselective catalytic processes at an unrivalled small scale. This is achieved by seamless on-chip integration of micro-sized chemical reactors and high performance liquid chromatography (HPLC)-columns on a single device. In contrast to our previous work, we present advanced chip-layouts and adapted strategies to transfer effluent samples from the reactor to the integrated HPLC column while keeping the flow through the reactor constant. This newly presented fluidic setup allows for the operation of several packed bed reactors combined with chip-based HPLC and mass spectrometric (MS) detection enabling the investigations of newly designed catalysts
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