Influence of stationary phase solvation on shape selectivity and retention in reversed-phase liquid chromatography

In the past few decades, shape selectivity has drawn a great deal of attention from chromatographers. The chemistry and characteristics of bonded stationary phases such as phase type, length of bonded phase, surface coverage, and silica surface material have an effect on the shape selectivity of the columns. Although the effects of bonded phase shape selectivity are relatively well understood, one remaining question is the effect of intercalated solvent on shape selectivity. The intercalation of organic modifier and water molecules into the stationary phase is believed to introduce more rigidity into bonded alkyl chains in RPLC. The use of gas chromatography (GC) opens a new dimension to approach this question. C18 columns 4 cm in length were prepared in our laboratory and used in both LC and GC experiments. Shape selectivity and thermodynamic constants for the transfer of a solute from the mobile phase to the stationary phase have been determined as a function of monomeric octadecyl stationary phase bonding densities over the range of 1.44-3.43 micromol/m2 and a polymeric phase (nominal surface coverage 4.77 micromol/m2). Comparing LC and GC experiments, we observed: (a) similar relationships between shape and phenyl selectivities with monomerically bonded C18 phase densities; (b) different correlation of thermodynamic quantities (DeltaH degrees , DeltaS degrees , and DeltaG degrees ) versus bonded phase densities. The effects of high temperature and residual silanol groups are sources of difficulty in elucidation of the intercalated mobile phase role in selectivity and retention for GC measurements.     

Fabrication of microelectrodes using flow layer-by-layer self assembly of gold nanoparticles

Microelectrodes to be used in microfluidic devices were prepared from the layer-by-layer flow deposition of gold nanoparticles. Pre-designed microfluidic channels were used as templates for the flow driven deposition of the nanoparticles in sequence with poly (diallyldimethyl amonium chloride) (PDADMAC). The electrical resistivity of the gold nanoparticle assembly was found to be strongly dependant on the concentration of sodium citrate used in the gold nanoparticle synthesis. As the electrical properties of the film changed from insulating to conducting when decreasing the citrate concentration, a 4 point probe setup was used to measure the resistivity of the film. Near bulk conductivity (5.42 × 10⁻⁶ Ω cm) was achieved with only 10 layers of film. The thickness and morphology of the flow-printed multilayer microelectrode was characterized using atomic force microscopy (AFM) and a field emission scanning electron microscope (FE-SEM). To demonstrate its usefulness, the microelectrode assembly was then tested toward the detection of KCl in solution, having a concentration ranging from 1 to 20 mM using AC current detection in a simple setup. Good linearity and stability of the electrode confirmed that this method could be very convenient for the fabrication of microelectrodes for lab-on-chip applications.     

Epoxy oligomer and cyclen polymerization on the air-water interface. Receptor properties of a monolayer

Two-dimensional functional polymer systems have been obtained for the first time based on an epoxy oligomer and 1,4,7,10-tetraazacyclododecane (cyclen). The effect of UV radiation on the efficiency of polymerization in the system has been studied. The morphology of polymer materials formed on the liquid surface as a result of thermo- and UV-induced polymerization has been studied. It has been shown that a polymer Langmuir monolayer based on an epoxy oligomer-cyclen mixture can be used as a sensor for copper ions, and the conditions for the system regeneration have been determined. Changes in the pH of the subphase have been established as causing structural rearrangements in the polymer monolayer, which lead to changes in the optical properties of the latter.     

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Über die Erregung infolge der Periodizität von Turbomaschinen

Übersicht In der vorliegenden Arbeit werden die Erregungsphänomene untersucht, die von der stator- und rotorseitigen Periodizität, wie sie in allen Turbomaschinen vorhanden sind, hervorgerufen werden. Es wird davon ausgegangen, daß bei jedem Vorbeifahren einer Lauf schaufel an einer Leitschaufel eine Last in radialer, tangentialer und achsialer Richtung entsteht. Die aus der Gesamtheit dieser Vorgänge resultierenden Kräfte und Momente, insbesondere ihre Frequenzen, werden anhand von drei einfachen mathematischen Modellen hergeleitet. Je nach zeitlicher und räumlicher Lage der einzelnen Pulse, und je nachdem, ob man die Resultierenden auf die Welle, auf eine Schaufel oder auf eine Radscheibe betrachtet, sehen diese Resultierenden anders aus. Außerdem kommt bei den Wellen- und Scheibenschwingungen zur gewöhnlichen Resonanzbedingung, daß Erregerfrequenz = Eigenfrequenz ist, noch eine zusätzliche Bedingung über die Leit- und die Laufschaufelzahl hinzu.

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On the excitations due to the periodic structure of turbomachines

Summary In the present paper the excitation caused by the periodical character of the rotor and the stator of turbomachines of any kind is investigated. The basic assumption is that, each time a runner blade passes a guide vane, a radial, a tangential and an axial load are produced. With the help of three simple mathematical models the forces and moments and especially their frequencies resulting from all these loads are determined. Depending on the position in time and space of each impulse and on the considered resultant, namely that acting on the shaft, on a blade or on the disk, these resultants have other properties. Furthermore, in the case of vibrations of shafts and disks, in addition to the known resonance condition, i.e. excitation frequency = natural frequency, an existence condition depending on the number of blades has to be taken into account.

     

Selective fluorescent sensor for mercury ions in aqueous media using a 1,4-dihydropyridine derivative

Novel 1,4-dihydropyridine (DHP) derivatives containing 3 carboxylic acid units are synthesized via cyclotrimerization of N-substituted β-aminoacrylates followed by basic hydrolysis of the triester. These DHP derivatives are readily soluble in aqueous media buffered at pH 8.0 and the solutions give blue fluorescent signals with quantum yields of 7–23%. One of these compounds, bearing a p-methoxyphenyl N-substituted group, shows specific fluorescent quenching with the mercuric ion (Hg²⁺). The fluorescent signal of the DHP derivative decays over a period of minutes to hours depending on the Hg²⁺ concentration, which implies that the sensing mechanism involves chemical reaction between the Hg²⁺ ion and the DHP compound. The ¹H NMR and MS data suggest that Hg²⁺ mediates the oxidation of the DHP ring into a pyridinium ring. The event is useful for fluorescent detection of Hg²⁺ at the micromolar level within 30 min, with a detection limit of 0.2 μM in aqueous medium.     

Green synthesis of silver nanoparticles for ammonia sensing

Silver nanoparticles synthesized by a reagent less method involving only UV radiation have been used in colorimetric assay for the detection of ammonia in solution. The silver nanoparticles were synthesized by the exposure of a silver nitrate solution to a low-power UV source in the presence of poly(methacrylic acid) (PMA), which acted both as reducing and capping agent. The synthesis of the silver nanoparticles was studied by monitoring the changes in position and amplitude of the localized plasmon resonance (LSPR) band using UV-vis spectroscopy. The morphology of the particles was studied using transmission electron microscopy which confirmed the formation of spherical particles with an average particle size around 8 nm. Interestingly, the silver nanoparticles solution was found to display a strong color shift from purple to yellow upon mixing with increasing concentration of ammonia ranging from 5 to 100 ppm. Hence, the nanoparticles prepared with this method could be used as colorimetric assay for sensing applications of ammonia in water.