Photoracemization broadening of selective reflection and polarization band of glassy chiral-nematic films

Glass-forming liquid crystals consisting of a cyclohexane central core with (S)-1-phenyl-ethylamine and (4-cyanophenyl)naphthalene pendants were synthesized as hosts for racemizable (R)-dinaphtho[2,1-d:1',2'-f][1,3]dioxepin, a chiral dopant. Chiral-nematic films 14, 22, and 35 mum thick were prepared for thermal and photoinduced racemization at temperatures from 95 to 130°C, i.e. in the mesomorphic temperature range, over a period of hours to days. Spatially modulated photoracemization was accomplished with an insignificant contribution from the thermal process at temperatures around 100°C over a period of up to 3 h. With an absorbance per unit thickness of 6.2 mum^-1 at 334 nm, the photochemical process was essentially confined to the irradiated surface, thereby setting up counter-diffusion of the two enantiomers through the film, and hence the pitch gradient as visualized by atomic force microscopy. The significantly widened selective reflection band was interpreted with the Good-Karali theory extended for a gradient-pitch film. Furthermore, the bandwidth was found to increase with decreasing racemization temperature or with increasing film thickness, further validating the presence of a pitch gradient as a result of controlled photoracemization.     

Effects of polymer network on electrically induced reflection band broadening of cholesteric liquid crystals

The reflection band of polymer stabilized cholesteric liquid crystals with negative dielectric anisotropy can be broadened by DC electric fields, which was ascribed to the pitch gradient caused by the motion of the structural chirality, that is, the polymer network. They systematically varied the mixture components, such as the photo‐initiator concentration, the monomer functionality, and the chiral dopant, to explore their influences on the reflection band broadening behavior. They learned how to control the polymer network morphology and ion density, which in turn determined the reflection bandwidth. By optimizing the mixture, they have greatly enhanced the broadening effect and achieved large bandwidth at low voltages. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 835–846     

Investigation and interpretation of band broadening in size exclusion chromatography

Study of Band Broadening occurring in Size Exclusion Chromatography (SEC) is reported using very narrow PS standards obtained and characterised by Temperature Gradient Interaction Chromatography (TGIC). Chromatograms are fitted by Exponentially Modified Gaussian functions (EMG) and mapping of band broadening is obtained for different column sets. Interpretation of the skewing of the chromatograms is proposed with a new model using Brownian motion properties inside the pores. That explains why band broadening and tailing become so important near total exclusion volume.     

Accurate determination of extra-column band broadening using peak summation

The present contribution addresses the problem of the accurate determination of the extra-column band-broadening dispersion by applying the method of moments (MOM) on pulse response experiments. The MOM provides the only mathematically rigorous way to determine the variance of a pulse response, but suffers from the fact that the obtained variance values usually depend very strongly and unpredictably on the selected integration boundaries. In the present study, it is investigated whether the MOM cannot be made more accurate by repeating the pulse response experiment a number of times, then add all measured peaks (after retention time alignment) and subsequently perform the integration on this summed peak. Testing this approach for a number of different integration boundary detection methods consistently more accurate results are obtained than when the different repeat response pulses are integrated individually and the variances are averaged afterwards. It was also found that adding five repeats already leads to a significant improvement of the variance estimate, whereas the addition of 10-20 repeats is needed before the variance estimate converges to a steady value.     

Theory of band broadening during cycling temperature capillary electrophoresis

A two-state transport model is presented for cycling temperature CE. In this high-throughput method of mutation detection, the temperature oscillates in time, causing the DNA to periodically transit between an annealed state and a partially melted (denatured) state. The change in state alters the electrophoretic mobility, and the presence of a mutation changes the temperature dependence of the denaturing/annealing kinetics. Asymptotic formulae for the mean velocity and effective diffusivity (dispersivity) of the DNA are computed by a multiple-time scales scheme in the limit where the DNA have experienced many temperature cycles before reaching the detector. Explicit analytical results are presented for the case where the temperature cycle consists of one interval with irreversible annealing, followed by a second interval with an infinitely fast, irreversible denaturation. The lag in the annealing leads to a reduction in the mean velocity and an enhanced dispersion compared to the idealized case where the DNA respond instantaneously to the changes in temperature, with the dispersion scaling quadratically with the electric field. The predicted plate height scales linearly with the electric field, and the optimal separation resolution is predicted for moderate values of the cycle frequency that allow the DNA to relax during each temperature oscillation.     

Direct determination of band broadening in size exclusion chromatography

A simple method to correct the measured extent of band broadening in size exclusion chromatography for the contribution of narrow (polydisperse) standards is presented. It is based on the assumptions that commercial polymer standards can be described by a Poisson distribution and the additivity of peak variances. Two sets of standards (polystyrene from two suppliers) were investigated under normal working conditions, i.e. a combination of four columns with different porosities and a flow rate of 1 ml/min. Furthermore, the polystyrene standards were used to determine the extent of band broadening for four additional combinations of columns (varying in their separation range and porosities) as a function of the elution volume. The assumption of a constant peak variance for band broadening turned out to be a (very) rough approximation for some combinations of columns, but all results taken together demonstrate that this assumption is not generally applicable. Qualitative agreement between theory and experiment was found with a rearranged van Deemter equation.     

Characterization of modified glassy carbon films

The composition and structure of the thin conversion films at pre- and breakdown state are investigated. The breakdown leads to a modified structure and composition. Raman spectra of modified films show the formation of glassy carbon with a defined size of crystallites. A typical morphology characterized by a strong structured surface indicates this breakdown state. Electron diffraction patterns of the pre-breakdown state show the presence of titanium nitride, titanium carbide and titanium oxide.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Experimental determination of the helicoidal electric field inside a twisted mesophase, in the selective reflection band

We consider the interface between a twisted birefringent mesophase and an isotropic medium. In the selective reflection band, when illuminated at normal incidence, the interface creates a coupling between the located dielectric reflection and the spread selective reflection. This coupling enables experimental measurement of the standing electric field orientation inside the liquid crystal.     

聚邻苯三酚修饰电极选择性预富集与测定铋(Ⅲ)

在pH 7的磷酸盐缓冲溶液中 ,氧化聚合邻苯三酚制得一稳定的水不溶性的膜粘附于玻碳电极的表面。此邻苯三酚膜修饰的玻碳电极被用于在一定电位下选择性预富集Bi(Ⅲ )。预富集的Bi(Ⅲ )在一定的电位下还原 ,再用微分脉冲阳极溶出伏安法测定。讨论了预富集、还原、溶出和清洗过程中的各种因素的影响。在最佳条件下 ,Bi(Ⅲ )浓度在 2 .5× 1 0 - 9～ 6.5× 1 0 - 6 mol/L范围内与阳极氧化峰电流呈线性 (富集 1 0min) ,相关系数为 0 9997。检测限为 1 .4× 1 0 - 1 0mol/LBi(Ⅲ ) (S/N =3,富集 1 0min)。对 5× 1 0 - 8mol/LBi(Ⅲ )平行测定 8次 ,相对偏差为 3.4%。方法可用于人发和指甲样品的测定。     

Diffuse reflection and polarization modulation FT-IR spectrometry of surface species

Summary Two techniques for measuring the infrared spectra of adsorbed species on surfaces of different types are described. Diffuse reflectance spectra of species adsorbed on powdered substrates with a high surface area can be measured with an exceptionally high signal-to-noise ratio. The technique is illustrated by the adsorption of carbon monoxide on rhodium supported on alumina at metal loadings between 0.1 and 10%. Some doubt is cast on previous band assignments for adsorbed CO molecules. Grazing incidence reflection-absorption spectrometry can be used to measure the spectra of molecules on flat metallic substrates of very low surface area. Attempts to develop a technique for observing the intermolecular vibration between the adsorbate and the substrate using a unique triple modulation approach are discussed.

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FT-IR-Spektrometrie mit diffuser Reflexions- und Polari-sations-Modulation von Adsorbaten an Oberflächen

Zusammenfassung Zwei Techniken zur Messung von Infrarot-Spektren von Adsorbaten verschiedener Art werden beschrieben. Diffuse Reflexionsspektren von Adsorbaten an pulverförmigen Substraten mit einer großen Oberfläche können mit einem außerordentlich großen Signal-zu-RauschVerhältnis gemessen werden. Als Beispiel dient die Adsorption von Kohlenstoffmonoxid an Rhodium auf Aluminiumoxid bei Metallgehalten von 0,1 bis 10%. Einige Zweifel an früheren Bandzuweisungen für adsorbierte CO-Moleküle werden geäußert. Reflexions-Absorptions-Spektrometrie mit streifendem Einfall kann zur Messung der Spektren von Molekülen an flachen metallischen Substraten mit sehr geringer Oberfläche benutzt werden. Versuche zur Entwicklung einer Technik für die Beobachtung der intermolekularen Schwingungen zwischen dem Adsorbaten und dem Substrat mittels einer einzigartigen Dreifach-Modulationstechnik werden diskutiert.

     

Experimental study of band broadening and solute interferences in preparative supercritical fluid chromatography

The effect of the amount injected on the elution profile of a single solute was used to investigate the shape of the distribution isotherm in overloaded supercritical fluid chromatography. Subsequently, the role of competition between solutes when the column is overloaded with a binary mixture was studied. The band broadening pattern is explained by the difference between the solubilities of the solutes in the supercritical fluid mobile phase.     

Dynamics of ultrathin films in the glassy state

We report hole growth experiments in free-standing polystyrene (PS) films at temperatures up to 10 degrees C below the bulk glass transition. The data show an unexpected result: the growth rate of nucleated holes increases with increasing molecular weight, up to a limiting value beyond which the rate is approximately constant. Film thicknesses of 45, 80, and 100 nm were studied, using PS molecular weights ranging from 65K to 11.4 Mg/mol. Hole diameters grew linearly with time, and no growing rims were observed to form around the developing holes. Possible explanations in terms of elasticity, yield, and influence of sample preparation and confinement effects are discussed.     

Analysis of band broadening in vibrational high-resolution electron-energy-loss spectra of condensed methane

High-resolution vibrational electron-energy-loss spectra of multilayer condensed films of methane recorded at 20 K show a strong tailing of the vibrational bands that clearly exceeds the instrumental resolution. At low incident electron energy, this tailing is remarkably less important for the dipole-allowed bending vibration (nu(4)) than for other bands. Also, the tailing becomes less pronounced with increasing size of the molecule as demonstrated by spectra of ethane and heptane recorded under the same conditions. Dipole coupling, rotational broadening, and multiple inelastic scattering have been considered as origins of this band broadening. While the first two effects can be excluded, multiple scattering involving a low-frequency phonon band provides a reasonable explanation as demonstrated by simulations of the spectrum of methane using a classical two-stream model. A lower phonon frequency in the cases of the larger molecules is held responsible for the better resolved vibrational signals in the spectra of ethane and heptane.     

Fluid mechanics of electroosmotic flow and its effect on band broadening in capillary electrophoresis

Electroosmotic flow (EOF) usually accompanies electrophoretic migration of charged species in capillary electrophoresis unless special precautions are taken to suppress it. The presence of the EOF provides certain advantages in separations. It is an alternative to mechanical pumps, which are inefficient and difficult to build at small scales, for transporting reagents and analytes on microfluidic chips. The downside is that any imperfection that distorts the EOF profile reduces the separation efficiency. In this paper, the basic facts about EOF are reviewed from the perspective of fluid mechanics and its effect on separations in free solution capillary zone electrophoresis is discussed in the light of recent advances.     

Estimation of the band broadening contribution of HPLC equipment to column elution profiles

Summary Methods to determine the contribution of the chromatographic equipment to the total band broadening which involve replacing the column by a union or a capillary tube are not suitable as they involve a fundamental change in the chromatographic system. The linear extrapolation method, based on the estimation of the relative influence of the instrument variance on solutes with different capacity factors, is a more attractive alternative method since the column remains in the chromatographic system. This method is only valid when a number of conditions are satisfied. By meeting these conditions the error in the instrument variance by using the linear extrapolation method was determined. At the same time, ways to minimise these errors were studied. Use of the linear extrapolation method in combination with conventional columns of 4.6 mm i.d. appears to yield inaccurate results. In combination with microbore columns the method can be used, provided the columns have a maximum length of 5cm and contain a packing material with a particle size of 2 or 3μm. The error in the determined instrument variance is then of the order of 2μl².     

Self-assembly of PbTe quantum dots into nanocrystal superlattices and glassy films

Monodisperse lead telluride (PbTe) nanocrystals ranging from approximately 4 to 10 nm in diameter are synthesized to provide quantum dot building blocks for the design of novel materials for electronic applications. Two complementary synthetic approaches are developed that enable either (1) isolation of small quantities of nanocrystals of many different sizes or (2) the production of up to 10 g of a single nanocrystal size. PbTe nanocrystals are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and optical absorption. Assembly of PbTe nanocrystals is directed to prepare nanocrystal solids that display either short-range (glassy solids) or long-range (superlattices) packing order by varying deposition conditions. Film order and average interparticle spacing are analyzed with grazing-incidence small-angle X-ray scattering (GISAXS) and high-resolution scanning electron microscopy (HRSEM). We perform the first optical and electronic studies of PbTe solids and demonstrate that chemical activation of these films enhances conductivity by approximately 9-10 orders of magnitude while preserving their quantum dot nature.     

The effect of electroosmotic and hydrodynamic flow profile superposition on band broadening in capillary electrophoresis

The effect of the superposition of electroosmotic flow and pressureinduced hydrodynamic counterflow on efficiency has been investigated for different capillary electrophoretic systems. Results are shown for 50 and 75 μm internal diameter capillaries at several voltage and counterpressure levels. Hydrodynamic counterflows were successfully applied in electrokinetic chromatography in order to delay the entry of a UV-active pseudostationary phase, tetraphenyl porphyrintetrasulfonate, into the detection zone allowing the separation of neutral nitroaromatics. The separations are based on the weak charge-transfer interactions between the porphyrin and the analytes.     

A general framework for the measurement of band broadening properties of GC capillary columns

In the approach to the developement of a practical procedure for the evaluation of band broadening properties of gas capillary columns, the expression for the variance of a solute band is cast in a dimensionless form. This expression supplies the basis for a valid bench mark for column evaluation. Experimental results demonstrate the applicability of the method.