Sulfur speciation by capillary electrophoresis with indirect spectrophotometric detection: In search of a suitable carrier electrolyte to maximize sensitivity

Various probes have been evaluated as alternative ions to chromate, which is most frequently used in the analysis of small inorganic anions by capillary electrophoresis with indirect UV detection. Sulfur species (S₂O₃²⁻, SO₄²⁻, S₄O₆²⁻, S(−II)) have been determined. The optimization of the method was particularly focused on S(−II) since this species rapidly yields S₂O₃²⁻ and SO₄²⁻ in the presence of oxidizing agents. Therefore, it could not be analysed by capillary electrophoresis with chromate as the background electrolyte. The alternative probe ions all contain aromatic rings (benzene or naphthalene) to provide the intrinsic background absorbance for indirect detection. They are all fully ionized at the pH chosen for this application (TRIS buffer, pH=8) and the range of their mobilities is large enough to suit the analytes mobilities. They have no oxidizing properties. Transfer ratios have been determined experimentally and compared to calculated values derived from the Kohlrausch regulation function. All experimental values were lower than expected from the calculations, which proves the limitations of the Kohlrausch theory concerning the configuration (electrolyte/analyte) of this study. However, maximizing (z_A/z_E)·ε_E (with z_A and z_E being the charges of the analyte and the probe, respectively, and ε_E the molar absorptivity of the probe) and keeping the mobility of the probe close to those of the analytes will give a good hint for the choice of the most suitable UV-absorbing probe. Pyromellitate and naphthalenetrisulfonate, the mobilities of which are close to that of S(−II), give the best sensitivity for this species, with good resolution and sensitivity for all other species.     

An algorithm for mass matrix calculation of internally constrained molecular geometries

Dynamic models for molecular systems require the determination of corresponding mass matrix. For constrained geometries, these computations are often not trivial but need special considerations. Here, assembling the mass matrix of internally constrained molecular structures is formulated as an optimization problem. Analytical expressions are derived for the solution of the different possible cases depending on the rank of the constraint matrix. Geometrical interpretations are further used to enhance the solution concept. As an application, we evaluate the mass matrix for a constrained molecule undergoing an electron-transfer reaction. The preexponential factor for this reaction is computed based on the harmonic model.     

Modeling of PEG grafting and prediction of interfacial force profile using X‐ray photoelectron spectroscopy

Poly(ethylene glycol) (PEG)‐grafted Sephadex? derivatives were prepared by a covalent amine conjugation method and characterized using XPS. PEG‐grafting kinetics were studied using both Langmuir and Langmuir–Freundlich isotherm models by correlating fractional C? O intensities obtained from high‐resolution C 1s scans with the grafting period. Theoretical values were compared with experimental results to confirm the reliability of the modeling predictions. Detailed surface characterization of PEG‐grafted Sephadex derivatives was performed using XPS data, and the results were used to predict the interaction free energy and stick force exerted at the matrix interface. Copyright © 2011 John Wiley & Sons, Ltd.     

Kinetics of RNA refolding in dynamic equilibrium by 1H-detected 15N exchange NMR spectroscopy

By implementing new NMR methods that were designed to map very slow exchange processes we have investigated and characterized the refolding kinetics of a thermodynamically stable 34mer RNA sequence in dynamic equilibrium. The RNA sequence was designed to undergo a topologically favored conformational exchange between different hairpin folds, serving as a model to estimate the minimal time required for more complex RNA folding processes. Chemically prepared RNA sequences with sequence-selective (15)N labels provided the required signal separation and allowed a straightforward signal assignment of the imino protons by HNN correlation experiments. The 2D version of the new (1)H-detected (15)N exchange spectroscopy (EXSY) pulse sequence provided cross-peaks for resonances belonging to different folds that interchange on the time scale of longitudinal relaxation of (15)N nuclei bound to imino protons. The 34mer RNA sequence exhibits two folds which exchange on the observable time scale (tau(obs) approximately T(1){(15)Nu} < 5 s) and a third fold which is static on this time scale. A 1D version of the (15)N exchange experiment allowed the measurement of the exchange rates between the two exchanging folds as a function of temperature and the determination of the corresponding activation energies E(a) and frequency factors A. We found that the refolding rates are strongly affected by an entropically favorable preorientation of the replacing strand. The activation energies are comparable to values obtained for the slow refolding of RNA sequences of similar thermodynamic stability but less favorable topology.     

A numerical study of highly-diluted,burner-stabilised dimethyl ether flames

Recently, a new burner was designed by Zhang et al. (Proc. Combust. Inst. 34 [2013], 763–770) to enable the investigation of 1D, premixed flames at atmospheric pressure with a temperature in the burnt gases near 1500 K. It consists of a matrix burner plate with alternating fuel and oxidiser feeds that, because of small-scale nozzles, mix quite rapidly. Flames at high dilution and reduced temperatures such as realised here are of relevance for the understanding of low-temperature combustion strategies. In this work, we examine the burner with regard to the validity of the 1D assumption for the investigated flames. Experimental measurements are conducted and 1D and 3D simulations are performed in which the chemistry is described by a detailed chemistry approach based on a reduced reaction scheme derived from the mechanism of Fischer et al. (Int. J. Chem. Kinetics 32 [2000], 713–740). The experimental results are compared to 1D simulations with two different temperature treatments. First, the unburnt temperature is set to the measured temperature closest to the burner surface; second, the experimental temperature profile is prescribed in the whole simulation domain without solving the energy equation. The comparison shows that the 1D simulation predicts the experimental results reasonably well, if the experimentally obtained temperature profile is prescribed in the simulation domain. Differences are found in the mole fractions of methyl and formaldehyde. Further comparisons of the experimental data with 3D simulation results and comparisons of 3D and 1D simulation results indicate that the differences between measured and computed mole fractions of these species are not a result of the 3D nature of the experimental flame and might be attributed to the chemical mechanism. The conclusion is that the measurement data can be used for validation purposes with the 1D simulation setup shown here if the measured temperature profiles are prescribed in the 1D simulation domain.     

An efficient approach of unsteady flamelet modeling of a cross-flow-jet combustion system using LES

Steady flamelet models have been widely used in turbulent combustion simulations because of their simplicity, efficiency, yet physics-based nature. They are, however, unable to handle slow chemical and physical processes such as pollutant formation. Unsteady flamelet models have been shown to be able to provide accurate predictions especially for pollutants, but their implementations are usually not as straightforward as for the steady models, and additional assumptions are involved. One relatively straightforward approach of implementing the unsteady flamelet model is to tabulate the time history of unsteady flamelet solutions. This often leads to flamelet libraries of large sizes because of increased dimensions for the new physics. The purpose of this paper is to introduce a new and efficient approach of tabulating unsteady flamelet solutions in the LES of complex systems, here demonstrated in simulations of a cross-flow-jet combustion system. This approach employs Taylor series expansions to represent the time history of unsteady flamelet solutions. Compared with other approaches, the new approach retains the efficiency and simplicity benefits of steady flamelet models but possesses the accuracy of unsteady flamelet models. Various issues associated with the formulation and implementation of this approach are discussed, which include the selection of the base solution, the order of accuracy of the expansion, and the treatment of simultaneous wall heat losses and heat transfer through thermal radiation. This approach is validated in large eddy simulations of a cross-flow-jet combustion system. Good agreement with experiments is obtained for both temperature and NO concentration, as well as for major species.