Biomedical subjects

A Seidel-Morgenstern

Publications and source records attributed to A Seidel-Morgenstern.

7 recordsLinked to original sources

PubMed2006

Exploiting the bootstrap method for quantifying parameter confidence intervals in dynamical systems.

A quantitative description of dynamical systems requires the estimation of uncertain kinetic parameters and an analysis of their precision. A method frequently used to describe the confidence intervals of estimated parameters is based on the Fisher-Information-Matrix. The application of this traditional method has two important shortcomings: (i) it gives only lower bounds for the variance of a parameter if the solution of the underlying model equations is non-linear in parameters. (ii) The resulting confidence interval is symmetric with respect to the estimated parameter. Here, we show that by applying the bootstrap method a better approximation of (possibly) asymmetric confidence intervals for parameters could be obtained. In contrast to previous applications devoted to non-parametric problems, a dynamical model describing a bio-chemical network is used to evaluate the method.

Algorithms↗

PubMed2006

Comparison of possible arrangements of five identical columns in preparative chromatography.

Goal of the paper is to demonstrate how configuration of a certain number of identical HPLC columns influences on the effectiveness of the isolation of a target component. There are two extreme configurations: all the available columns are operated in parallel (provided there are enough pumps available) or all the columns are connected in series. Various combinations of columns connected parallel and in series are possible. Besides batch operation more sophisticated multicolumn arrangements allowing continuous separation are considered. In particular, in this work different configurations of five columns were examined. For illustration the separation of a binary mixture was considered. As the objective function the productivity of the less retained component has been used. A standard model capable to describe the propagation of chromatographic bands has been used to optimize the operating conditions for various possible multicolumn arrangements.

Algorithms↗

PubMed2005

Effect of mobile phase composition on the SMB processes efficiency. Stochastic optimization of isocratic and gradient operation.

The solvent composition was adjusted in a theoretical study in order to maximize the efficiency of a simulated moving bed (SMB) process. The isocratic realization of the process as well as the solvent gradient mode were considered. The solvent composition and the flow rates were used as decision variables in a random search optimization algorithm known to be a reliable tool for nonlinear programming problems. The results of the optimization indicate that the optimal composition of the mobile phase depends strongly on the feed concentration. The asymmetry of the internal concentration profiles, which has a negative effect on the separation efficiency, can be partly damped by an increase of the solvent strength. In the cases studied the optimal solvent strength determined for concentrated feed streams is higher than that for diluted ones. Moreover, the optimum is strongly influenced by the value of the selectivity factor and its dependency on the mobile phase composition. Different results were obtained for cases, in which the separation factor increases with increasing the modifier concentration, than for cases, in which the separation factor decreases with increasing the modifier concentration. A similar analysis was performed for a solvent gradient SMB process, in which different solvents are used at the two inlet ports: a weak solvent in the feed stream and a strong solvent in the desorbent stream. Again the optimal mobile phase composition was strongly affected by the type of the isotherms and their non-linearity. The potential of a gradient SMB process in terms of increasing the productivity and reducing the eluent consumption is exemplified.

Chromatography, Liquid↗

PubMed2002

Measurement and modeling of the equilibrium behavior of the Tröger's base enantiomers on an amylose-based chiral stationary phase.

The binary isotherms of the two enantiomers of Tröger's base were measured on a system made of pure 2-propanol as the mobile phase and of Chiralpak AD, a chiral stationary phase (CSP) based on amylose tri-(3,5-dimethylphenyl carbamate). The experimental data were acquired using both frontal analysis and the perturbation method. The results obtained are most unusual. The adsorption of the more-retained (-)-enantiomer is not competitive: the amount adsorbed onto the CSP at equilibrium with a constant concentration of the (-)-enantiomer is independent of the concentration of the (+) enantiomer. On the other hand, the adsorption of the less-retained enantiomer is cooperative: the amount of this (+)-enantiomer adsorbed by the CSP at equilibrium with a constant concentration of this enantiomer increases with increasing concentration of the (-)-enantiomer. Such a phenomenon has hardly ever been reported. A model equation is proposed that accounts well for all these isotherm data.

Adsorption↗

PubMed2001

Frontal analysis method to determine competitive adsorption isotherms.

In order to design and to optimise preparative separations, the knowledge of the underlying thermodynamic functions, i.e., the adsorption isotherms, is the most essential information. Usually these functions cannot be predicted and various methods have been suggested to determine them experimentally. In particular, dynamic methods are attractive regarding time requirements and reliability. Frontal analysis (FA) is frequently applied to measure single solute isotherms. The theoretical background of this method is offered by the classical equilibrium theory of adsorption. Although this theory also explains the shape of multicomponent breakthrough curves, FA is only seldom applied to determine competitive isotherms. In this work FA was used to measure adsorption isotherms of three components as single solutes and in binary and ternary mixtures in a reversed-phase system. The obtained equilibrium data were correlated using the competitive Langmuir isotherm equation, a bi-Langmuir model, the ideal adsorbed solution theory and the real adsorbed solution theory. No substantial improvement of the predictions was achieved using the more complex models instead of the most simple Langmuir model.

Adsorption↗

PubMed2001

Coupling of simulated moving bed chromatography and fractional crystallisation for efficient enantioseparation.

An optimised coupling of liquid chromatography and fractional crystallisation is suggested for efficient enantioseparation. As a first stage, a chromatographic separation, preferably simulated moving bed (SMB) chromatography, is applied to achieve an enantiomeric enrichment sufficient for a subsequent crystallisation. First results of the experimental and modelling work for the model system (+)-/(-)-mandelic acid in an aqueous solution are described. Chromatographic investigations involve the estimation of adsorption isotherms on a suitable chiral stationary phase and the simulation and optimisation of a corresponding SMB process. From the ternary phase diagram measured for the (+)-/(-)-enantiomer/ solvent system, the conditions required to crystallise a pure enantiomer from an asymmetric mixture can be derived. The productivity gains achievable from the combined process compared to the application of chromatography alone are discussed.

Chromatography, Liquid↗

PubMed1999

Quantification of single solute and competitive adsorption isotherms using a closed-loop perturbation method.

A modification of the classical method of perturbation chromatography for measuring isotherms of the adsorption of dissolved components is suggested. The general principle of the method consists in analyzing responses of the chromatographic system to small perturbations at different equilibrium concentrations. Essential advantages of the method are: (a) only retention times or volumes have to be measured and no detector calibration is required and (b) experiments with mixtures can be performed and analyzed efficiently. The modification suggested in this paper is the application of a closed-loop arrangement allowing the efficient exploitation of the sample. Experimental data for four different chromatographic systems are presented to illustrate the method. With the determined adsorption isotherms elution profiles could be predicted satisfactorily.

Adsorption↗