Biomedical subjects

Andreas Seidel-Morgenstern

Publications and source records attributed to Andreas Seidel-Morgenstern.

11 recordsLinked to original sources

PubMed2004

Influence of moderate Joule heating on electroosmotic flow velocity, retention, and efficiency in capillary electrochromatography.

The influence of Joule heating on electroosmotic flow velocity, the retention factor of neutral analytes, and separation efficiency in capillary electrochromatography was investigated theoretically and experimentally. A plot of electrical current against the applied electrical field strength was used to evaluate the Joule heating effect. When the mobile phase concentration of Tris buffer exceeded 5.0 mM in the studied capillary electrochromatography systems using particulate and monolithic columns (with an accompanying power level of heat dissipation higher than 0.35 W/m), the Joule heating effect became clearly noticeable. Theoretical models for describing the variation of electroosmotic flow velocity with increasing applied field strength and the change of retention factors for neutral analytes with electrical field strength at higher Tris buffer concentrations were analyzed to explain consequences of Joule heating in capillary electrochromatography. Qualitative agreement between experimental data and implications of the theoretical model analysis was observed. The decrease of separation efficiency in capillary electrochromatography with macroporous octadecylsilica particles at high buffer concentration can be also attributed to Joule heating mainly via the increased axial diffusion of the analyte molecules and dispersion of solute bands by a nonuniform electroosmotic flow profile over the column cross-section. However, within a moderate temperature range, the contribution of the macroscopic velocity profile in the column arising from radial temperature gradients is insignificant.

Chromatography, Micellar Electrokinetic Capillary↗

PubMed2004

Analysis of the isolation of a target component using multicomponent isocratic preparative elution chromatography.

The separation of a certain target component from a multicomponent mixture using isocratic preparative elution chromatography was studied theoretically. In particular, the important and most complicated case was considered that the target component does not elute in the first or last position. To specify the productivity of collecting this component different options are suggested to identify suitable times for fractionation. Using a conventional Craig model, capable to quantify chromatographic processes, the impact of several essential parameters (e.g. threshold concentration, desired purity, injection volume, separation factor between neighboring components, composition of the mixture) is evaluated for a ternary system based on parametric calculations. The paper provides simple tools to evaluate and optimize the productivity and other objective functions relevant in multicomponent preparative chromatography.

Chromatography, Liquid↗

PubMed2004

Experimental determination of single solute and competitive adsorption isotherms.

In order to design and to optimise preparative liquid chromatography, the knowledge of the underlying thermodynamic functions, i.e. the adsorption isotherms, is of large importance. Usually these functions can not be predicted and various techniques have been suggested to determine them experimentally. In this paper, several important methods to measure adsorption equilibrium data are discussed and evaluated. The main focus is set on dynamic methods analysing concentration profiles that could be detected at the outlet of fixed-beds packed with the stationary phase of interest. The theoretical background of the different methods is explained using classical equilibrium theory and the equilibrium dispersion model. Each method is illustrated based on experimental data collected in our laboratory. Based on these personal experiences recommendations are given regarding the potential and the applicability of the methods discussed.

Adsorption↗

PubMed2004

Online monitoring of preferential crystallization of enantiomers.

Polarimetry is used for continuous online monitoring of optical resolution by preferential crystallization. In combination with refractometry the liquid phase composition is determined, allowing one to follow the resolution progress quantitatively. The measurement techniques were calibrated up to relatively high solution concentrations and combined with the crystallizer. The resolution of DL-threonine was performed by preferential crystallization experiments in aqueous solution varying several process parameters like supersaturation, seed amount, initial enantiomeric excess, and scale. The resolution progress can be conveniently described by profiles of the optical rotation (polarimetric signal) and the crystallization pathway in the corresponding ternary phase diagram. The method outlined is applicable for dynamic process optimization and control purposes in "quasi-continuous" chiral separation processes.

Calibration↗

PubMed2003

Simulated moving bed process with cyclic modulation of the feed concentration.

The improvement of the simulated moving bed (SMB) process based on the introduction of a cyclic modulation of the feed concentration is described. It is demonstrated that such a feed concentration gradient during the shifting cycle can improve the performance significantly. The productivity and the product concentrations can be increased while simultaneously the solvent consumption can be decreased compared to the conventional SMB process with constant feed parameters.

Chromatography↗

PubMed2003

Concentration dependence of lumped mass transfer coefficients linear versus non-linear chromatography and isocratic versus gradient operation.

The general rate model provides a reliable platform to predict elution bands in both linear and non-linear chromatography provided the required equilibrium functions and the coefficients quantifying the mass transfer in and around the particles are available. If further the variation of the equilibrium functions with changes in the mobile phase composition is known, this model is also able to predict gradient elution chromatography. Significant disadvantages of the model are the need to specify three kinetic coefficients and the amount of computing time required for the numerical solution of the underlying equations. Thus, several simplified models have been suggested lumping mass transfer resistances together. In this work the accuracy of predicting chromatographic bands based on the numerical solution of two lumped models has been analyzed. Elution profiles calculated by (a) the transport-dispersive and (b) the equilibrium-dispersive models were compared between each other and with the solution of the more detailed general rate model. In the analysis performed both linear and non-linear chromatography was considered under isocratic and gradient conditions.

Chromatography↗

PubMed2003

Linear two-step gradient counter-current chromatography analysis based on a recursive solution of an equilibrium stage model.

The implementation of gradients in continuously operated chromatographic counter-current processes has recently attracted considerable interest as a method to improve the performance of this effective separation method. If liquid mobile phases are applied it is advantageous to set the solvent strength in the desorbent stream higher than that in the feed stream. As a consequence. the components to be separated are more retained in the adsorption zones and more easily eluted in the desorption zones. Due to the additional degrees of freedom the design and the optimization of such a two-step gradient counter-current process is difficult. In this paper a steady state equilibrium stage model is used to simulate the process under linear conditions. A simple solution of the underlying model equations is presented capable to describe efficiently the unit for large stage numbers typically encountered in chromatographic columns. Due to the rapidity of the algorithm developed a broad range of operating conditions can be evaluated systematically for different types of gradients. The impact of (a) the functional dependence of the adsorption equilibrium constants on the solvent composition, (b) the number of equilibrium stages and (c) the specification of purity requirements is illustrated and discussed based on results of parametric calculations. The results achieved emphasize the potential of two-step gradient counter-current chromatography.

Countercurrent Distribution↗

PubMed2003

Mathematical modelling of adsorption and transport processes in capillary electrochromatography: open-tubular geometry.

A mathematical modelling approach for open-tubular capillary electrochromatography is presented. The spatially one-dimensional model takes into account (i) a coupling of (non)linear adsorption of positively or negatively charged analyte molecules (at a negatively charged capillary inner surface) with the equilibrium electrokinetics at this solid-liquid interface, (ii) mobile phase transport by electroosmosis and pressure-driven flow, as well as (iii) transport of species by electrophoresis and molecular diffusion. Under these conditions the local zeta-potential and electroosmotic mobility become a function of the concentration of the charged analyte. The resulting inhomogeneity of electroosmotic flow through the capillary produces a compensating pore pressure as requirement for incompressible mobile phase flow (i.e., for constant volumetric flow along the capillary). The results of the simulations are discussed in view of the surface-to-volume ratio of the capillary lumen, the analyte concentration (in combination with a Langmuir isotherm for the adsorption process), and buffer effects.

Adsorption↗

PubMed2002

Impact of adsorption isotherm parameters on the performance of enantioseparation using simulated moving bed chromatography.

Often there are several chromatographic systems, i.e., combinations of mobile and stationary phases, available to solve a certain separation problem. Essential differences of these chromatographic systems are the separation factors and the efficiencies. For preparative applications in addition also the column saturation capacities and solubility limits are of importance. The impact of all these parameters appears to be rather well understood for conventional overloaded elution chromatography using a single column. In the last years the continuous simulated moving bed (SMB) process was increasingly used as a powerful alternative to batch elution since increased productivities and reduced solvent consumptions could be realised. However, the selection of suitable chromatographic systems is more sophisticated for this process. In this paper five different chromatographic systems capable of separating the enantiomers of mandelic acid are compared based on the achievable productivities using SMB chromatography. For these five systems the adsorption isotherms have been determined experimentally. Subsequently, an analysis of the SMB process was performed numerically using a well-established model.

Adsorption↗

PubMed2002

Two-step solvent gradients in simulated moving bed chromatography. Numerical study for linear equilibria.

The application of gradients in simulated moving bed (SMB) chromatography has recently attracted interest as a method for further improving the performance of this continuous separation process. One possible implementation of gradients consists in setting the solvent strength in the desorbent stream higher than that in the feed stream. As a result, the components to be separated are more retained in the zones upstream of the feed position and more easily eluted in the zones downstream of the feed position. If a liquid mobile phase is used, gradients can be created by dosing different solvents into the feed and desorbent ports. In a closed-loop gradient SMB arrangement the solvent strength within the unit will depend on the two feed compositions and on the characteristic flow-rates of the process. In this work an equilibrium stage model describing a true moving bed process is used to analyze numerically the main features of a two-step gradient SMB process. The adsorption isotherms are assumed to be always linear under isocratic conditions. The relevant Henry constants depend in a nonlinear manner on the composition of the solvent. Based on numerical simulations the impact of the two inlet solvent compositions is demonstrated in terms of the size and shape of regions of applicable flow-rates. Different strategies of designing the process are discussed and compared with respect to maximizing productivities and minimizing desorbent requirements.

Chromatography, Liquid↗

PubMed2002

Effect of the homogeneity of the column set on the performance of a simulated moving bed unit. II. Experimental study.

Previous studies of the simulated moving bed (SMB) process assume identical characteristics of all the columns incorporated in a given unit. Due to the practical impossibility to manufacture identical columns, numerical applications of the theory to modeling and optimization use for each of the needed column parameter the average value for the entire column set. In this study, the effects of these simplifications on the actual productivity of the SMB process are evaluated by making exact calculations, i.e., by taking the differences in the porosity values into account. We apply a revised set of separation conditions previously introduced and derived from the equilibrium theory. Earlier theoretical results are compared to experimental results obtained in the study of the enantiomeric separation of Tröger's base on Chiralpak AD. Due to the nonLangmuirian character of the adsorption isotherms of these two compounds on the packing material used, the separation area cannot be determined analytically. As an alternative, a reliable numerical algorithm was used to scan a wider region and to define the separation area. The form of this area depends on the applied porosity values. A UV detector and a laser polarimeter located at one node of the SMB monitor on-line the internal concentration profiles. Excellent agreement between the calculated and the experimentally determined concentration profiles was obtained under nonlinear conditions. The influence of column-to-column variations on the performance of the SMB process was found to be more significant under nonlinear than under linear conditions.

Adsorption↗