Preparative chromatography adsorption isotherms

Contemporary development of chromatography theory has tended to concentrate on dispersion in electro-chromatography and the treatment of column overload in preparative columns. Under overload conditions, the adsorption isotherm of the solute with respect to the stationary phase can be grossly nonlinear. One of the prime contributors in this research has been Guiochon and his co-workers, [27-30]. The form of the isotherm must be experimentally determined and, from the equilibrium data, and by the use of appropriate computer programs, it has been shown possible to calculate the theoretical profile of an overloaded peak. 

Nieoud R. M., Seidel-Morgenstem A. (1993) Adsorption Isotherms Experimental Determination and Applieation to Preparative Chromatography Simulated Moving Bed Basics and Applications, R. M Nieoud (ed.), Institut National Polyteehnique de Lorraine, Naney, Pranee, p. 4-34. 

Classical liquid chromatography based on adsorption- desorption was essentially a non-linear process where the time of retardation (retention time) and the quantitative response depend on the position on the adsorption isotherm. Essentially, it was a preparative technique the aim was to obtain the components present in the sample in pure form which could then be submitted to further chemical or physical manipulations [3]. 

Validation issues are especially important in the analytical field when quantifications are made. However, it is also important to validate the adsorption isotherm model and its estimated parameters used in preparative chromatography. In contrast to the situation for the bioanalytical area in the pharmaceutical industry, there are few published reports on validation in the analytical biotechnological and in the isotherm parameter determination field, and there exists no detailed validation guidelines [16, 17], This is why much effort in this thesis is focused on the development of validated methods in both these fields. 

In this section a short overview is given of preparative chromatography and the determination of adsorption isotherm parameters - single and competitive - to be used for computer-assisted optimization of separations. 

In nonlinear chromatography the concentration of a component in the stationary phase at equilibrium is no longer proportional to its concentration in the mobile phase [13], Under such conditions, the adsorption isotherms also depend on the concentrations of all other components in the sample mixture. Thus, the retention time, peak height and band shape depend both on the sample composition and amount [13], This is the situation found in practically all preparative applications [13], Nonlinear chromatography is extremely complex due to the interdependence of the individual band profiles that are caused by the dependence of the amount of any component adsorbed on the stationary phase on all the components in solution [13],... 

Preparative chromatography is widely used for the purification of different compounds, but this procedure needs to be optimized to achieve the minimum production costs. This can be done by computer-assisted modeling. However, this approach requires a priori determination of accurate competitive adsorption isotherm parameters. The methods to determine this competitive information are poorly developed and hence often a time limiting step or even the reason why the computer-assisted optimization is still seldom used. In this thesis in papers IV-VI, a new injection method was developed that makes it possible to determine these competitive adsorption isotherm parameters more easily and faster than before. The use of this new... 

This chapter introduces fundamental aspects and basic equations for the characterization of chromatographic separations. Starting from the simple description of an analytical separation of different compounds the influences of fluid dynamics, mass transfer and thermodynamics are explained in detail. The important separation characteristics for preparative and process chromatography, e.g. the optimization of resolution and productivity as well as the differences compared with chromatography for analytical purposes, are described. Especially, the importance of understanding the behavior of substances in the nonlinear range of the adsorption isotherm is highlighted. 

The most dramatic difference between analytical and preparative chromatography is the extension of the working range of the adsorption isotherm into its nonlinear region. The behavior of single components as well as their mixtures over the complete range of the adsorption isotherm has, therefore, to be determined with great accuracy. 

Thermodynamic effects are the main source of increasing productivity in preparative chromatography. The most important parameter is the selectivity, which describes the ratio of the initial slopes H of the adsorption isotherms for both components (Eq. 2.46). 

For preparative chromatography, where we almost always have to deal with high feed concentrations and nonlinear adsorption isotherms, the following approach to the appropriate choice of feed concentration during model-based optimization is recommended ... 

Boysen, H., Wozny, G., Laiblin, T., Arlt, W. CDF simulation of preparative chromatography columns considering adsorption isotherms,... 

Seidel-Morgenstern, A., Nicoud, R.-M., Adsorption isotherms experimental determination and application to preparative chromatography, Isolation s[ Purific., 1996, 2, 165-200. 

The FA method gives isotherm data. To be useful in preparative chromatography, these data must be fitted to an isotherm model. There are presently no numerical procedures available to smooth the data from multidimensional plots, similar to the 2-D splines or French curves and obtain purely empirical isotherms. Therefore, the major difficulty is the selection of adequate models. The Langmuir isotherm is too simplistic in most cases, and the LeVan-Vermeulen isotherm is complicated and difficult to use as a fitting fimction. Several methods have been described to extract the "best" set of Langmuir parameters which could accormt for a set of competitive adsorption data [108]. These methods have been compared. The most suitable method seems to depend on the aim of the determination and on the deviation of the system from true Langmuir behavior [108]. 

While analyses made in gradient elution often involve the use of small and dilute samples, the column is often overloaded in preparative gradient elution chromatography. This causes the adsorption isotherms to be nonlinear and competitive. Therefore, interference effects become important. Furthermore, the mass transfer resistances can be very significant, especially for macromolecules. Various dispersive effects, such as axial dispersion and the mass transfer resistances often coimter-balance the thermodynamic effects of adsorption and desorption... 

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