Retention Modeling as Function of Mobile Phase Composition

Raw experimental data would be meaningless without some form of analysis to provide understanding. We need models to obtain meaning from observable fact. A model combines interpretation and a depiction of a phenomenon. The two fundamental types of models are theoretical and empirical. The former follows from known theoretical laws or principles they convert raw data to knowledge and are predictive in their own right. If sound new data are at variance with theoretical predictions, the theory must be improved to take experimental evidence into account. Conversely, anpirical models do not adhere to any theoretical basis raw data are used to describe the system response. 

Dnring the infancy of IPC, retention prediction commonly faced trial-and-error procednres that attempted to make the problem univariate, holding all experimental conditions constant except one. This one-at-a-time changing of parameters, without regard to parameter interactions, is still practiced and may, in a time consuming way, improve performance. The description of the dependence of retention on the mobile phase composition parameters is the focus of interest of model makers becanse an a priori retention prediction is highly desirable. Optimization is finding the nnique combination of values of adjustable parameters that yields the best performance possible for a set of requirements. 

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Q Retention Modeling as Function of Mobile Phase Composition... 

The solid lines in Figure 4.28 show the retention times of the perturbation pulses as functions of the mobile phase composition (concentration of each enantiomer, the solution being prepared from the racemic mixture) calculated from the isotherm model while the symbols show the experimental measurements. 

The mobile phase is a solution of a weak solvent and of one or several additives, often referred to as the strong solvent. The retention factors of the sample components decrease with increasing concentration of the strong solvent. We assume that the mechanism of adsorption is not so affected by the change in mobile phase composition that the isotherm model of any one component itself would vary from one composition to another. Accordingly, we assiune that the coefficients of the isotherm are functions of the mobile phase concentration. If the equilibrium isotherm is accormted for with the Langmuir model, for example, we can write the isotherm as... 

We have performed such calculations for samples of non-functional polybutadienes 66) (Fig. 11) and, using the found Ax and X0 values, we calculated Kd0) for a cubic lattice model and a slit-like pore within the whole experimentally accessible eab range using Eq. (3.16). The result presented in Fig. 12 shows a good agreement of the experimental data with the calculated curves. Even such a crude model as the lattice-like model and a slit-like pore can be successfully applied to assess the change in the retention volume as a function of the composition of the mobile phase. 

S. Espinosa, E. Bosch, and M. Roses, Retention of ionizable compounds in high-performance hquid chromatography. IX Modeling retention in reversed-phase liquid chromatography as a function of pH and solvent composition with acetonitrile-water mobile phases,/. Chromatogr. A 947 (2002),47-58. 

A single solvent only rarely provides suitable separation selectivity and retention in normal-phase systems, which should be adjusted by selecting an appropriate composition of a two- or a multi-component mobile phase. The dependence of retention on the composition of the mobile phase can be described using theoretical models of adsorption. With some simplification, both the Snyder and the Soczewinski models lead to identical equation describing the retention (retention factor. A) as a function of the concentration of the stronger (more polar) solvent, (p. in binary mobile phases comprised of two solvents of different polarities [,121 ... 

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