Chromatography transport model

In the frontal analysis experiment described in Section 5.3.2, the transport model of chromatography was used to fit the experimental data [40]. Neglecting axial and eddy diffusion, band broadening was accounted for by one single mass transfer rate coefficient. The mass transfer rate coefficients estimated were small and strongly dependent on the temperature and solute concentration, particularly the rate coefficients corresponding to the imprinted L-enantiomer (Fig. 5.12). Above a concentration of ca. 0.1 g/L the mass transfer rate coefficients of the two enantiomers are similar. 

Frontal chromatography can be used in combination with chromatographic models to study mass-transfer and dispersion processes (e.g., the equilibrium dispersive or the transport model of chromatography [7]). 

Based on a microscopic model of gas-solid thermochromatography in open columns by Zvara (1985), a Monte Carlo code has been made available (Tiirler 1996), which allows to generate thermochromatographic deposition zones as well as yield versus temperature curves as observed in isothermal chromatography. This model accommodates the influence of the carrier gas flow, the actual temperature profiles, and the different half-lives of the investigated species. For each isothermal temperature, the transport through the column is modeled for a large number of sample molecules. These calculations result in a curve of yield versus temperature for each value of the adsorption enthalpy The curve for the particular... 

This agrees to internal VolSurf models derived for PAMPA membrane transport [163] to understand passive transcellular transport across membranes. One of our internal models based on 29 compounds characterized by immobilized artificial membrane chromatography by Salminen etal. ]164] shows an of 0.81 and = 0.70 for two PLS components derived using VolSurf descriptors. This is one of the rare examples where ionized starting molecules led to slightly better PLS statistics, while the general chemical interpretation is not affected. 

The model of ideal linear gas chromatography (Equation 1) [12] also describes the transport of a chemical species in the temperature gradient of a vacuum tube. At molecular flow conditions the linear velocity of the carrier gas, which is identical to the transport velocity of the adsorbate in the gas phase, has to be substituted by the fraction of the column length over the average retention time of the species in the column ... 

Capillary electrochromatography is a more complicated system than CE and HPLC due to the combination of both electrophoretic and chromatographic transport mechanisms. It is difficult to define an effective selectivity (separation factor) as in the case of general chromatography or general electrophoresis. To better illustrate the interactions that control selectivity, we defined a relative selectivity a,. =. tjt 2), and postulated a model that illustrates the effect of separation parameters on the enantioselectivity [10]. 

In the following the most relevant models for liquid chromatography are derived in a bottom-up procedure related to Fig. 6.2. To illustrate the difference between these models their specific assumptions are discussed and the level of accuracy and their field of application are pointed out. The mass balances are completed by their boundary conditions (Section 6.2.7). For the favored transport dispersive model a dimensionless representation will also be presented. 

The simplest model takes into account convective transport and thermodynamics only. It assumes local equilibrium between mobile and stationary phase. This model, also called the ideal or basic model of chromatography, was described first by Wicke (1939) for the elution of a single component. Subsequently, De Vault (1943) derived the correct form of the mass balance. 

Transport dispersive model Adsorption chromatography for products with low molecular weights Generally high accuracy Chiral separation... 

Equation 6.138 defines a formal connection between the effective mass transport and the film transport, the pore diffusion and the adsorption rate coefficient. It illustrates that keff is a lumped parameter", composed of several transport effects connected in series. This also gives reasons for the use lumped rate models as it proves that the impact of the lumped parameters on the most important peak characteristics, retention time and peak width, is identical to the effect described by general rate model parameters in linearized chromatography. 

In contrast to the work quoted so far, Strube and Diinnebier applied a different approach by optimizing elution chromatography using cut strategy II. Strube uses a transport-dispersive model for his simulation studies and optimized the productivity... 

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