Overloaded elution chromatography

Purified zones of analytes can be obtained under favorable conditions in a far more efficient way than can be attained using overloaded elution chromatography. 

Displacement chromatography is seen primarily as an alternative to overloaded elution chromatography. The advantages of this mode are as follows (1) concentrated product pools are obtained (2) the stationary phase is utilized more effectively (3) large... 

Felinger, A., Guiochon, G., Optimization of the experimental conditions and the column design parameters in overloaded elution chromatography, J. Chromatogr. A, 1992a, 591, 31-45. 

The theory of simple waves applies to large-volume injections, i.e., to the profiles obtained upon injection of rectangular profiles which are so wide that the injection plateau has not been entirely eroded when the band elutes. Then, simplifications of the solution occur because there is a constant state, the concentration plateau. This solution is not valid in overloaded elution chromatography when the injection volume is sufficiently small that the injection plateau has eroded and disappeared by the time the band elutes from the column. It is important to discuss this solution, however, because it takes a finite time for the profile of even a narrow rectangular injection to decay, and the band profile during that period is given by the simple wave solution. Also, this solution is the basis for a method of determination of competitive equilibrium isotherms (Chapter 4, Section 4.2.4). 

Thus, it will be extremely difficult at best to separate the influences of the various phenomena that may be responsible for the effects of a slow kinetics of mass transfer and a slow kinetics of the retention mechanism. The fitting of experimental data obtained in overloaded elution chromatography to various models of chromatography will not permit the choice of a best model, nor the identification of the slowest step in the chromatographic process. Independent measurements of the kinetic parameters are necessary. 

Overloaded elution chromatography Name given to elution chromatography when a large sample is used, so the column is operated imder nonlinear isocratic conditions. This distinguishes the preparative applications of elution from its analytical applications, which use small samples. 

In most cases, chromatography is performed with a simple initial condition, C(f = 0,z) = q t = 0,z) = 0. TTie column is empty of solute and the stationary and mobile phases are under equilibrium. There are some cases, however, in which pulses of solute are injected on top of a concentration plateau (see Chapter 3, Section 3.5.4). The behavior of positive concentration pulses injected xmder such conditions is similar to that of the same pulses injected in a column empty of solute and they exhibit similar profiles. Even imder nonlinear conditions (high plateau concentration), a pulse that is sufficiently small can exhibit a quasi-linear behavior and give a Gaussian elution profile. Its retention time is linearly related to the slope of the isotherm at the plateau concentration. Measuring this slope is the purpose of the pulse method of measurement of isotherm data. Large pulses may also be injected and they will give overloaded elution profiles similar to those obtained with a column empty of solute. 

In the equilibrium-dispersive model of chromatography, however, we assume that Eq. 10.4 remains valid. Thus, we use Eq. 10.10 as the mass balance equation of the component, and we assume that the apparent dispersion coefficient Da in Eq. 10.10 is given by Eq. 10.11. We further assume that the HETP is independent of the solute concentration and that it remains the same in overloaded elution as the one meastued in linear chromatography. As shown by the previous discussion this assxunption is an approximation. However, as we have shown recently [6], Eq. 10.4 is an excellent approximation as long as the column efficiency is greater than a few hundred theoretical plates. Thus, the equilibriiun-dispersive model should and does account well for band profiles under almost all the experimental conditions used in preparative chromatography. In the cases in which the model breaks down because the mass transfer kinetics is too slow, and the column efficiency is too low, a kinetic model or, better, the general rate model (Chapter 14) should be used. 

We compare in Figures 10.16a to 10.16d [70] the experimental band profiles in overloaded elution (symbols) and the profiles calculated for elution performed in normal and reversed phase chromatography (solid lines). Figure 10.16a corresponds to the elution of large bands of benzyl alcohol on silica with a THF/n-hexane solution. Figure 10.16b corresponds to the elution of acetophenone on silica with a (97.5 2.5) mixture of n-hexane and ethyl acetate. Figure 10.16c illustrates the profiles of bands of benzyl alcohol eluted on CIS silica by a methanol/water solution. Figure 10.16d corresponds to the elution of phenol on C18 chemically bonded silica with a (20 80) mixture of methanol and water. In all four cases. 

In the abundant literature regarding either system peaks or nonlinear chromatography, we find few papers discussing system peaks imder conditions of overloaded elution. Helfferich and Klein [8] discussed the phenomena that take place upon injection of a large sample of a single component in a binary mobile phase. 

Analytical Solution for Volume-overloaded Gradient Elution Chromatography. 703... 

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... 

In the case of a step input, the numerical solution of the system of Eqs. 16.30 and 16.31 has been discussed in the literature for multicomponent mixtures [16]. The numerical solution of Eqs. 16.30 and 16.31 without an axial dispersion term i.e., with Di = 0) has been described by Wang and Tien [17] and by Moon and Lee [18], in the case of a step input. These authors used a finite difference method. A solution of Eq. 16.31 with D, = 0, combined with a liquid film linear driving force model, has also been described for a step input [19,20]. The numerical solution of the same kinetic model (Eqs. 16.30 and 16.31) has been discussed by Phillips et al. [21] in the case of displacement chromatography, using a finite difference method, and by Golshan-Sliirazi et al. [22,23] in the case of overloaded elution and displacement, also using finite difference methods. 

The Pr xY objective fxmction can successfully be applied to the optimization of overloaded gradient elution chromatography [40,43]. Figure 18.22 compares the chromatograms obtained rmder the optimum conditions given by the Pr and... 

The optimization of the experimental conditions in displacement chromatography for maximiun production rate has been studied less than the optimization of overloaded elution, reflecting the lesser importance of this method in industrial practice. Frenz et al [52] performed an experimental study of the dependence of the throughput on the operational parameters in reversed-phase displacement systems. They demonstrated that both the nature and the concentration of the displacer must be appropriately selected to optimize the throughput. Jen and Pinto [53] have used the ideal model, and the fe-transform approach to maximize... 

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