Isotherms overload conditions

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. 

The lower isotherm represents the overload condition that can occur in liquid/liquid or gas/liquid systems under somewhat unique circumstances. If the interactions between solute molecules with themselves is stronger than the interactions between the solute molecules and the stationary phase molecules, then, as the concentration of solute molecules increases, the distribution coefficient of the solute with respect to the stationary phase also increases. This is because the solute molecules interact more strongly with a solution of themselves in the stationary phase than the stationary phase alone. Thus, the higher concentrations of solute in the chromatographic... 

Nonlinearity of the Langmuir adsorption isotherms is observed even in noncompetitive chromatographic processes. Individual adsorption isotherms can be found experimentally using frontal analysis at overload conditions however, the adsorption isotherms in the separation of mixtures are different because of the interference of other compounds in the mixture. In PHPLC method development, it is necessary to optimize separation conditions and column loading experimentally. 

In analytical TLC, linear adsorption isotherms and compact spots are obtained for the loading of sample below 10 g mixture/1 g adsorbent. To increase throughput, PLC is operated under overloaded conditions, at 1 mg/1 g adsorbent or more. The overloading can be of concentration or volume type the former is more advantageous [2]. 

Because of the limited surface capacity of the stationary phase, the column often operates under overloaded conditions. This is why a further increased sample load results in a smaller amount of the fraction adsorbed. Thus, nonlinear conditions prevail. A further complication in the preparative case is that the different types of components compete with each other for the same binding site, an effect that ultimately results in strong band interactions and band contaminations. The functions describing this complex behavior are called competitive adsorption isotherm parameters. Because of... 

The term. .overload" has, notably, been introduced by analytical chemists. Here a column should not be. .overloaded" in order to achieve a constant retention time for a reproducible analytical detection of each component peak. Preparative chromatography has a different aim, which is called. .productivity". To achieve this goal the columns are operated under so-called. .overloaded" conditions. From the engineering view point overloaded systems are nonlinear because of nonlinear isotherms as well as dispersion and mass transfer effects. 

In order to calculate band profiles and compare the results with experimental profiles, we need to know the detailed experimental conditions. These conditions include (i) the adsorption isotherm, (ii) the HETP under linear conditions, (iii) the sample size, (iv) the mobile phase flow rate, (v) the hold-up volume, and (vi) the column dimensions. The column HETP can be obtained easily by injecting a very small amormt of sample and measuring the band width. The band profile im-der overloaded conditions is very sensitive to the adsorption isotherm, which is why isotherms must be measured accurately in order to achieve good agreement... 

Once separation conditions are established, the maximum sample load is determined. To minimize sample waste this is determined for the analytical column. The sample size (volume or mass) is increased in increments until the product bands are just touching, but still separated from impurity peaks [60,64-66]. This corresponds to the upper region of the linear portion of the sorption isotherms for the products, or to a slight overload condition. For difficult separations, partially overlapped peaks may have to be accepted to obtain acceptable sample throughput, in which case, product purity is maintained by collecting the outside portions of the product peaks and recycling the middle portion. Figure 11.6. 

To describe the peak shapes of a separation under overload conditions a clear understanding of how the competitive phase equilibria, the finite rate of mass transfer, and dispersion phenomena combine to affect band profiles is required [ 11,66,42,75,76]. The general solution to this problem requires a set of mass conservation equations appropriate initial and boundary conditions that describe the exact process implemented the multicomponent isotherms and a suitable model for mass transfer kinetics. As an example, the most widely used mass conservation equation is the equilibrium-dispersive model... 

Before starting a preparative separation (e.g. with an SMB system) the chromatographic separation has to be optimized on the analytical scale. For overloading conditions, parameters like adsorption isotherms have to be determined before realizing an experimental separation. As the SMB process is very complex an optimization of the separation in terms of productivity, solvent consumption and costs have to be carried out by simulation. Therefore models of chromatography are needed. The fundamentals of modeling and simulation of chromatography are shown in Section 9.3. 

In addition, in comparison to conventional batch chromatography, the application of the SMB principle allows the continuous operation of processes, which makes it easier to obtain products of constant purity without analytical controls for each single batch. Even more importantly, a 90% reduction in the overall amoimt of solvents needed for a separation can be achieved [32]. The overloaded conditions make it possible to optimize productivity with regard to the inventory of stationary phases. This optimization requires knowledge of the adsorption isotherms (Eq. 8) and the use of special simulation programs [33]. 

This is the simplest non-linear relation which is exhibited by single solutes under mass-overloaded conditions. The relation in Eq. (3) is the Langmuir Adsorption Isotherm. Other isotherms relating the stationary and mobile phase concentrations are possible, depending upon the individual properties of the solutes, mobile phases and packing materials. Very many solutes follow the Langmuir isotherm, which is one... 

This form of the isotherm results in a linear rate of change of the capacity factor with mobile phase concentration and gives rise to a truly triangular peak shape. Knox also concluded that the efficiency of a peak under mass overloaded conditions... 

A study of the effects of mobile phase composition on retention and selectivity of some carboxylic acids and amino acids was performed on a commercially available teicoplanin CSP, under analytical conditions, on the profile of the adsorption isotherms of the enantiomers and on the overloaded separation [87]. 

The method appears to be particularly suitable for the study of expensive compounds and/or species available in very low amounts. Figure 10.19 shows the application of IM to the study of the adsorption equilibria of a polypeptide, nociceptin/orphanin FQ, on a C,g column [40]. Isotherms (Figure 10.20) were obtained by numerical procedure through the fitting of the overloaded band profiles obtained under different conditions. Adsorption data allowed for the prediction of the band profile under overloaded gradient conditions (Figure 10.21) with a minimal amount of compound consumption. 

Band dispersion from sample mass overload is a direct result of the chromatographic process proceeding under conditions, where the adsorption isotherm of the solute on the stationary phase, is no longer linear. The development of an equation that describes the extent, of band spreadinn as a function of mass of sample placed on the column, is complex. This problem has been elegantly approached by 6uiochon and his co-workers (15-18) from the basis of the adsorption isotherm of the solute on the stationary phase. 

Tailing peaks may also originate if the amount of the solute in the chromatographic system is too great. If the linear range of the adsorption isotherm of the solute is exceeded, the chromatographic sorbent is overloaded, which results in asymmetry of the elution peak. By conversion into a derivative with other sorption properties, conditions may be attained that are suitable for operation in the linear range. 

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