Displacement chromatography displacer concentration

The concentration depends only on that of the leading ion it is independent of the initial concentration in the sample. Therefore, ITP can act as an enrichment method, analogous to displacement chromatography and in contrast to zone electrophoresis and elution chromatography. The concentration in the steady state is adjusted to the value given in Eq. (2). If the concentration of the analyte species is lower in the initial sample, the higher steady-state concentration is established. This concentration is independent of the migration distance there is no dilution with a BGE as there is in capillary zone electrophoresis (CZE). 

Figure 18.28 Comparison of the performance of displacement and elution chromatography. Maximum production rate in the separation of a certain mixhure on a given column 25-cm- long column packed with 20 jim particles. 1 3 mixture, cc = 1.70 k 2 = 9.78. (a) plot of maximum production rate of second component versus reduced velocity of the mobile phase. Figures by the symbols on lines are the loading factor and, in the case of displacement, the displacer concentration, (b) concentration of the collected fractions (mg/mL). Reproduced with permission from A.M. Katti et al, J. Chro-matogr., 540 (1991) 1 (Fig. 9).

Overdisplacement A phenomenon which takes place in displacement chromatography when the displacer concentration is too high, and, although the isotachic train forms, the band plateaus are very narrow or nonexistent, the retention times of the bands are dose to the holdup time and/or the shock layers are too thick. Little or no separation of the bands take place. 

In displacement chromatography, a pulse of mixture is injected, and this is followed by a step of a single component called a displacer, which is adsorbed more strongly than any of the mixture components. After a certain period of time, during which the profiles of each pulse become reorganized, an isotachic pulse train is formed. In the isotachic train, each component forms its own concentration boxcar, with a height that depends on the component and displacer isotherms as well as on the displacer concentration, and a width proportional to the amount of the corresponding component in the sample [14], [15]. 

Displacement chromatography is suitable for the separation of multicomponent bulk mixtures. For dilute multicomponent mixtures it allows a simultaneous separation and concentration. Thus, it permits the separation of compounds with extremely low separation fac tors without the excessive dilutiou that would be obtained in elution techniques. 

TABLE 16-15 Concentrations and h-Function Roots for Displacement Chromatography of a Mixture of M-1 Components Numbered in Order of Decreasing Affinity for the Stationary Phase (Adapted from Frenz and Horvath/ 1985). 

FIG. 16 36 Dimensionless time-distance plot for the displacement chromatography of a binary mixture. The darker lines indicate self-sharpening boundaries and the thinner lines diffuse boundaries. Circled numerals indicate the root number. Concentration profiles are shown at intermediate dimensionless column lengths = 0.43 and = 0.765. The profiles remain unchanged for longer column lengths. 

Conventional elution chromatography has the serious disadvantage of dilution, and usually a concentration step must follow. The technique of displacement chromatography circumvents dilution and may even result in an eluant more concentrated than the feed. A displacer compound breaks the desired product from the chromatographic material sharply, and a column heavily loaded with several biochemicals will release them one at a time depending on their adsorption equilibria. However, the displacers tena to be expensive and can be troublesome to remove from the product. 

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