Dispersion stationary phase

The more dispersive solvent from an aqueous solvent mixture is adsorbed onto the surface of a reverse phase according to Langmuir equation and an example of the adsorption isotherms of the lower series of aliphatic alcohols onto the surface of a reverse phase (9) is shown in figure 9. It is seen that the alcohol with the longest chain, and thus the most dispersive in character, is avidly adsorbed onto the highly dispersive stationary phase, much like the polar ethyl acetate is adsorbed onto the highly polar surface of silica gel. It is also seen that... 

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. 

In a chromatographic separation, the individual components of a mixture are moved apart in the column due to their different affinities for the stationary phase and, as their dispersion is contained by appropriate system design, the individual solutes can be eluted discretely and resolution is achieved. Chromatography theory has been developed over the last half century, but the two critical theories, the Plate Theory and the Rate Theory, were both well established by 1960. There have been many contributors to chromatography theory over the intervening years but, with the... 

Alhedai et al also examined the exclusion properties of a reversed phase material The stationary phase chosen was a Cg hydrocarbon bonded to the silica, and the mobile phase chosen was 2-octane. As the solutes, solvent and stationary phase were all dispersive (hydrophobic in character) and both the stationary phase and the mobile phase contained Cg interacting moieties, the solute would experience the same interactions in both phases. Thus, any differential retention would be solely due to exclusion and not due to molecular interactions. This could be confirmed by carrying out the experiments at two different temperatures. If any interactive mechanism was present that caused retention, then different retention volumes would be obtained for the same solute at different temperatures. Solutes ranging from n-hexane to n hexatriacontane were chromatographed at 30°C and 50°C respectively. The results obtained are shown in Figure 8. 

The induced counter-dipole can act in a similar manner to a permanent dipole and the electric forces between the two dipoles (permanent and induced) result in strong polar interactions. Typically, polarizable compounds are the aromatic hydrocarbons examples of their separation using induced dipole interactions to affect retention and selectivity will be given later. Dipole-induced dipole interaction is depicted in Figure 12. Just as dipole-dipole interactions occur coincidentally with dispersive interactions, so are dipole-induced dipole interactions accompanied by dispersive interactions. It follows that using an n-alkane stationary phase, aromatic... 

Alternatively, using a polyethylene glycol stationary phase, aromatic hydrocarbons can also be retained and separated primarily by dipole-induced dipole interactions combined with some dispersive interactions. Molecules can exhibit multiple interactive properties. For example, phenyl ethanol possesses both a dipole as a result of the hydroxyl group and is polarizable due to the aromatic ring. Complex molecules such as biopolymers can contain many different interactive groups. 

This is one approach to the explanation of retention by polar interactions, but the subject, at this time, remains controversial. Doubtless, complexation can take place, and probably does so in cases like olefin retention on silver nitrate doped stationary phases in GC. However, if dispersive interactions (electrical interactions between randomly generated dipoles) can cause solute retention without the need to invoke the... 

This relationship depends on the assumption that two similar stationary phases, irrespective of their polarity, can be considered to differ by measuring the ratio of the activity coefficients of two noncomplexing solutes (this basically implies the solute is nonpolar and will only interact with the stationary phase by dispersion forces). If this were true then. 

Scott and Kucera [4] carried out some experiments that were designed to confirm that the two types of solute/stationary phase interaction, sorption and displacement, did, in fact, occur in chromatographic systems. They dispersed about 10 g of silica gel in a solvent mixture made up of 0.35 %w/v of ethyl acetate in n-heptane. It is seen from the adsorption isotherms shown in Figure 8 that at an ethyl acetate concentration of 0.35%w/v more than 95% of the first layer of ethyl acetate has been formed on the silica gel. In addition, at this solvent composition, very little of the second layer was formed. Consequently, this concentration was chosen to ensure that if significant amounts of ethyl acetate were displaced by the solute, it would be derived from the first layer on the silica and not the less strongly held second layer. 

It follows that stationary phases of intermediate polarities can be formed from binary mixtures of two phases, one strongly dispersive and one strongly polar. This procedure is not used extensively in commercial columns, although is the easiest and most economic method of fabricating columns having intermediate polarities. 

The dispersion of a solute band in a packed column was originally treated comprehensively by Van Deemter et al. [4] who postulated that there were four first-order effect, spreading processes that were responsible for peak dispersion. These the authors designated as multi-path dispersion, longitudinal diffusion, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase. Van Deemter derived an expression for the variance contribution of each dispersion process to the overall variance per unit length of the column. Consequently, as the individual dispersion processes can be assumed to be random and non-interacting, the total variance per unit length of the column was obtained from a sum of the individual variance contributions. 

Theoretically, dispersion can take place by diffusion in the stationary phase but, as will be seen, in practice, is much less in magnitude than that in the mobile phase. The theoretical treatment is similar to that for dispersion in the mobile phase using equation (10). 

In a packed column, however, the situation is quite different and more complicated. Only point contact is made between particles and, consequently, the film of stationary phase is largely discontinuous. It follows that, as solute transfer between particles can only take place at the points of contact, diffusion will be severely impeded. In practice the throttling effect of the limited contact area between particles renders the dispersion due to diffusion in the stationary phase insignificant. This is true even in packed LC columns where the solute diffusivity in both phases are of the same order of magnitude. The negligible effect of dispersion due to diffusion in the stationary phase is also supported by experimental evidence which will be included later in the chapter. 

In summary, equation (13) accurately describes longitudinal dispersion in the stationary phase of capillary columns, but it will only be significant compared with other dispersion mechanisms in LC capillary columns, should they ever become generally practical and available. Dispersion due to longitudinal diffusion in the stationary phase in packed columns is not significant due to the discontinuous nature of the stationary phase and, compared to other dispersion processes, can be ignored in practice. 

Dispersion caused by the resistance to mass transfer in the stationary phase is exactly analogous to that in the mobile phase. Solute molecules close to the surface will leave the stationary phase and enter the mobile phase before those that have diffused further into the stationary phase and have a longer distance to diffuse back to the surface. Thus, as those molecules that were close to the surface will be swept along in the moving phase, they will be dispersed from those molecules still diffusing to the surface. The dispersion resulting from the resistance to mass transfer in the stationary phase is depicted in Figure 8. 

At the start, molecules 1 and 2, the two closest to the surface, will enter the mobile phase and begin moving along the column. This will continue while molecules 3 and 4 diffuse to the interface at which time they will enter the mobile phase and start following molecules 1 and 2. All four molecules will continue their journey while molecules 5 and 6 diffuse to the mobile phase/stationary phase interface. By the time molecules 5 and 6 enter the mobile phase, the other four molecules will have been smeared along the column and the original 6 molecules will have suffered dispersion. 

The resistance to the mass transfer term for the stationary phase will now be considered in isolation. The experimentally observed plate height (variance per unit length) resulting from a particular dispersion process [e.g., (hs), the resistance to... 

It is clear that, in LC, the resistance to mass transfer in the mobile phase (albeit within the pores of the particle) is much greater than the resistance to mass transfer in the stationary phase and, thus, simplifying equations (23) and (24), by ignoring the dispersion contribution from the resistance to mass transfer in the stationary phase, will be quite valid. 

Unfortunately, any equation that does provide a good fit to a series of experimentally determined data sets, and meets the requirement that all constants were positive and real, would still not uniquely identify the correct expression for peak dispersion. After a satisfactory fit of the experimental data to a particular equation is obtained, the constants, (A), (B), (C) etc. must then be replaced by the explicit expressions derived from the respective theory. These expressions will contain constants that define certain physical properties of the solute, solvent and stationary phase. Consequently, if the pertinent physical properties of solute, solvent and stationary phase are varied in a systematic manner to change the magnitude of the constants (A), (B), (C) etc., the changes as predicted by the equation under examination must then be compared with those obtained experimentally. The equation that satisfies both requirements can then be considered to be the true equation that describes band dispersion in a packed column. 

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