Mobile phase adsorption isotherms

In LC, at very low concentrations of moderator in the mobile phase, the solvent distributes itself between the two phases in much the same way as the solute. However, as the dilution is not infinite, the adsorption isotherm is not linear and takes the form of the Langmuir isotherm. 

The mobile phase was water in which the moderator alcohols were dissolved. It is seen that the linear relationship is completely validated and the data can provide the adsorption isotherms in the manner discussed. The mean surface area was found to be 199 m /g with a standard deviation between the different alcohols of 11 m /g. 

It is seen that at high concentrations (a) becomes unity and the surface is completely covered with the more strongly adsorbed solvent. The adsorption isotherm of chloroform on silica gel, determined by Scott and Kucera (5) is shown in figure 1. It is seen that the monolayer of chloroform collects on the surface continuously until the chloroform content of the mobile phase is about 50%. At this concentration the monolayer appears complete. Thus, between 0 and 50% chloroform in the n-heptane, the interactions between the solute and the chloroform in the mobile phase are continuously increasing. 

As the solvent concentration increases, the PIC reagents will interact more strongly with the mobile phase and will be less strongly adsorbed on the reverse phase surface. As a consequence, there will be less ion exchange material on the stationary phase surface. This is clearly demonstrated by the adsorption isotherm of octane sulfonate shown in figure 10. 

Volume overload can be treated in a simple way by the plate theory (8,9). In contrast, the theory of mass overload is complicated (10-12) and requires a considerable amount of basic physical chemical data, such as the adsorption isotherms of the solutes, before it can be applied to a practical problem. Volume overload is useful where the solutes of interest are relatively insoluble in the mobile phase and thus, to apply a sample of sufficient size onto the column, a large sample volume is necessary. If the sample is very soluble in the mobile phase then mass overload might be appropriate. 

The recommended mobile phase must assure a proper adsorption isotherm for a given stationary phase. Chapter 2 discusses the adsorption planar chromatography in the nonhnear region. 

According to this relation, the distribution function K can be estimated from the chromatographic data of a solute using pure solvents as the mobile phase. Equation 4.25 shows that the difference j for each component of the mobile phase can be calculated without the adsorption isotherm data. 

To optimize a given preparative chromatographic process (highest productivity, lowest mobile phase consumption, and product dilution) the separation has to be performed with the highest product concentrations still compatible with the system. One immediate consequence of this is that each column has to be operated in the non-linear range of the adsorption isotherm. 

The retention of analyses in RP-HPLC markedly depends on the adsorption of the organic constituent of the mobile phase on the surface of the stationary phase. The excess adsorption isotherms of ACN, THF and methanol were measured on silica support modified with C, C6, C8, C10, C12 and C18 monomeric phase and a model was developed for the description of the retention of solutes from the binary mobile phase. The dependence of the retention factor on the partition coefficient can be described by... 

As is the case with adsorption isotherms, those curves in Figure 18.3 which are concave to the concentration axis for the mobile phase are termed favourable and lead to self-sharpening ion exchange waves. 

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]. 

Two assumptions essential to obtain Eqs. (82) or (84) are (i) the hetaeron adsorption has an asymptotic limiting value, and (ii) complexes between the eluite and hetaeron form in the mobile phase. The Langmuir adsorption isotherm is used here because it is simple to manipulate mathematically and because most data on the adsorption of detergents used as hetaerons are reported to obey this relationship as determined directly (207, 209) or from analyses of its effect on chromatographic retention ( 4). 

When the amount of the sample is comparable to the adsorption capacity of the zone of the column the migrating molecules occupy, the analyte molecules compete for adsorption on the surface of the stationary phase. The molecules disturb the adsorption of other molecules, and that phenomenon is normally taken into account by nonlinear adsorption isotherms. The nonlinear adsorption isotherm arises from the fact that the equilibrium concentrations of the solute molecules in the stationary and the mobile phases are not directly proportional. The stationary phase has a finite adsorption capacity lateral interactions may arise between molecules in the adsorbed layer, and those lead to nonlinear isotherms. If we work in the concentration range where the isotherms are nonlinear, we arrive to the field of nonlinear chromatography where thermodynamics controls the peak shapes. The retention time, selectivity, plate number, peak width, and peak shape are no longer constant but depend on the sample size and several other factors. 

In analytical applications where the concentrations of analytes in mobile phase are very small the adsorption isotherm is usually considered to be linear, i.e., for each component involved in the separation the relationship between mobile (C) and stationary phase (q) concentrations is linear ... 

The assumption of linear chromatography fails in most preparative applications. At high concentrations, the molecules of the various components of the feed and the mobile phase compete for the adsorption on an adsorbent surface with finite capacity. The problem of relating the stationary phase concentration of a component to the mobile phase concentration of the entire component in mobile phase is complex. In most cases, however, it suffices to take in consideration only a few other species to calculate the concentration of one of the components in the stationary phase at equilibrium. In order to model nonlinear chromatography, one needs physically realistic model isotherm equations for the adsorption from dilute solutions. 

FIGURE 10.12 Adsorption data of buspirone (o), doxepin (V), and diltiazem ( ) and best Bilangmuir isotherms (continuous and dotted lines). Mobile phase is acetonitrile buffer=35 65 buffer is 0.1 M phosphate, pH 3.0, T = 25°C. (Reproduced from Quinones, I. et al., J. Chromatogr. A, 877, 1, 2000. With permission from Elsevier.)... 

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