Retention in LSC

Snyder [350] has given an early description and interpretation of the behaviour of LSC systems. He explained retention on the basis of the so-called competition model . In this model it is assumed that the solid surface is covered with mobile phase molecules and that solute molecules will have to compete with the molecules in this adsorbed layer to (temporarily) occupy an adsorption site. Solvents which show a strong adsorption to the surface are hard to displace and hence are strong solvents , which give rise to low retention times. On the other hand, solvents that show weak interactions with the stationary surface can easily be replaced and act as weak solvents . Clearly, it is the difference between the affinity of the mobile phase and that of the solute for the stationary phase that determines retention in LSC according to the competition model. 

Eqn. (3.72) describes retention in LSC in terms of separate parameters for the adsorbent ( a a) solute (S9, At) and the solvent ( °). As such, it has proved invaluable for the interpretation of retention and selectivity phenomena in LSC. For example, the effect of a change in the solvent using the same stationary phase and the same solute can easily be understood in terms of a variation in °. 

Equation (8) is a fundamental relationship for retention in LSC as a function of the solvent strength of the mobile phase. It states that log A values for different solutes will yield linear plots against values of for different mobile phases, and the slopes of these plots will be proportional to the molecular size A, of the solute. Numerous data are summarized or referenced in Ref. /. showing the validity of Eq. (8) when applied to LSC systems where the solute and solvent molecules are nonlocalizing (nonpolar or moderately polar compounds—see Section II,B below). Similar data showing the applicability of Eq. (8) to amino-phase polar-bonded-phase columns are given in Ref. 17. 

Various workers have considered the effect of the log Q term on retention in LSC systems (usually for silica) (2, 8, 31, 32). Soczewinski (2)... 

Solvent selectivity effects in LSC are accounted for by terms (i), (ii), and (lil) of Eq. (34). We will first discuss solvent-strength selectivity term (i) and solvent-solute localization selectivity term (ii), leaving hydrogenbonding selectivity term (iii) to the following section. As already indicated, solvent-strength selectivity term (i) is of limited value in optimizing retention in LSC. This effect is directly based on the validity of Eq. (8),... 

The various aspects of displacement and localization are now well understood, and predictions of their effects on retention in LSC can be made with some confidence. Hydrogen bonding between solute and solvent molecules requires further investigation, and it is likely that such studies will contribute to our understanding of hydrogen bonding in solution as well. On the basis of the present model it should prove possible to systematically explore new stationary phase compositions for unique separation potential. However, this subject falls outside the area of mobile-phase effects per se, and will be reserved for another time. 

Retention and selectivity in LSC are dramatically influenced by the presence of even low concentrations of polar additives in the mobile phase, particularly water [20,22,253-255]. Their influence is most pronounced when the mobile phase is nonpolar. However, when used in controlled amounts (in which case they are... 

The relevance of LSC data to reverse osmosis stems from the physicochemical basis (adsorption equilibrium considerations) of liquid-solid chromatography (52), and the principle that the solute-solvent-membrane material (column material) Interactions governing the relative retention times of solutes in LSC are analogous to the interactions prevailing at the membrane-solution Interface under reverse osmosis conditions. The work already reported in several papers on the subject (53-58) indicate that the foregoing principle is valid, and hence LSC data offer an appropriate means of characterizing interfacial properties of membrane materials, and understanding solute separations in reverse osmosis. 

The solubility parameter model appears to work very well for the prediction of iso-eluotropic mixtures in LLC and RPLC. However, in LSC the retention mechanism is very different from the one that was assumed at the outset of this section, and hence a different model should be applied to allow the description and possibly prediction of the eluotropic strength in LSC. This model will be described in section 3.2.3. 

Figure 3.19 Variation of retention with composition in LSC according to the simplified linear relationship of eqn.(3.74). Stationary phase Lichrosorb ALOX T (Alumina). Mobile phase n-propanol (

volume fraction) in n-heptane. Solutes lumisterol (1), tachysterol (2), calciferol (3) and ergosterol (4). Figure taken from ref. [357]. Reprinted with permission. 

Figure 3.19 shows an example of the linear variation of retention with composition according to eqn.(3.74). In this figure the logarithm of the volume fraction () of the stronger solvent is plotted on the horizontal axis. Plotting In XB will lead to a similar linear plot. The simple equation of Soczewinski (eqn.3.74) often yields a very good description of experimental data in LSC [353,355,356]. 

The concentration of the counterion can be used to control the retention in IEC. It plays a role similar to that of the eluotropic strength of the eluent in RPLC or LSC, in that it affects retention much more than it does selectivity. The capacity factor can be related to the distribution coefficient of the solute (Dx) ... 

Both pentane and carbon dioxide are solvents of low polarity. The polarity may be increased by the addition of suitable modifiers to the mobile phase. Such modifiers have a pronounced effect on the retention. The decrease in retention upon the addition of modifiers seems to resemble what is observed in LSC (see section 3.2.3). Apart from the effect on retention, the addition of polar modifiers to the mobile phase also has a marked effect on the peak shape. Especially in the case of more polar solutes, the addition of... 

Eqn.(5.6) defines a so-called linear gradient. Indeed, linear gradients are most popular in RPLC [527]. In LSC, retention varies much more strongly with mobile phase composition than in RPLC, especially when small amounts of organic modifier are added to the mobile phase (see section 3.2.3). Therefore, concave gradients are to be preferred [527]. 

The ratio of retention volumes in the two solvents depends on their difference in eluent strength (e° - e ), since a and As are constant. Thus, if one has tabulated values for the eluent strength parameter e°, one can predict the effect on retention of changing e°. This concept forms one basis for the selection of mobile phases in LSC. 

Second, previous tests of the displacement model have focused mainly on its ability to correlate and predict retention data in terms of derived correlational equations. Such correlations are based on various free energy relationships, and it is often found that comparisons of this kind can be insensitive to differences in the underlying physical model. That is, correlations of experimental retention data with theory may appear acceptable, in spite of marked deficiencies of the model. In some cases (e.g.. Ref. 12, sorption versus displacement models), radically different models can even yield the same or similar correlational equations. Here we will further test the proposed model for LSC retention in the following ways (1) application of the model to a wide range of LSC systems, involving major variations in solute, solvent, and adsorbent (2) examination of the various free energy terms that individually contribute to overall retention. 

The present model has so f2ir assumed that interactions between molecules of solvent or solute in either phase can be ignored. Now we will examine the effects of these interactions on retention in various LSC systems. Equation (4) for the retention of a solute X in a mobile phase M recognizes intermolecular interactions in the mobile phase (n), but assumes that adsorbed-phase free energies ( ia) are not a function of intermolecular interactions within the adsorbed phase. We can recognize these adsorbed-phase intermolecular interactions by adding an energy term Eja" to Eq. (4) for each adsorbed species i ... 

The mobile phase competes with the sample components for adsorption sites on the stationary phase and thereby decreases the number of adsorption sites which are available for the solutes (i.e. sample components). Consequently, use of increasingly polar mobile phases decreases the retention times of solutes. Several solvents used in LSC in order of increasing polarity are Fluoroalkanes, petroleum ether, carbon tetrachloride, cyclohexane, toluene, benzene, esters, chloroform, ethyl ether, dichloroethane, methyl ethyl ketone, acetonitrile, alcohols, water, pyridine, organic acids. 

Many of the adsorbents normally used in LSC (Chapter 7) have found application in GSC. In addition, several adsorbents have been developed specifically for GSC, usually for the purpose of overcoming the unique problems of GSC, but occasionally to provide unusual separation selectivity. For the most part, these adsorbents are based on the creation of a uniform surface, which in turn tends to promote isotherm linearity, lower retention volumes, and decreased sample reaction, and also makes adsorbent standardization easier. One approach, pioneered by Scott 14), is based on coating a conventional adsorbent or chromatographic support... 

Polarity is a key word in many chromatographic separations since a polar mobile phase will give rise to a low solute retention in normal phase LC (liquid-solid chromatography, LSC, of adsorption chromatography), or to a high solute retention in reversed-phase LC (RPLC). Nevertheless, it is often unclear exactly what this term means. One way to define the concept of polarity is to consider the Hildebrand solubility parameter another is to consider the Snyder solvent parameter. 

Every chromatographic separation should give sufficient resolution Rs and good reproducibility, which have great importance for qualitative and quantitative analysis. Good reproducibility in LSC can be achieved only in the range of a linear sorption isotherm. In this case the retention time Ir is independent of sample size and sample vol-... 

In liquid-solid adsorption chromatography (LSC) the column packing also serves as the stationary phase. In Tswett s original work the stationary phase was finely divided CaCOa, but modern columns employ porous 3-10-)J,m particles of silica or alumina. Since the stationary phase is polar, the mobile phase is usually a nonpolar or moderately polar solvent. Typical mobile phases include hexane, isooctane, and methylene chloride. The usual order of elution, from shorter to longer retention times, is... 

Using different polymeric materials in the chromatographic columns and LSC data on retention times (t) of suitably chosen reference solutes, three interfacial parameters (o(p, Ojj, and S), defined below, have been generated for characterizing polymeric membrane materials (53,56)... 

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