Elution strength

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

Consequences of the Snyder and Soczewinski model are manifold, and their praetieal importance is very signifieant. The most speetaeular conclusions of this model are (1) a possibility to quantify adsorbents ehromatographic activity and (2) a possibility to dehne and quantify chromatographic polarity of solvents (known as the solvents elution strength). These two conclusions could only be drawn on the assumption as to the displacement mechanism of solute retention. An obvious necessity was to quantify the effect of displacement, which resulted in the following relationship for the thermodynamic equilibrium constant of adsorption, K,, in the case of an active chromatographic adsorbent and of the monocomponent eluent ... 

Assuming that the adsorbent surface is occupied by an adsorbed solute molecule (Ag) and a molecule of a stronger solvent (n ) which are equal to one another, the elution strength of a binary eluent, shows the following dependence on its quantitative composition ... 

Relative strength of the solvents on polar adsorbents arranged as an eluotropic series in ehromatographie elution strength order for pure solvents or mixtures are given in literature [13-16]. The eluotropic series of pure solvents are generally referred to a partieular adsorbent. 

Arranging the solvents in separation-strength order, the so-called eluotropic series appeared. This term, introduced by Trappe, was related to the experience with bare silica, where a strong solvent is able to move polar solutes on a polar stationary phase. Later this was improved by the discovery of a direct proportion between the elution strength and the dielectric constant. Because silica is hydrophilic and highly polar, there was a correlation between the eluotropic series and the polarity of a solvent [16,18]. 

Figure 5. TLC retention factor Rp for poly(methyl methacrylate) on silicagel, as a function of the elution strength of binary solvent (carbontetrachloride, CCl )/ displacer (1,4-dioxane) mixtures. Note the steep increase in Rj, at Efj w 0.38, indicating a sharp adsorption/ desorption transition.

Electrical field perturbation variance. It is well established in HPLC that when the injected sample solution has a different eluting strength than that of the mobile phase, peak deformation is bound to occur. A similar effect is observed in CE. Instead of eluting strengths, it concerns here differences in conductivity between the sample zone and the bulk electrolyte in the capillary [9,32], The conductivity (y, Ohm 1 m ) of a solution is given by the cumulative effect of the contributions of different ions ... 

In the case of a binary eluent system the dependence of the capacity factor on the volume fraction of the component with the higher elution strength (C) can be calculated by... 

Fig. 3.4. Potency values (related to the elution strength) 2 standard deviations of solvents, simultaneously taking into consideration each tetrazolium salt. Reprinted with permission from E. Forgacs et al. [85],...

The addition of surfactants to the mobile phase will reduce surface tension and increase eluting strength. For this reason, surfactants might be expected to be useful in the chromatography of polypeptides. This has proven not to be the case... 

Grumbach et al. [100] recommended the use of acetonitrile with bare silica columns, with concentration not greater than 95% or less than 70%. At least 5% of the mobile phase should be water to allow for the formation of the aqueous layer and to allow solubility of buffer, if one is used. In some cases, methanol can be used to form the polar layer. It was noted that while bare silica can be used at pH < 1 (no bonded ligands to hydrolyze, as in RP-HPLC) it is more susceptible to dissolution at intermediate pH (presumably since it not protected by a C18 layer), and should not be used above pH 6. Buffers such as ammonium acetate at pH 5 and ammonium formate at pH 3 were recommended at 5-20 mM concentrations. They reported the elution strength of various solvents using silica and HlLlC conditions as... 

TABLE 2 Equal Elution Strength of Mobile Phases for RPLC... 

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