Eluent strength

For the third case, in which the sample is dissolved in solvent different from the eluent, it may disturb the separation process owing to a change of eluent strength and the precipitation of the solutes. The substances that are stagnant in the adsorbent s pores are gradually eluted, forming elongated zones from the start line. 

In this technique, the development distance is increased linearly in 10- or 20-mm steps (with evaporation of the mobile phase from the plates after each step) by using the same solvent or a series of solvents for modified IMD technique. In gradient IMD, the eluent strength is rednced stepwise. 

Solvent Eluent strength Dielectric constant (Debye) Viscosity at20"C (cP) Refraictive inde x at 20"C Cut-off wavelength (nm)... 

Eluent strength, 4, 70, 169, 173, 181 adjustment of, 73-77 calculation of, 87 of commonly used solvents, 72 dependence on composition, 73-77 effect on retention, 102 modulation of, 39 of mixed eluents, 73-77 of neat eluents, 72 optimization, 87... 

Solvent properties, 16S-166 Solvent strength parameter, 175, 176 Solvent strength, 73 see also Eluent strength... 

The concept of entropy-enthalpy compensation resulting in the critical conditions of enthalpic interactions and the molar mass independent sample retention turned out useful also for the understanding several other coupled methods of polymer HPLC. It is accepted [195,196] that the polymer species tend to elute at the critical conditions also when either eluent strength or quality change within the HPLC system in the course of the HPLC experiment that is in the continuous and local gradient methods (Sections 16.5.3, 16.5.4, and 16.5.6). Irrespective of the problems and limitations of LC CC, its concept belongs to the important breakthroughs in polymer HPLC. 

The identification of critical conditions for the given polymer-column packing system needs series of preliminary experiments. In the most practical LC CC experiments, the eluent strength or quality is adjusted tentatively to attain the molar mass independent elution at constant temperature. 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

Figure 12.9 Chromatogram of a complex mixture (a) using the same eluent strength, and (b) using a gradient to desorb the most retained product (marked with a ), reducing its retention times.

While pressure has no effect on the eluent strength when liquid solvents are used, under supercrihcal conditions the solvent density increases greatly with increased pressure. This offers an additional degree of freedom when selecting the chromatographic conditions of the process. The impact of the column back pressure is illustrated in Figure 12.21. The impact of the pressure on retention... 

Figure 25-1 ] Solvent molecules compete with solute molecules for binding sites on the stationary phase, the greater the eluent strength of the solvent, the more easily it displaces the solute.

Adsorption chromatography on bare silica is an example of normal-phase chromatography, in which we use a polar stationary phase and a less polar solvent. A more polar solvent has a higher eluent strength. Reversed-phase chromatography is the more common... 

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