Mobile-phase composition, influencing shape

Mobile phases are usually binary or ternary mixtures of solvents. Selectivity is affected mostly by mobile phase composition rather than strength, and peak shape and retention are both influenced by the addition of organic modifiers.101 Some compounds naturally have 77-donor or 77-acceptor groups and can be resolved directly. In many cases, however, introduction of 77-donating groups by derivatization steps is necessary. Figure 2.20 shows the proposed three-point interaction of 3-aminobenzo[a]pyrene, a polycyclic aromatic hydrocarbon (PAH), with a Pirkle-type stationary phase.111 Two possible interactions are illustrated, showing the best orientations for maximum interaction. 

The mobile-phase composition in chromatography is important because (1) it determines the thermodynamic properties of the separation, and (2) it may effect the mobile-phase viscosity and thus the operating pressure. The thermodynamic properties are characterized by the adsorption isotherm. The adsorption isotherm directly influences the shape and rate of migration of the individual components of the feed and thus the degree of separation. How strongly the mobile-phase composition effects the adsorption isotherm needs to be ascertained for each separation problem experimentally. This can be done by measuring the individual band profiles at various compositions or by measuring the... 

Resolution of the three HBCD diastereomers is easily achieved by reversed-phase LC on C18 columns, often in less than 15 min. The usual elution order for the diastereomers is a-, P-, y-HBCD however, an alternate elution order (i.e., a-, y-, P-HBCD) was reported by Dodder et al. [Ill] for shape-selective columns operated with methanolic mobile phases (see Figure 13.8). The reversed elution order for P-HBCD and y-HBCD provides the basis for the development of orthogonal methods that may help eliminate matrix interferences. Yu et al. [112] also studied the influence of mobile phase composition on HBCD diastereomer selectivity and show that MS sensitivity was also affected by this parameter. 

It will be clear from figure 6.7 that the nature of the mobile phase (compare figures 6.7a and 6.7b) and the stationary phase (compare figure 6.7c with figures 6.7a and 6.7b) have a great effect on the character of the retention vs. composition plots and hence on the shape of the required (optimum) gradient. It will also be clear that, unlike the situation in GC, the selectivity may be greatly influenced by variations in the mobile phase. 

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