Quaternary mobile phases

The Window diagram method is seldom used for ternary or quaternary mobile phases because of a large variety of intermoleeular interaetions that appear in... 

R-P separation of alkaloids Quaternary mobile phase composition and flow rate 81)... 

The same group demonstrated the use of the step gradient to improve speed of the CEC analysis of DNA adducts derived from syn benzo[g]chrysene-11,12-dihydrodiol-13,14-epoxide. They used a three-step gradient, which started with a quaternary mobile phase followed by two ternary mobile phases. The analysis was completed in... 

A series of secondary parameters may be exploited. Changing the nature of the organic modifier is the most common and probably the most rewarding parameter to use. If ternary and quaternary mobile phases are considered, then the ratio between the concentrations of different modifiers becomes a continuous parameter that may be optimized. 

Figure 5.2 (a) Pseudo-isometric three-dimensional response and (b) iso-response contour plot for a two-parameter optimization problem. Parameters (in triangular representation) quaternary mobile phase composition. Criterion normalized resolution product (see section 4.3.2). O, is the location of the optimum. For further details see section 5.5.2. Figure taken from ref. [502]. Reprinted with permission. 

The second approach is the one followed by Drouen et al. [502]. It is based on the experience that only in very few cases does the optimization of a quaternary mobile phase composition in RPLC yield an optimum that is truly quaternary, i.e. contains all four solvents. Hence, the procedure discussed before for ternary solvents usually leads to the global optimum. This argument, correct though it may be, only applies to the particular problem of mobile phase optimization in RPLC, and prohibits the application of the same method to other two-parameter optimization problems [582]. 

Binary mixtures, however, have only limited abilities for controlling mobile-phase selectivity. Therefore, ternary and even quaternary mobile phases that contain two or more different polar solvents along with a nonpolar solvent are often used to achieve the required selectivity. If the ratio of the concentration of two polar solvents is constant but the sum of the their concentration is being changed with respect to that of the nonpolar solvent, the effect on retention is much the same as when the concentration of the single strong solvent... 

The mobile phase in RPC contains water and one or more water-soluble organic solvents. The most useful are, in order of decreasing polarities, acetonitrile, methanol, dioxane, tetrahydrofuran and propanol. By the choice of the type of the organic solvent, selective polar interactions, dipole-dipole, proton-donor or proton-acceptor, with analytes can be either enhanced or suppressed and the selectivity of the separation adjusted. For simplicity, binary mobile pha.ses are used more often than those containing more than one organic solvent in water, as they often make possible an adequate separation of various samples. However, ternary or less often quaternary mobile phases offer advantage of fine-tuning the optimum selectivity of more difficult separations. This is discussed in more detail in Section 1.4.6. 

Fig. 1.18. Examples of chromatographic separation of a ihree-componcnt sample mixture and possible ways lo improve the separation during HPLC melhtxl development. tA) Satisfactory separation. (B) Unsatisfactory separation — ttw low retention. The elution strength of the mobile phase should be decreased. (C) Good resolution, but too long time of separation. The elution strength of the mobile phase should be increased. (D) Unsatisfactory separation — too low column efficiency. The plate number should be increased by using finer packing panicles or a longer column. (E) Unsatisfactory separation — gixxl retention and column efficiency, but too low separation selectivity. The components of the mobile phase can be changed, a ternary or a quaternary mobile phase, selective mobile phase additives, or another type of the stationary phase can be used.

The use of optimization methods makes it possible to separate complex mixtures in a short analysis time with binary, ternary, or quaternary mobile phases. Only procedures concerning the optimization of selectivity are described in this section. 

Another approach in taxoid preparative separation included solid-phase extraction (alumina, silica, or RP-8 cartridges) followed by preparative TLC on silica gel plates with quaternary mobile phase consisting of dichloromethane-dioxane-acetone-methanol (83 5 10 2, v/v). In this way, 10-DAB III, paclitaxel, and cephalomannine as well as two further taxoids could be easily isolated with relatively high efficiencies from yew materials (Fig. 2). Multiple development technique or fiuther separation of the isolated taxoid fractions (especially less polar ones) on RP-2 silica bond stationary phase with methanol-water mixtures as mobile phases was applied for purification of the compounds isolated. 10-DAB III isolated in this way was relatively pure, as was shown in reversed-phase (RP)-HPLC analysis (Fig. 3). 

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