Selectivity triangle

In Figure 4.9 Snyder s solvent selectivity triangle is presented. The solvents of Table 4.2 are marked in the plot with triangular coordinates for the eight groups. 

Figure 4.28 Selectivity triangle for solvents. (Reproduced with permission from ref. 517. Copyright Preston Publications, Inc.)...

The Snyder-solvent-selectivity triangle concept can be combined with a mixture-design statistical technique to define the optimum mobile-phase composition for a particular separation A feature of this mixture-design... 

There are several systems which can be used to select the solvents of the mobile phase. The number of selected solvents and the solvents which are selected not only depend on the chromatographic problem but also on the method which will be used to optimize the system. With response surface methodology it is appropriate to use a minimum number of solvents. For reasons stated below this minimum number of solvents was four. The second question is, which solvents will be selected, is more difficult to answer when a small number of solvents is used because the consequences of a wrong selection are large. Several approaches are possible to select the solvents. The most simple method is comparison with common solvent systems for the solutes under investigation. A more general approach is to use the selectivity triangle of Snyder [4] in the selection of the solvents. 

Solvents used here for a general liquid-liquid extraction method were selected from Snyders solvent selectivity triangle. As extraction liquids have to be composed of mixtures of three solvents which may enter into maximum interaction with the analyte, three solvents had to be selected that represent a wide variety of selective interactions. In addition, the solvents should be sufficiently polar to ensure quantitative extraction. Besides selectivity and polarity requirements, the solvents should also meet a few other criteria, mainly for practical reasons they should not be miscible with water, have low boiling points (for relatively fast evaporation procedures) and have densities sufficiently different from the density of water, for pure solvents as well as for selected binary or ternary mixtures of solvents. 

In GC, the mobile phase acts only as a carrier. In LC, solute undergoes interaction with liquids or mixtures of liquids used as the mobile phase. Selection of the mobile phase is critical. The most useful criteria are the solubility parameter concept, Snyder s selectivity triangle, and solva-tochromic parameters. 

Figure 5.23 Illustration of preferred modifiers for NPLC (dashed triangle) and for RPLC (dashed and dotted triangle) in the Snyder selectivity triangle (see section 2.3.3). Figure taken from ref. [542). Reprinted with permission.

The separation and identification of natural dyes from wool fibers using reverse-phase high-performance liquid chromotog-raphy (HPLC) were performed on a C-18 column. Two isocratic four-solvent systems were developed on the basis of the Snyder solvent-selectivity triangle concept (1) 10% acetonitrile, 4% alcohol, and 2% tetrahydrofuran in 0.01 M acetic acid and (2)7% acetonitrile, 8% alcohol, and 5% tetrahydrofuran in 0.01 M acetic acid. Samples were also eluted in 30% acetonitrile. Spot tests and thin-layer chromatography were performed on all samples to confirm HPLC results. The systems also were found to be potentially useful in the identification of early synthetic dyes. A system of sample preparation that minimizes the reaction of samples was discussed. The application of this HPLC separation technique to samples from 20th century Caucasian rugs and American samples unearthed from the foundation of Mission San Jose was examined. 

The technique used to develop the four-solvent systems was based on procedures elucidated by Lehrer (6), Rohrschneider (7), and Glajch (8). After trials with individual solvents chosen from the comers of the Snyder solvent-selectivity triangle—a system of classification of solvents by the degree to which they function as proton donors, proton acceptors, or dipole interactors—an ideal solvent system was calculated. Ethanol, acetonitrile, and tetrahydrofuran were the reverse-phase solvents used, and water was the carrier solvent. Once the ideal solvent strength of one solvent-water combination was empirically determined, that of the other combinations could be estimated by use of the following equation (9) ... 

Fig. 23. The solvent selectivity triangle for separations on silica LD, localized dipole LB, localized base, (a) general form (b) seven compositions for retention optimization. Reprinted with permission from Glajch el a . (20),...

Figure 5-2. Snyder s selectivity triangle for solvents. (Reprinted from reference 9, with permission.)...

Fig. 1.19. Selectivity triangle for three- and four-componeni mobile phases in reversed-phase HPLC MeOH = methanol (predominant proton-donor interactions) ACN = acetonitrile (predominant dipole-dipole interaaions) THF = tetrahydrofuran (predominant proton-acceptor interactions).

Figure 9 Solvent selectivity triangle approach forthe selectivity optimization in HP-RPC. First, three initial experiments (1-3) with three mobile phases (binary mixtures of ACN/water, MeOH/water, and THF/water, respectively) are performed.

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