Solvents 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. 

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... 

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. 

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 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.

If these parameters are used for the construction of a diagram as shown in Figure 5.1, a solvent selectivity triangle is obtained which clearly shows the differences between the individual solvents with regard to their dipolar (re ), acidic (a) and basic (/3) properties.2 The largest differences in the elution pattern can be expected if solvents are chosen which are as far apart from each other as possible. Because mixtures of two solvents, A and B, are used in most cases, only such solvents can be chosen which are miscible with each other. The usual A solvent in normal-phase separations is hexane, in reversed-phase separations it is water. Therefore the possible B solvents are limited in number. With regard to selectivity, it makes no real sense to try a normal-phase separation with diethyl ether as well as with tert, butyl methyl ether because all aliphatic ethers are located at the same spot in the selectivity triangle. Likewise it is not necessary to try several aliphatic alcohols for reversed-phase separations. 

Basicity is one of the axes of the solvent selectivity triangle of Figure 5.1. The most basic of the common HPLC solvents are the ethers. 

The literature of QSRR with LSS is dominated by a specific SSD, the I ER solute parameters V, E, S, A, and B, as defined in Equation 15.2. An extraordinary amount of attention has been paid to predict retention (24,25) and to establish phase selectivity in MEKC using LSER (5, 7, 26-31). Attempts to classify and to contrast micellar phases with basis on the LSER coefficients have been pursued by many researchers (5,26,27,29). Interesting approaches comprise the classification of micellar phases by the combined use of LSER parameters and retention indexes (32), the clustering of micellar systems by principal component analysis (26), the use of LSER parameters to compose vectors for characterization of lipophilicity scales (33), and, more recently, the establishment of micellar selectivity triangles (34,35) in analogy to the solvent selectivity triangle introduced by Snyder to classify solvents and ultimately mobile phases in liquid chromatography. 

Fig. 18 Snyder solvent selectivity triangle for solvents of Table 3.

Solvents can be grouped on the basis of their properties as proton donors (acidic), proton acceptors (basic), and dipole interactions. Solvents can be positioned within the Solvent Selectivity Triangle (Fig. 17) on the basis of the relative involvement of each of these three factors as parameters of solubility. Mobile phases consisting of a mixture of three solvents can be optimized by... 

Several mobile phase optimization strategies in liquid chromatography are based on the use of isoeluotropic solvents, that is, solvent mixtures of identical strength but different selectivity. Suitable binary (and higher order) solvent mixtures are usually selected based on their location in the solvent selectivity triangle (Table 4.14). To... 

Figure 6.16. Solvent-selectivity triangle based on normalized solvatochromic parameters for some common water miscible organic solvents. TEA = triethylamine, THF = tetrahydrofiiran, DMF = N,N-dimethyl-formamide ISP = 2-propanol MeOH = methanol ACN = acetonitrile and TFE = 2,2,2-trifluoroethanol. (From ref. [95] Research Instimte for Medicinal Plants).

FIGURE 28-17 Solvent selectivity triangle lor optimization of reversed-phase separati<5ns. Ten mixtures of the three organic solvents (methanol, acetonitrile, and tetrahydro-furan) are shown with the relative proportions indicated in parentheses (MeOH, MeCN, and THF). Water is used to maintain solvent strength and keep the k value within an appropriate range,... 

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