Snyder s solvent 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 7 Snyder s solvent selectivity triangle. Solvent are classified in eight groups (I-VIII), according to the proton acceptor (Xe), proton donor (Xj), and strong dipole (X,i) contribution.

A chemometric approach where the /ty-values of forty-seven flavonoids in seven TLC systems were studied using principal component and cluster analyses, has made it possible to choose the minimum number of chromatographic systems needed to perform the best separation (20). Another method (the PRISMA model) based on Snyder s solvent selectivity triangle has been described to aid mobile phase optimization (21). This model is reported to give good separation of flavonol glycosides from Betula spp. (1). When tested in our laboratory no improvements were obtained in comparison with established systems (22) such as the solvent ethyl acetate-formic acid-acetic acid-water (100 11 11 27) on silica support, which can be used for separation of a wide range of flavonoids. 

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. 

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

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

To maximize the differences in selectivity, solvents must be selected from different selectivity groups that are situated close to the Snyder s triangle apexes. For example, for NPTLC, a suitable selection could be solvents from several groups of Snyder s classifications (I, VII, and VIII), mixed with hexane to control solvent strength. 

Solvent strength and selectivity parameters based on Snyder s selectivity triangle. (Si is an empirical solvent strength parameter for reversed-phase chromatography)... 

Rutan, S.C. Carr, P.W. Cheong, W.J. Park, J.H. Snyder, L.R. Re-evaluation of the solvent triangle and comparison to solvatochromic based scales of solvent strength and selectivity. J. Chromatogr. 1989, 463, 21-37. 

---