Mobile phase selectivity triangle

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. 

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. 

Application of a computer program to these data of Fig. 24 yielded calculated values of a for every possible solute pair in the seven mobile phases used. The program then interpolated these data over the entire compositional triangle by fitting to a quadratic curve, to yield values of a as a function of mobile-phase composition. Finally, these a values are plotted in trilinear form (Fig. 25) in such a manner as to indicate mobile-phase compositions of optimum selectivity. Figure 25a shows such a plot for band pair 6-8, where the white region indicates resolution of the two bands (on one 25-cm silica column) greater than the minimum desired Rs > 1.0). 

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).

Once the different solvents have been selected, a first experiment is done using the mobile phase composition corresponding to the centre of the cover triangle (1/3 solvent A, 1/3 solvent B and 1/3 solvent C). If the observed Rf values are too high, additional experiments are performed to adjust the solvent strength by the addition of zero strength solvent. 

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.

Within the last several years HPLC separations have been optimized in terms of the most appropriate mobile phase composition for a particular set of solutes by exploring the whole plane of solvent selectivities using this solvent classification scheme with a minimal number of measurements in statistically-designed experiments. For reversed phase HPLC systems, the selectivity triangle is often defined by methanol, acetonitrile, and tetrahydrofuran with water as the diluent (37). 

Therefore, in liquid-solid chromatographic systems selectivity in a separation is determined by the mobile phase solvent strength and interaction with the adsorbent. The general selectivity triangle... 

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. 

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... 

Figure 6.50 Mobile phase optimisation/selectivity triangle. The comers are isoelutropic mobile phases chosen to provide different selectivity. Intermediate points are mixtures of these binary eluants in the indicated proportions.

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