Solvent triangle

Solvent triangle for optimizing reverse-phase HPLC separations. Binary and ternary mixtures contain equal volumes of each of the aqueous mobile phases making up the vertices of the triangle. 

However, not withstanding the above objections, further discussion of the Snyder solvent triangle classification method is justified by its common use in many solvent optimization schemes in liquid chromatography. The polarity index, P, is given by the sum of the logarithms of the polar distribution constants for ethanol, dioxane and nltromethane and the selectivity parameters, X, as the ratio of the polar distribution constant for solute i to... 

Isohydric 385 localizing 378 mixed solvents strength 465 isoeluotropic 467 properties 460 solubility parameters 460 solvatochromic parameters 461 solvent triangle classification 463 types 458... 

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... 

Carry out multiple experiments based on solvent triangle... 

Another approach, which is often used in combination with modelling methods, is the so-called threshold method. In many cases, it is possible to define for each response a part of the experimental domain where the response is at least adequate. By combining these areas, one may then find a set of conditions where all responses are adequate. This is for instance the approach followed when the solvent triangle (see Section 6.5) is applied for multicomponent separations. The quality of the separation of each successive pair of substances in the chromatogram is determined. A limit is imposed (e.g. resolution should be at least 1.5) and for each pair the area in the triangle is obtained where this is the case. The part of the triangle where the criterion is not reached is shaded and this is done for each pair (see Fig. 6.4). The triangles for each binary separation are then superimposed and the area, which stays blank after this operation, is the one where all separations yield acceptable results. 

The solvent triangle approach can be applied in two ways. One way is of course to determine all seven chromatograms, to compute the model using Eq. (6.16), the isoresponse curves and to apply the threshold principle of Section 6.2, i.e. what we called earlier a simultaneous approach. The resulting map is called the overlapping... 

Figure 10.4 Solvent triangle for reversed-phase chromatography.

For normal-phase separations, a similar solvent triangle is used in which the selectivity solvents are ethyl ether, methylene chloride, and chloroform -" the solvent strength adjustment is then made with n-hcxanc. With these solvent systems, optimization can be accomplished with a minimal number of experiments. 

Figure 2. Representative turbulent intensity distributions. Closed circles solvent. Triangles Re = 7,200. Open circles Re = 11,000. Dash-dot Re = 9,500.- 35,000. Solid curve Re = 20,000 - 52,000.

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. 

Figure 28-16 illustrates the systematic, solvent-triangle approach to the development of a separation of six steroids by reversed-phase chromatography. The first two chromatograms show the results from initial experiments to determine the minimal value of k required. Here, the last eluting component is used to calculate k. With k for component 6 equal to 10, room exists on the time scale for discrete peaks here, however, the a value for components I and 3 (and to a lesser extent 5 and 6) is not great enough for satisfactory resolution. Further experiments were then performed to... 

A second means of obtaining Is from the sum of If k is really the only viscosity sensitive rate constant, then the kQ+ks sum should be constant in all solvents and equal to the value obtained by extrapolation. This was not the case for either the perbenzoate ( ) or the peracetate (3) Figure 2 also shows the Pp values from ko/kg which takes into account the variation of both k and kg with solvent (triangles). The descrepancy the two means of estimating k is very large and negates any conclusions concerning the N2 effect. 

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