Solvent strength estimation

The practical consequence of linear solvent strength theory is that, in principle, it is possible to estimate the retention times of all peaks at any... 

The selection of a solvent for a new separation problem, even today, is a matter of trial and error. The application of theory (2) with the additional application of the solubility parameters (6J-65) makes it possible to estimate the composition of appropriate solvent mixtures for the separation of relatively simple compounds. In order to calculate the necessary solvent strength, however, a set of experimental data concerning the behavior of the sample components, the adsorbent, and the elution strength of the eluents with the specific adsorbent are necessary. Others (J5) recommend a graphical method as a time-saving alternative to bi th calculation and the trial-and-error approach to obtain a first approximation of the eluent composition appropriate for the separation of a givin sample. 

Eqnation 16.10 holds for mixed eluents A plus B with not very low concentration of stronger component B. The role of intermolecular interactions between eluent components A and B is not considered. The solvent strength of a binary elnent can be roughly estimated also from the relation... 

A simple way to estimate the appropriate isocratic conditions from the result of a gradient elution chromatogram is provided by the theory of linear solvent strength (LSS) gradients of Snyder (for a review, see ref. [528] or [527]). By definition, an LSS gradient obeys the following relationship ... 

In Figure 5-50, each horizontal line corresponds to a full range (0-100% v/v) of binary solvent mixtures. The first five lines represent mixtures of pentane with other solvents (line 1, pentane with isopropyl chloride line 2, pentane with methylene chloride line 3, pentane with ethyl ether line 4, pentane with acetonitrile and line 5, pentane with methanol). For the top line of this series (mixtures of pentane and isopropyl chloride) any solvent strength intermediate between pentane (e° = 0) and isopropyl chloride (e° = 0.22), can be estimated by reading from a vertical line dropped from the e° scale. Thus, e° = 0.10 is obtained by a 26% v/v isopropyl chloride in pentane and e° = 0.20 is obtained by 80% v/v isopropyl chloride in pentane. The... 

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) ... 

As an initial approximation of the ideal solvent strength for HPLC, the results of TLC studies on acetylated cellulosic plates with various methanol-water mixtures and natural dye extractions were graphed (R/ vs. methanol concentration). An acetylated cellulosic TLC system is not directly comparable to a C-18 HPLC system, but TLC results were nonetheless useful as a rough estimate of solvent strength. In practice, approximately two-thirds of the solvent strength required to elute natural dyes with TLC was necessary to achieve a similar separation of major sample components with HPLC. After individual solvent concentrations were determined, samples of known dyes extracted from wool were eluted in each of the three pairs of solvents (the concentration used for each individual solvent-water system was reduced by one-half) and adjustments were made until each sample eluted with a kf value no larger than 10. Retention times and kf values are equivalent expressions of relative retention of a sample on the column if the flow rate is the same for all trials. With a flow rate of 1.4 mL/min, the maximum time... 

Eq. 10 can be useful in estimating the effect on sample Rf values of a change in solvent strength. 

Equations (8-6a)-(8-6c) in conjunction with Table 8-1 permit the estimation of solvent strength values for a large number of solvents on the adsorbents silica, Florisil, and magnesia. The standard deviation of the ... 

Calculation of AAe Values for Estimation of Binary Solvent Strength... 

Values of (the solvent strength parameter) for pure solvents on alumina can be obtained from Table 8-1. Similar values for adsorption on silica or other adsorbents are given in Tables 8-2 and 8-3, or can be estimated from e values for alumina through Eqs. (8-6a)-(8-6c). Values of e for a few binary solvents are listed in Appendix III. Other values can be calculated through Eq. (8-10) ... 

A RAPID PROCEDURE FOR ESTIMATING THE SOLVENT STRENGTHS OF A SERIES OF BINARY MIXTURES. TABULATED VALUES FOR SEVERAL BINARY SOLVENT SYSTEMS... 

Estimation of Binary Solvent Strength from e and Binary Composition... 

PROCEDURE FOR ESTIMATING SOLVENT STRENGTHS Table III-3 (Continued)... 

Estimation of the optimum solvent strength for isocratic separations from gradient elution... 

The solvent strength of binary solvents providing adequate retention can also be estimated from a series of sequential isocratic chromatograms of decreasing solvent... 

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