Solvent strength calculation

Solvent optimization in reversed-phase liquid chromatography is commenced by selecting a binary mobile phase of the correct solvent strength to elute the seuaple with an acceptable range of capacity. factor values (1 < k <10 in general or 1 < k < 20 when a larger separation capacity is required). Transfer rules (section 4.6.1) are then used to calculate the composition of other isoeluotropic binary solvents with complementary selectivity. In practice, methanol, acetonitrile and tetrahydrofuran are chosen as the selectivity adjusting solvents blended in different... 

The solvent strength for any solvent mixture can be calculated from this equation ... 

The linear solvent strength (LSS) model combined with QSRR calculations has been applied for the prediction of retention in gradient RP-HPLC. It was established that total dipole moment (jd), electron excess charge of the most negatively charged atom (<5Mm) and water-accessible molecular surface area (Awas) exert the highest impact on the retention ... 

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. 

In gradient-elution NPC, the solvent strength of the mobile phase gradually increases with increasing concentration of the polar solvent. In NP systems where Equation 5.10 applies nnder iso-cratic conditions, the elution volnme of a sample solnte in NP LC with linear gradients described by Equation 5.4 can be calculated using [28] ... 

The most important features of this equation are the emergence of a eluotropic series, i.e., an eluent-strength series based on e°. The key solvent-strength parameter, e°, which relates to the eluotropic series, was calculated by Snyder (14) for a wide variety of pure solvents. Table 2 lists some values. 

The local density of solvent about the solute may be determined by comparing the experimental and calculated curves. Consider points A and B in Figure 5 at a constant value of E, i.e., 55 kcal/mol. A hypothetical homogeneous fluid at point B gives the same "solvent strength" as he actual fluid at point A. The local density about the solute exceeds the bulk density due to compression, such that... 

The optimization procedure is similar to that used in reverse phase. The concentration of methylene chloride in hexane is adjusted to get partition ratios in the range of 1 to 10. The solvent strength e° of this solution is calculated and used in preparing the other solvent mixtures to the same value. Consequently, all mixtures used in this process will have the same... 

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) ... 

The present model predicts how solvent selectivity will vary with mobile-phase composition, and this allows the selection of extreme solvents for maximum differences in selectivity. This information plus the ability to calculate solvent strength versus composition of the mobile phase then allows development of a general strategy for optimizing retention of any sample, so as to maximize resolution. This four-solvent approach can be further refined by use of computer-assisted procedures, such as the overlapping-resolution-mapping technique. 

These deficiencies were addressed by revising Snyder s model as follows [8]. To account for the preferential adsorption of solute and solvent onto the strong sites, empirical As and Ne values larger than those calculated from molecular dimensions are used based on experimental observation. The revised model acknowledges the tendency of polar molecules to localize on the strongly adsorbing active site and expresses solute retention in terms of the solvent strength as follows ... 

The solvent strength parameter for a binary mobile phase, e°a,b, may be calculated from the equation (Snyder, 1968)... 

There are a number of ways of calculating overall solvent strength in order to give isoeluotropic mixtures. One method is to express the strength as the product of the sum of the strengths of the component solvents with their volume fractions, i.e. 

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