Solvents isoeluotropic mixtures

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. 

The composition of isoeluotropic solvents is simply calculated using the additive nature of the volume fractions and their respective solvent weighting factors. If, the solvent strength of the methanol-water (60 40) mixture was about optimum for a particular separation but not its selectivity, the band spacing (resolution) could be further optimized by using isoeluotropic mixtures containing water-acetonitrile and water-tetrahydrofuran. The composition of these mixtures is calculated as indicated for acetonitrile-water... 

Optimization of the chromatographic process with aid of the Snyder concept of solvent polarity and selectivity in fact means optimization of the separation selectivity. This goal can be attained with help of the so-called isoeluotropic mixtures, i.e. of the mixed eluents which in spite of composition different from that of the original eluent preserve equal elution strength. 

Let us consider the adsorption and the normal-phase partition chromatography systems employing binary mobile phases composed of the solvents A and B (with the nonpolar solvent A, for which P 0). If we want to change the separation selectivity of this system, the simplest way is to employ the isoeluotropic mixture in which solvent B is replaced by solvent C. The volume fraction of solvent C can be estimated from the relationship ... 

The data in Table 5-1 also demonstrate several other behaviors typical of reverse-phase chromatography. First, the elution order of the sample components is from the most polar, eluting first, to the most nonpolar, eluting last. Second, as the eluent is made more polar (more water), the sample compounds are less soluble in the mobile phase, and hence more soluble in the stationary phase. Therefore, retention is increased. The third behavior is that for every compound and stationary phase combination, a variety of solvent mixtures can be used as mobile phases to elute a compound with approximately the same k value. Mobile phases that have identical eluent strengths are referred to as isoeluotropic or equieluotropic for that compound. The general trend is for similar retention to exist in isoeluotropic mobile phases. However, isoeluotropic mobile phases sometimes have a different selectivity (a) for a pair of compounds due to secondary interactions between the solvent and analyte. 

Several mobile-phase optimization strategies in TLC are based on the use of isoeluotropic solvents (i.e., solvent mixtures of identical strengths but different selectivities). Selecting mobile phase will be achieved based on the eluotropic series (Figs. 1 and 2). These considerations are... 

Alternatively, the following empirical relationships can be used to estimate the equivalent solvent composition of isoeluotropic binary solvent mixtures given that experimental conditions have been established using methanol as the organic modifier in reversed-phase liquid chromatography... 

If we want to change selectivity of the mixed mobile phase in the reversed-phase partition chromatography mode, we also need to employ the isoeluotropic mobile pha.se. The common strategy is to replace the less polar component of the starting binary mixture by another solvent. E.g., if the starting eluent was the methanol-water mixture, methanol has to be replaced by acetonitrile or tetrahydrofuran. Then the volume fraction of the new solvent can be estimated from the relationship ... 

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