Isoeluotropic solvents

Fig. 6.7. (a) k values for four analytes in two isoeluotropic solvents A and B with respectively 0% and 100% of A. t l is the level at which the best result is expected, (b) The experimental results at level I l are included, i2 is the level at which the best results arc now expected. 

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... 

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... 

For separating the pyrethrins and synergists, sifica gel layers (standard method, chamber saturation) and various so-called chloro-form-isoeluotropic solvents have been employed for development 1. benzene-butanone (90+10) 2. benzene-ethyl acetate (85+15) 3. carbon tetrachloride-ethyl acetate (80 + 20) 4. hexane-butanone (80 + 20) 5. hexane-ethyl acetate (75 + 25). 

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

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 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. 

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. 

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... 

The isoeluotropic series obtained for quinoline on various adsorbents is developed with solvents from different selectivity groups (Table 4). It is clearly seen that the highest ARm values were obtained for quinoline bases on Horisil with dichloromethane, 2-propanol, and other modifiers, which proves highest selectivity of separation in this system. 

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