Eluotropic series binary

Equation 4.13 and Equation 4.14 were tested for a series of mobile phases on alumina [29-31] and silica gel [32]. Two eluotropic series of solvent binary mixtures for alumina (a = 0.6) and silica gel (a = 0.7) have been calculated by using Equation 4.13, and the obtained data can be used to establish many such series or series of other selectivities [13,28],... 

Based on the chromatographic profiles observed on both plates, suitable binary solvent mixtures are chosen. Using another TLC plate, an eluotropic series of these solvents is prepared and tested. The start and end composition of this eluotropic series depends on the R values measured for the strongest solvent of the mixture. 

The calculation of r" values for a large number of binary solvents is occasionally required for gradient elution systems or for the prediction of an extended eluotropic series of binary solvents. In these cases the application of Eq. (8-10) can be tedious. Appendix III describes an approximate treatment which is essentially as accurate as Eq. (8-10), but which is much simpler and easier to apply in the case of a series of solvents A-B of varying composition. 

Binary Solvent Eluotropic Series for Adsorption on Water-Deactivated Alumina (a = 0.6) Calculated Values from Eq. (8-10)... 

Whereas the e" values of pure solvents do not change with adsorbent activity, the relative strength of a binary solvent is a function of a [see Eq. (8-10)]. As a increases, the e" value of a binary solvent increases. For example, the experimental e value of lOvol.% benzene-pentane is 0.066 on 16% HaO-SiO . (small pore), increasing to 0.098 for 1.6% Hc O-SiOa (29). When the activity of an adsorbent is unknown, some uncertainty will exist concerning the relative strengths of a series of pure and binary solvents. Similarly, it is not possible to define an eluotropic series of both pure and binary solvents that will be precisely applicable for all adsorbent activities. 

Binary Solvent Eluotropic Series for Adsorption on Water-Deactivated... 

Several mobile-phase optimization strategies in TLC are based on the use of isoeluotropic solvents (i.e., solvent mixmres of identical strengths but different selectivities). Selecting mobile phase will be achieved based on the eluotropic series (Figs. 1 and 2). These considerations are very general. The selection and optimization of one system of eluent is a more complex problem and must be discussed for each particular system. For example, the separation of 13 phenylurea and s-triazine herbicides was performed by overpressured layer chromatography (OPLC) with a binary... 

A classification of solvents to their ability to adsorb on the stationary phase is called an eluotropic series (Table III). Retention of solute is reduced with mobile phase solvents of higher solvent strength parameter, . Solvent polarity parameters ((f)) are similar to e values. They can be used to estimate the overall polarity of a binary solvent as = ( a PX + Pg For example, (p values for... 

Figure 3.20 Nomogram illustrating the solvent strength of various binary solvent mixtures for LSC. Vertical (dashed) line illustrates a series of iso-eluotropic solvents (see text). Figure taken from ref. [358]. Reprinted with permission.

The points on the sides of the triangle (A, B, and C) represent three iso-eluotropic binary mixtures (solvents A, B and C). The composition of one of the three binary mixtures (i.e. the appropriate eluotropic strength) should be determined by either a scanning gradient or a stepwise series of isocratic scans (see section 5.4). Once one of the compositions is known, the compositions of the two iso-eluotropic binary mixtures can be calculated using the conversion factors given in table 5.4b. 

Fig. 126. Arrangement of solvents and their binary mixtures in an equieluotropic series based on the average /-values of 20 steroids. Vertical lines join solvent systems of approximately equal eluotropic properties. The first division of the horizontal mixture lines, e.g., cyclohexane-ethyl acetate, from the left corresponds to 99% cyclohexane and 1% ethyl acetate the first from the right, 10% cyclohexane and 90% ethyl acetate etc. See also text [129, 130]...

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