Elution power

When the solvent has nearly reached the top of the adsorbent layer, the components should be well separated. The relative distance travelled by the components can be increased by using a solvent of higher or lower polarity in the order of increasing eluting power ... 

The column is eluted with dry solvents of gradually increasing eluting power. The order of eluting power of the common dry solvents is shown in Table A3.5. The compounds are eluted from the column in order of their increasing polarity. The usual order of elution of organic compounds is shown in Table A3.6, The progress of the... 

Silica gel or alumina is usually used as the stationary phase. For these active phases a solvent of weak elution power is recommended for dissolution of the sample. A... 

These aspects of solvent property similarly apply to precoated impregnated silica gel plates, e.g., by ammonium sulfate, silver nitrate, or magnesium acetate, as well as to microcrystalline cellulose precoated plates. On preparative RP phases, water has the lowest elution power. Therefore, more polar or aqueous solvents should be preferred. In contrast to HPTLC RP-18 layers, on which such aqueous solutions remain as a drop on the surface and are not able to penetrate through the lipophilic layer, on preparative RP phases, pnre aqneons application solutions can be apphed owing to the minor degree of C18 modification. 

Solvents for prewashing should be of high purity and eluting power to be able to desorb impurities from the layer otherwise prewashing is useless. The following techniques for prewashing of layers can be employed. 

The use of extraction cartridges in the separation of azines, discussed in the last Section, is an example of on-column concentration using off-line column switching. A chromatogram can be cut off-line by collecting the zones of interest at the detector outlet followed by reinjection of the collected fraction onto a secondary column. The mobile phases used with the two columns should be compatible, eg they should be miscible and the mobile phase used with the first column should not have too high an eluting power in the second column. If the mobile phases are incompatible it may be possible to evaporate the primary mobile phase and redissolve the sample in a suitable solvent. 

The eluting power of a solvent is determined by its overall polarity, the polarity of the stationary phase and the nature of the sample components. Table 4.9 lists some widely used solvents in order of their eluting power, this being known as an eluotropic series. In practice, better separations are achieved with the least polar solvent possible and mixtures of solvents are often used to achieve optimum separation conditions. 

The choice of solvent or a mixture of solvents used in TLC is solely guided by two important factors (a) the nature of the constituent to be separated i.e., whether it is polar or non-polar and (b) the nature of the process involved i.e., whether it is a case of adsorption or partition chromatography . It has been observed that the rate of migration of a substance on a given adsorbent depends upon the solvent used therefore, the latter may be arranged in order of the elutive power, usually termed as the elutropic series as shown in the following Table 28.1. 

The analyte is dead stopped on the SPE medium by loading it onto the cartridge in a solvent of low eluting power. It may then be washed with other solvents of low eluting power and is then finally eluted with a small volume of a strong solvent. 

In order to be successful with only four solvents it is necessary to use solvents which have all different properties. The most important properties are solvent strength and solvent selectivity. The solvent strength is the overall elution power. The solvent selectivity is the way in which different components are eluted over different distances. Two solvents with the same solvent strength and different selectivity have the same average elution distance on the plate but the solutes are eluted in a different pattern. By choosing solvents with extremely different properties, intermediate... 

Eluting power roughly parallels the dielectric constants of solvents. The series also reflects the extent to which the solvent binds to the column material, thereby displacing the substances that are already adsorbed. This preference of alumina and silica gel for polar molecules explains, for example, the use of percolation through a column of silica gel for the following purposes-drying of ethylbenzene, removal of aromatics from 2,4-dimethylpentane and of ultraviolet absorbing substances from cyclohexane. 

Fig. 27. The dependence of In K.d/M for polybutadienes of different molecular weight on the relative elution power of binary solvents...

Let us recollect Fig. 10 showing the dependence between the eluting power of a binary solvent, eab and the molar fraction of the polar component, Ch. T wo characteristic parts can be seen on the curve a rapid increase in eab at small Cb and a plateau at larger Cb. In the plateau region one can very accurately fix the critical conditions, since there eab depends but very slightly on Cb. This is the case for PDEGA and it is necessary that r.0h is a little greater than s. ... 

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