Eluting power of solvents

Eluting power of solvents as function of polarity of stationary phases. 

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... 

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. 

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. ... 

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 A-11. Eluotropic solvent series for hydrophilic adsorbents such as alumina or silica, listed in order of increasing eluting power of the solvent , quantitatively measured by Snyder s empirical eluant strength parameter °. ...

Fortunately the method is not limited to colored substances. Any organic compound capable of being eluted from alumina will form a spot, which soon becomes visible when the solvent is let evaporate and the plate let stand in a stoppered 4-oz bottle containing a few crystals of iodine. Iodine vapor is adsorbed by the organic compound to form a brown spot. A spot should be outlined at once with a pencil because it will soon disappear as the iodine sublimes away brief return to the iodine chamber will regenerate the spot. The order of elution and the elution power for solvents are the same as for column chromatography. 

In both normal phase and reversed phase HPLC, the eluting power or solvent strength of the mobile phase is mainly determined by its polarity. Although most analysts have a good idea of what the term polarity implies and could rank most common solvents in order of their polarity, a more quantitative description would be very useful in chromatography. 

The polarity index or the solubility parameter may be used as a measure of solvent strength, which would be a measure of polarity in those cases. For reversed phase HPLC, solvent strength parameters have been proposed for the four most common solvents used, i.e. water (Si = 0), methanol (Si = 2.6), acetonitrile (Si = 3.2) and THF (Si = 4.5). Using these values water makes no contribution to the eluting power of the mobile phase and the solvent strength is measured by the volume fraction of organic modifier. 

The eluting power of different solvents can be approximated by a modification of this procedure. Some of the substance to be processed is dissolved in each of the solvents to be studied. Each solution is then treated with carbon, and the solvent from which the poorest adsorption is obtained can be expected to be the best eluting agent. 

Various workers have recorded elutropic series, e.g. Trappe [4], Snyder [5], Strain [6], BickofT [7] and Knight and Groennings [8]. Generally, for polar adsorbents such as alumina and silica gel, the strength of adsorption increases with the polarity of the adsorbate. For carbon the order is reversed. Trappe s elutropic series found that the eluting power of a series of solvents for substances adsorbed in columns such as silica gel decreased in the order ... 

Fig. 4. Scheme of the Universal Scientific gradient former. A = solvent Of low eluting power, B = solvent of high eluting power, a = divider, b = gasket, c = valve, d = to pump. 

In normal-phase column chromatography, which solvent has more eluting power petroleum ether or dichloromethane In what way is the eluting power of a solvent related to its polarity ... 

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