High solvent elution strength

Arranging the solvents in separation-strength order, the so-called eluotropic series appeared. This term, introduced by Trappe, was related to the experience with bare silica, where a strong solvent is able to move polar solutes on a polar stationary phase. Later this was improved by the discovery of a direct proportion between the elution strength and the dielectric constant. Because silica is hydrophilic and highly polar, there was a correlation between the eluotropic series and the polarity of a solvent [16,18]. 

The elution strength should be adjusted so that the sample components of interest are eluted within a reasonable time. Generally > 0.3 are necessary to separate the sample components from unretained substances including the solvent. On the other hand, values of A < 10 are desired to reduce analysis time and minimize sample dilution which increases with increasing k value. In high-speed analysis optimum conditions are obtained if the k values of the sample components vary between 0.3 and 3. 

A more efficient use of a step gradient is the injection of a plug of mobile phase with high elution strength into the part of the chromatogram where the component of interest elutes (Fig. 5.12e). This eluent plug forces the component of interest to desorbed from the column. It thus increases the fraction concentration but not the cycle time due to solvent readjustment. (Chapter 4.3). 

There is one way to avoid the limitations in injection volume. If the sample solvent is markedly weaker than the mobile phase for the HPLC analysis, i.e. if its elution strength is low, the solutes are concentrated at the top of the column. In this case the injection volume may be unusually high, in the range of millilitres or even litres. However, a prerequisite is that the amount of solute is small enough to prevent the adsorption isotherm from becoming nonlinear. 

Solvent strengths are also measured in terms of polarity, and dielectric constants are generally used to quantify relative strengths (Table 1). A high dielectric constant indicates a polar solvent with a strong power of elution, and a low dielectric constant indicates a nonpolar solvent with a lower ability to elute a component from a sorbent. This elution strength applies to normal phase adsorption chromatography. 

After the complete fraction is collected in the column, it is washed with a mobile phase of low solvent strength to remove the salts. In the last step, the sample is eluted with a low volume of a solvent with high elution strength (e.g., methanol). 

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