Manipulation of Retention of Acids in Ion Suppression Chromatography

Solute retention results from solvophobic effects occurring between the mobile phase, the stationary phase and the solutes. For the separation of monocar-boxylic acids, the pH of the mobile phase influences the retention behaviour according to the following equation:

Figure 1 Plot of the retention factor of a weak monoprotic acid vs. (pH - pK).

where k0 is the retention factor of the undissociated acid, k-1 is the retention factor of the conjugate base, and Ka is the acid dissociation constant in the mobile phase. This retention behaviour is illustrated in Figure 1, which shows the retention factor of a weak acid versus (pH-pKa). The curve is sigmoidal in shape

Figure 1 Plot of the retention factor of a weak monoprotic acid vs. (pH - pK).

and the inflection point is located at the point where the pH of the mobile phase is equal to the pKa of the solute in the mobile phase. At pH values substantially less than its pKa value, the acid is present in its neutral form and has a large retention factor. Further decreases in mobile-phase pH show no effect on retention factor, since there will be no further change in the ionization of the acid. Conversely, mobile-phase pH values substantially greater than the pKa value will result in complete ionization of the solute, leading to a small retention factor. At intermediate pHs, the solute charge, and hence its retention, will be dependent on the particular pH used and its proximity to the pKa value.

In the case of dicarboxylic acids, the shape of the curve is largely determined by the difference between the two pKa values. When pKa1 and pKa2 are very close, sigmoidal curves are obtained and the behaviour of dicarboxylic acids is almost the same as that of monocarboxylic acids. When the two pKa values are well separated, the curve is a composite of two sigmoidal curves.

Both the ionic strength and organic modifier content of the mobile phase may be varied in order to manipulate retention in ion suppression chromato-graphy. Increasing the ionic strength of the mobile phase causes an apparent increase in the dissociations, leading to a decrease in the retention factor.

This effect is more pronounced in nonaqueous media. In the approximate range of ionic strengths from 0 to 0.5 mol the higher the ionic strength of the mobile phase, the greater the increase in ApKa. The addition of organic modifiers influences retention behaviour in two ways. Firstly, increasing the organic modifier content of the mobile phase decreases the retention factor, as is generally the case in reversed-phase liquid chromatography. However, the apparent pKa of the solute increases as organic modifier is added to the mobile phase, leading to an increase in the degree of ionization of the solute and therefore reduced retention.

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