MODELING RETENTION OF IONOGENIC SOLUTES IN LIQUID-CHROMATOGRAPHY AS A FUNCTION OF PH FOR OPTIMIZATION PURPOSES

In liquid chromatography, the retention of ionogenic solutes is a strong function of the pH of the mobile phase, with different solutes showing different behaviour, both qualitatively (e.g., acids vs. bases) and quantitatively (values of dissociation constants). Thus, pH will also affect selectivity and it can be used as a parameter for optimizing separations. In many instances, such optimization studies will require an accurate description of retention as a function of pH. In this paper, attention is focused on basic models describing retention as a function of pH and their use in practice. The theoretical, sigmoidal curve is discussed and a number of possible causes of deviations are considered. The inaccuracies introduced by linearly or quadratically interpolating part of a sigmoidal curve are addressed, in addition to the sensitivity of sigmoidal interpolation to experimental errors.