Influence of pH on retention and selectivity in micellar liquid chromatography: consequences of micellar-induced shifts of ionization constants

Abstract

The retention and selectivity of ionizable solutes in the two-surfactant-mediated reversed-phase LC techniques (micellar and ion pair) are compared through the use of a general retention equation for mono- and zwitterionic solutes. In RPLC with different mobile-phase modifiers (organic solvents, surfactants), the influence of pH can be quantitatively described by one general equation. The existing theory for the secondary chemical equilibria has been applied to compare selectivity effects in micellar and ion-pair chromatography. In order to predict the influence of different mobile-phase compositions on the selectivity of ionizable compounds, one should determine how a mobile-phase parameter influences the ionization equilibria (i.e., selective shifts of pKa) and self-selectivity (defined as the ratio of retention factors of the acid/conjugate base). For example, the first ionization constants of amino acids and peptides in aqueous mobile phase are between pH 2.3 and 3.4. Consequently, the pH required to maximize the retention of these solutes by ion suppression is less than the operational pH range of silica-based columns. In addition, since the pKa1 values of these solutes are similar, adjustment of pH has little effect on separation selectivity. In contrast, ion-pairing and micellar mobile phases with SDS surfactant increase the magnitude and range of the ionization constants. These trends are more pronounced with micellar mobile phase. The displacement of solute ionization constants to higher PH with micellar mobile phase allows the maximal, limiting, retention of zwitterionic solutes to be observed within the pH limits of silica-based columns.(ABSTRACT TRUNCATED AT 250 WORDS)