CPSA, October 18-21, 2004

Application of Structure-based pK_a Prediction to Reversed-Phase Chromatographic Method Development

Abstract

Selectivity in high performance liquid chromatography may be tuned by changing the mobile phase pH. Even small changes in mobile phase pH can alter the selectivity by an order of magnitude (or more), which could have a major impact on the robustness of the chromatographic method. Absolute retention times may also be affected by a change in mobile phase pH, which could affect the reproducibility of separations. Therefore, selection of an appropriate mobile phase pH is critical to chromatographic method development when developing stable, reproducible separations.

Mobile phase pH levels may be optimized experimentally, but this process can be time consuming. However, prior knowledge of the expected chemical structure(s) and/or functional groups may be used to estimate pK_a values, which can then be used to select a range of appropriate mobile phase pH. This paper will describe the practical application of pK_a prediction prior to, and during method development. Measurements, databases, and prediction systems are primarily based on aqueous pK_a values (predicted or experimental), but reversed-phase chromatography often uses organic solvents. The adaptation of aqueous pK_a values to the organic mobile phases will also be discussed.

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Relevant Products: LC Simulator, pK_a DB, Method Development Suite