Characterization of HPLC columns: a comparison of Tanaka and Abraham methods

Abstract

Characterization of the selectivity of HPLC columns is an essential need in many chromatographic development methods. Different tests and procedures have been developed for this purpose. In this paper, we analyze two of the most used methods, the Tanaka tests and the Abraham solvation parameter model, and compare the information obtained for neutral solutes in two columns with different stationary phases: a C18 and a CN columns. The Tanaka tests use pairs of solutes to characterize column selectivities caused by hydrophobicity, shape, and hydrogen bonding interactions of neutral solutes with the mobile and the stationary phases. Abraham model provides information on the selectivities caused by the interactions of cavity creation, hydrogen bond basicity, hydrogen bond acidity, and dipolarity/polarizability of the solute with the mobile and stationary phases from the retention of a relatively larger set of neutral solutes. Analysis of the solute’s characteristics and retention in the two columns shows that the Tanaka selectivity for solute hydrophobicity is equivalent to the selectivity obtained from the Abraham term for the differences in the cavity formation of the solute in mobile and stationary phases. Abraham model does not consider shape selectivity, but the apparent shape selectivity measured by the Tanaka test does not consider the influence on retention/selectivity caused by differences in the size, dipolarity/polarizability, and hydrogen bonding capabilities of the test solutes. If these differences are taken into account, the differences in the selectivity coefficient obtained for steric interaction may be considerable. Tanaka selectivity for hydrogen bonding is a mixing of selectivities for hydrogen bonding from solute to the chromatographic phases (column hydrogen bond basicity) and from the phases to the solute (column hydrogen bond acidity). The Abraham method differentiates between the two types of selectivities: hydrogen bond acidity and hydrogen bond basicity. Additionally, Abraham method provides information on the dipolarity and polarizability selectivities. Finally, new selectivity coefficients are proposed for the interactions of the Abraham model; solute cavity formation (or hydrophobicity), hydrogen bond basicity, hydrogen bond acidity, dipolarity/polarizability, and excess polarizability.