Development of Micellar Selectivity Triangle for Classification of Pseudo-stationary Phase Selectivity in Electrokinetic Chromatography

Abstract

A novel Micellar selectivity triangle (MST) was developed to rationalize and classify the chemical selectivities for a wide variety of pseudo-phases in Electrokinetic Chromatography (EKC). The descriptors of polar selectivities for constructing the MST were derived from the linear solvation energy relationships (LSER). The effects of solvent modification of micelles, mixed-micellar systems along with surfactant type on the pseudo-phase selectivities were investigated using the MST.

To quantitatively describe the similarities and differences of the chemical selecitivities, a phase selectivity ranking system was constructed on the basis of the Euclidean distance between objects. Chemometric methods including the principal component analysis (PCA), hierarchical clustering and k-means clustering were used to elucidate the classification and characterization of the micellar phases in different perspectives. The results form the MST and the chemometric methods were in generally good agreement.

The micellar selectivity triangle was used as a guideline for optimizing the separation of 20 Phenylthiohydantoin (PTH) amino acids. The effects of the surfactant type and concentration, temperature and organic solvent additives on the PTH amino acids separation were evaluated. Additionally, the substituent constants of these PTH amino side chain obtained from the EKC methods were compared to those obtained from octanol-water partitioning and free energy of transferring amino acid moieties from the globular proteins interior to surface.

Finally, the combined use of the unified selectivity triangle (UST), PCA and cluster analysis was explored for the comparison of the chemical selectivity between the two powerful chromatographic techniques, EKC and RPLC. The database consisted of 518 reversed-phase systems and 83 pseudo-phases. The primary factors to influence the phase selectivity of the two techniques were identified and compared. Suggestions for future works are also discussed.