Effect of the residual silanol group protection on the liquid chromatographic resolution of α-amino acids and proton pump inhibitors on a ligand exchange chiral stationary phase

A new ligand exchange chiral stationary phase (new CSP) containing residual silanol group-protecting n-octyl groups on the silica surface was prepared by treating a ligand exchange CSP (original CSP) based on sodium N-[(R)-2-hydroxy-1-phenylethyl]-N-undecylaminoacetate bonded to silica gel with excess n-octyltriethoxysilane. The new and original CSPs containing an identical amount of chiral selector were applied to the resolution of α-amino acids and proton pump inhibitors (PPIs) including omeprazole, pantoprazole, lansoprazole, and rabeprazole. The separation factors (α) and resolutions (RS) were greater on the new CSP than on the original CSP except for the resolution of asparagine. The trends of the retention factors (k1) for the resolution of α-amino acids on the new and original CSPs with the variation of the organic modifier content in aqueous mobile phase were opposite to those for the resolution of PPIs. Removal of the nonenantioselective interactions between the residual silanol groups and the analytes and the improved lipophilicity of the new CSP were proposed to be responsible for the improved chiral recognition ability of the new CSP and the different retention behaviors of the enantiomers between the new and original CSPs.