Effects of alcohol mobile-phase modifiers on the structure and chiral selectivity of amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase

Following a previous publication, the present paper reports additional results on the effects of alcohol mobile-phase modifiers on the structure and chiral selectivity of amylose tris(3,5-dimethylphenylcarbamate) (Chiralpak AD) chiral stationary phase (CSP). Solid-state NMR (1H/13C CPMAS) was utilized to identify and compare structural differences in Chiralpak AD caused by the various alcohol mobile-phase modifiers, many of which were not studied in the previous publication. The influences of the various alcohol modifiers (in hexane-based mobile phase) on the structure and chiral selectivity of the CSP were studied and compared. CPMAS spectra of Chiralpak AD flushed with the mobile phases displayed clear evidence of solvent incorporation into the CSP. When alcohol modifiers with varying size and bulkiness were used in the mobile phase, differences in structure and chiral selectivity were observed on Chiralpak AD based on solid-state NMR and chromatographic data. The change of t-butanol concentration in the t-butanol/hexane mobile phase caused changes of structure and chiral selectivity of the Chiralpak AD. These data further support our belief that the different chiral selectivities of the CSP associated with the use of different alcohol modifiers are due to different alterations of the steric environment of the chiral cavities in the CSP by the different mobile-phase modifiers.