Comparison study of Chiralpak AD-H with AD columns in chromatographic enantioseparation of dihydropyrimidinone acid and its methyl ester.

This paper reports a comparison study of the difference between Chiralpak AD-H and AD columns in enantioseparation of dihydropyrimidinone (DHP) acid and its methyl ester under normal phase LC conditions. Unlike those of the AD phase, the van't Hoff plots of retention factors for DHP acid on the AD-H phase were linear. The cyclic van't Hoff plots of selectivity factors for DHP acid on the AD-H phase were non-linear and slightly non-superimposable. No conformational transition was observed on the AD-H phase in the whole temperature range. A single-step temperature program on the AD-H phase showed that the selectivity factors of DHP acid only increased approximately 1.7% in 24 h (versus approximately 50% on the AD phase). For DHP ester, the single-step temperature program showed that the selectivity factors on the AD-H phase remained the same in 24 h while those on the AD phase increased around 3.1%. The enantioselectivity of DHP acid on the AD-H phase was lower than that on the AD phase while the enantioselectivity of DHP ester on the AD-H phase was higher than that on the AD phase. The resolution of DHP acid on the AD-H phase was about the same as that on the AD phase while the resolution of DHP ester on the AD-H phase was much higher than that on the AD phase. The results of DHP acid are opposite of what the vendor suggested while the results of DHP ester are the same as the vendor's application notes. This indicates that the differences between Chiralpak AD-H and AD columns are not only in their particle size, but also in the solvated conformations.