Resolution of π-acid racemates on π-acid chiral stationary phases in normal-phase liquid and subcritical fluid chromatographic modes : A unique reversal of elution order on changing the nature of the achiral modifier

The enantiomeric separation of a series of π-acid N-(3,5-dinitrobenzoyl) (DNB) derivatives of α-amino esters, α-aminoamides and α-amino alcohols was investigated in the normal-phase mode on several π-acid chiral stationary phases (CSPs) derived from ( S)-DNB-tyrosine, ( R)-DNB-phenylglycine and ( R)-DNB- p-hydroxyphenylglycine (chiral selectors, CSs). CSs were covalently grafted on to either γ-mercaptopropylsilica gel (type-1 CSPs) or γ-aminopropylsilica gel (type-2 CSPs). A comparison of the selectivities obtained under liquid (LC) and subcritical fluid chromatographic (SubFC) modes indicated important differences in the stereoselectivity of the separation of α-aminoamide test solutes. Studies of the relationship between the nature of the achiral mobile phase (in LC) and enantioselectivity showed a unique reversal of elution order on changing from hexane-ethanol to hexane-chloroform (or hexanemethylene chloride) mobile phases. Finally, chiral recognition models are discussed. Difficulties in correlating chromatographic data (selectivity, elution order) with the proposed mechanisms are outlined. This study emphasizes the importance of the part played by the mobile phase during the separation process; the mobile phase can induce major changes in the conformation of the molecules, thus leading to different chiral recognition processes.