How mobile phase composition and column temperature affect enantiomer elution order of liquid crystals on amylose tris(3‐chloro‐5‐methylphenylcarbamate) as chiral selector

Abstract

A comprehensive study into the effects of mobile phase composition and column temperature on enantiomer elution order was conducted with a set of chiral rod‐like liquid crystalline materials. The analytes were structurally similar and comprised variances such as length of terminal alkyl chain, presence of chlorine, number of phenyl rings, and type of chiral center. Experiments were carried out in polar organic and reversed‐phase modes using amylose tris(3‐chloro‐5‐methylphenylcarbamate) immobilized on silica gel as the chiral stationary phase. For all liquid crystals, reversal of elution order of enantiomers was observed based on type of used cosolvent and/or its content in the mobile phase; for some of the liquid crystals a temperature‐induced reversal was also observed. Both linear and nonlinear dependencies of natural logarithm of enantioselectivity on temperature were found. Tested mobile phases comprised pure organic solvents and binary and tertiary mixtures of acetonitrile with organic solvents and/or water. Effect of acidic/basic mobile phase additives was also tested. Effect of structure of chiral selector is briefly discussed.

Enantioseparations of polyhalogenated 4,4’‐bipyridines on polysaccharide‐based chiral stationary phases and molecular dynamics simulations of selector–selectand interactions

“…As a particular case, retention of the second eluted enantiomer of compound 1 increased moving from cC‐3,5diMe ( k 2 = 7.54) to iC‐3,5diCl ( k 2 = 8.26), whereas the two chloromethyl substituted cC‐3Cl,4Me and cC‐4Cl,3Me provided lower k 2 values (4.78 and 5.15, respectively). On the other hand, EEO reversal was observed on cC‐3Cl,4Me and cC‐4Cl,3Me ( M ‐ P ) for both compounds compared to the 3,5‐disubstituted cC‐3,5diMe and iC‐3,5diCl ( P ‐ M ), this evidence revealing the occurrence of a different adsorption mechanism [3, 28–31]. It is worth noting that EEO is also a key factor for the method development [32].…”

“…It is worth noting that EEO is also a key factor for the method development [32]. Indeed, as chiral separation methods are optimized for optical purity control of a chiral analyte, the possibility to modify the EEO may be advantageous in order to have the minor enantiomer eluted first [28, 31, 32].…”

“…It is worth noting that EEO is also a key factor for the method development [32]. Indeed, as chiral separation methods are optimized for optical purity control of a chiral analyte, the possibility to modify the EEO may be advantageous in order to have the minor enantiomer eluted first [28,31,32]. The addition of 5% MeOH to the MP was detrimental for retention and selectivity in almost all cases (Supporting Information, Table S3 and Fig.…”