Understanding the Transition from High-Selective to High-Efficient Chiral Separations by Changing the Organic Modifier with Zwitterionic-Teicoplanin Chiral Stationary Phase

Abstract

The retention behavior of small molecules and N-protected amino acids on a zwitterionic teicoplanin chiral stationary phase (CSP), prepared on superficially porous particles (SPPs) of 2.0 μm particle diameter, has shown that efficiency and enantioselectivity, and so enantioresolution, dramatically change depending on the employed organic modifier. In particular, it was found that while methanol permits the boost of enantioselectivity and resolution of the amino acids, at the cost of efficiency, acetonitrile allows for the ability to reach extraordinary efficiency even at high flow rates (with reduced plate height <2 and up to 300,000 plates/m at the optimum flow rate). To understand these features, an approach based on the investigation of mass transfer through the CSP, the estimation of the binding constants of amino acids on the CSP, and the assessment of compositional properties of the interfacial region between bulk mobile phase and solid surface has been adopted.

Novel insights into the dependence of adsorption-desorption kinetics on particle geometry in chiral chromatography

“…This indicates that the contribution to the intraparticle diffusivity of surface diffusion can be considered negligible, suggesting the localized nature of the adsorption process. This result is somehow expected since, from excess isotherm measurements, it has been demonstrated that a low-mobility water-rich layer covers the adsorbent surface hindering the mobility of the analyte molecules [ 7 ]. Little contributions of surface diffusion have been observed also in achiral HILIC mode [ 38 ], in chiral RP [ 13 ], chiral polar organic mode (POM) [ 10 ] and NP [ 6 ] chromatography.…”

“…This type of isotherm accounts for a bimodal energy distribution, given by two different adsorption sites: one selective (responsible for diastereomeric or enantioselective interactions) and one nonselective (where both enantiomers behave identically) [ 31 , 32 ]. For the reasons presented above it can be successfully applied to chiral separations [ 7 , 10 , 32 , 34 ]. For two competitive components, it is written as: with the saturation capacity (equal for both enantiomers) and the adsorption equilibrium (binding) constant.…”

“…Nevertheless, this innovation was delayed for chiral stationary phases (CSPs) due to a series of challenges, especially in the functionalization and packing procedures, but also related to a lack of fundamental information about mass transfer and chiral recognition mechanism [ 1 , 2 ]. As a matter of fact, conversely to common expectations, it has been experimentally observed that in chiral chromatography (for defined conditions) SPPs do not always outperform FPPs in terms of kinetic performance [ 3 – 7 ]. This could be attributable to the existence of slow adsorption-desorption kinetics between enantiomers and the chiral selector.…”

“…Even if the complete understanding of what happens at a molecular level has not been accessed yet, some clues helpful to explain this behavior can be found in the intrinsic differences between particles (e.g., selector loading and pore accessibility) coupled to important findings about molecular diffusion occurring on the surface of the chiral particle [ 7 , 10 ].…”

Synergetic Multichiral Covalent Organic Framework for Enantioselective Recognition and Separation

“…The separation of racemic mixtures of chiral molecules is critically important in the pharmaceutical chemistry, life science, and agrochemical industries since the toxicity and biological behavior of enantiomers differ significantly. , Among them, the efficient separation and recognition of polar chiral molecules are particularly important in the development of drug substances and life sciences. − This has promoted the development of chiral separation techniques. For enantiomer selective recognition and separation, chiral materials are critical to the process.…”