Thermodynamic and Kinetic Aspects of Enantioseparations on New Generation Fully Porous and Superficially Porous Chiral Stationary Phases

Thanks to the introduction ofvery efficient particle formats in chiral chromatography, such as superficially porous (SPPs) and sub-2μm fully porous particles (FPPs), important results, unimaginable until few years ago, have been achieved not only in termsof efficiency but also in terms of speed of separation. Indeed, the time required to elute a couple of enantiomers has been decreased from several minutes (required in the past with traditional 5 o 3.5μm FPP chiral column) to less than one second by using short columns packed with chiral SPPs or sub-2μm FPPs, with efficiency as high as 300,000 N/m. However, mass transfer phenomena in chiral chromatography have been barely investigated so far, mostly due to the intrinsic complexity of sample migration processes in chiral porous media. In particular, there are a few data available about the estimation of the impact of adsorption-desorption kinetics on the efficiency of separation.In this work, we evaluated kinetic performance of columns packed in house with four different chiral stationary phases (CSPs): 2.5 μm FPPs and 2.6 μm SPPs functionalized with Whelk-O1 chiral selector (which is usually referred to bea “fast” chiral selector) and 1.9 μm FPPs and 2.0 μm SPPs functionalized with teicoplanin (whichis considered a “slow” chiral selector). Whelk-O1 columns were operated in normal phase mode (hexane/ethanol 90:10), while teicoplanin ones in HILIC mode (acetonitrile/water 85:15 + 20mM ammonium formate). An approach based on the deconvolution of the eddy dispersion from the adsorption-desorption kinetics was adopted in order to evaluate the impact of this last term on the efficiency of separation. It was found that the adsorption-desorption kinetics is slower on SPPs columns, probably due to the larger bonding density found for this particle format, even if experimental conditions were the same for both SPPs and FPPs. Additionally, adsorption equilibria have been further investigated by means of the so-called “Inverse Method”, which allows to determine the adsorption isotherms through a numerical procedure in which the parameters of an isotherm model are derived from overloaded (nonlinear) band profiles ofenantiomers. The adsorption mechanism on both Whelk-O1 and teicoplanin CSPs was described by means of abi-Langmuir isotherm. It was found that even though SPPs have a lower saturation capacity than their FPP counterparts, they are characterized by an higher enantioselectivity if the contribution of nonselective sites is neglected.