Investigation of Mass Transfer Phenomena and Thermodynamic Properties of New Generation Porous Particles for Ultrafast High-Efficient Enantioseparations

During the last decade there has been a strong revolution in the field of chiral chromatography. Thanks to pressing requirements from medicinal and pharmaceutical industries, very efficient particle formats such as superficially porous (SPPs) and sub-2μm fully porous particles (FPPs) have been introduced also in chiral chromatography. Important results, unimaginable until few years ago, have been achieved with columns packed with these new generation chiral stationary phases (CSPs) not only in terms of efficiency but also in terms of speed of separation. The time required to elute a couple of enantiomers has been effectively decreased from several minutes (required in the past with traditional 5 o 3.5 μm FPP chiral column) to less than one second with 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 CSPs: 2.5 μm FPPs and 2.6 μm SPPs functionalized with Whelk-O1 chiral selector (which is usually referred to be a “fast” chiral selector) and 1.9 μm FPPs and 2.0 μm SPPs functionalized with teicoplanin (which is 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 + 20 mM 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, the 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 of enantiomers.