Study of mass transfer phenomena in core-shell and sub-2μm fully porous chiral stationary phases for ultrafast high-performance enantioseparations

During the last decade, column manufacturers have put much effort into the design and packing of materials (in particular, sub-2μm fully porous and core-shell particles) suitable for highly efficient and fast separations in ultra-high-performance liquid chromatography (UHPLC). As a matter of fact, however, this advancement has only partially interested the field of chiral liquid chromatography. This is essentially due, on the one hand, to the lack of interpretation of the complex mass transfer process in chiral chromatography and, on the other, to the empirical difficulties related to the functionalization of small particles with chiral selectors. In this study, a brush-type chiral selector (Whelk-O1) was used to functionalize both core-shell and fully porous silica particles of different diameter (including also sub-2μm fully porous ones). The chiral stationary phases (CSPs) were slurry packed at high pressure into columns of different geometries. The kinetic performance of these columns was evaluated in normal phase conditions for the separation of the two trans-stilbene oxide (TSO) enantiomers. The results of peak parking experiments were interpreted in the light of a proper model of diffusion in porous media (the so called time-averaged or parallel model was used) and each contribution to band broadening was individually determined. It was found that eddy dispersion and slow adsorption-desorption kinetics are the most important contributions that affect the efficiency of these columns.