Efficiency in chiral high-performance ligand-exchange chromatography : Influence of the complexation process, flow-rate and capacity factor

Chromatographic separations of enantiomers by ligand-exchange mechanisms are performed either by using chiral stationary phases (CSPs) or chiral mobile phase additives (CMAs). The two modes differ significantly with respect to their efficiency. It is shown that the process of chelate formation and dissociation at the CSP is the main source of enhanced plate height values when employing CSPs. When using highly soluble CMAs as selector ligands, the rate-determining ligand-exchange process takes place mainly in the mobile phase and therefore does not contribute much to the plate heights. These fundamental differences in the kinetics of the two modes of ligand-exchange chromatography are reflected in different dependences of the plate height data on the flow-rates. It is shown that for CSPs the plate height depends on the structure and configuration of the analyte and the composition of the mobile phase. Within a series of homologous amino acids a pronounced positive correlation is found between the kinetic plate height contribution and the capacity factor. However, this positive correlation is not a general dependence: on changing the capacity factor by changing the eluent composition, a negative correlation is found.