Phase system selectivity and peak capacity in liquid column chromatography - the impact on two-dimensional separations

Correlations between the separation selectivity in aqueous and non-aqueous reversed-phase systems and in normal-phase systems were investigated. The degree of correlation between the selectivity in separation systems can be useful indicator of suitability of phase systems for two-dimensional HPLC separations. The real gain in peak capacity improves as the orthogonality of separation systems increases and impairs as the correlation between the selectivity of coupled systems increases. Similarity between the systems was determined on the basis of the change in retention (selectivity) attributed to suitable structural descriptors, such as polar interaction indices, number of carbon atoms in the lipophilic parts of the molecules or on number of repeat monomer units in high-molecular compounds. Based on this approach, the degree of orthogonality of the HPLC separation systems was evaluated and correlated with the peak capacity in two-dimensional HPLC. The approach was tested with simple aromatic compounds and was used to characterise retention behaviour of the individual species in samples of synthetic EO-PO co-polymers.