Rational design of reversed phase stationary phases for liquid chromatography: Is the ubiquitous C18 phase really the best

Chromatography, as a separation technique, has received wide acceptance and achieved widespread application. Reversed-phase liquid chromatography (RPLC) is a versatile separation mode utilized in biotechnology, pharmaceutical and other industries. Some problems encountered in RPLC keep the chromatographers head-scratching and hair-pulling. One thorny problem, for many years, has been peak tailing of the bases, which has drawn considerable attention, partially because the bases comprise the majority of pharmaceuticals and important biomedical chemicals. Although numerous works have been carried out, tailing of the bases remains as a challenge. In RPLC, the properties of the stationary phase determine retention and selectivity. Conventional ODS (octadecylsilane, C18) stationary phase is the most generally used column packing due to its efficient and reproducible properties. The other commercially available columns are also even-numbered columns such as C8 and C30. The evolution of the stationary phase has revolved around the even-numbered columns, both in the cases of laboratory research and practical application. Little work has been done regarding the odd-numbered columns.;The first part of this work was to investigate the chromatographic even-odd effect. The even-odd effect means that physical and chemical properties of the molecules vary with the even or odd number of structure units in the molecules. We define the chromatographic even-odd effect as dependence of shape selectivity and retention behavior on the even or odd carbon number of chain length on stationary phase. In the work presented here, the bonding chemistries of the columns used included solution polymerization, monomeric synthesis and self-assembled monolayer (SAM). The carbon number of each type of column used ranged from C13-C18. Shape-constrained solutes (benzene, naphthalene and anthracene) were selected as analyte. The chromatography was operated in the highly organic mobile phase. The retention behavior was reflected by the enthalpy of transfer. The shape selectivity was thought to be associated with the entropy of transfer. Solution polymeric and monomeric stationary phases demonstrated that the enthalpy of transfer altered with the even or odd carbon number of chain length and both shape selectivity and the relative entropy of transfer depended on the even or odd carbon number of alkyl chain length. These results indicated the chromatographic even-odd effect in retention behavior and shape selectivity. The researchers involved in this study speculate that the chromatographic even-odd effect is attributed to the steric arrangement of carbon chain length and may be partially due to the bonding density.;The second part of this work was to investigate the effect of stationary phase bonding density/bonding chemistry on column efficiency N, which is associated with peak shape. Reversed-phase columns, in various bonding densities, were used, involving different bonding chemistries, which were the same as in the first part of the work. Strong base nortriptyline and weak base pyridine were chosen as targets. The chromatographic experiments were conducted in low pH conditions. Polymeric solution columns exhibited that low bonding density columns yielded higher efficiency than high bonding density columns. Self-assembled monolayer (SAM) or monomeric columns with the same chain length but different bonding densities also presented the same trend. It was concluded that changing bonding density was a feasible way to improve peak tailing.