ABSTRACT
Reversed-phase liquid chromatography (RPLC) separations provide the foundation in a wide range of chemical analytical techniques for the determination of individual components in complex mixtures and matrices. Separations of nonvolatile organic compounds, polar pharmaceutical compounds, and other larger-size biomolecules are routine, and selectivity can be controlled by the choice of stationary and mobile phases as well as by modification of operational parameters such as temperature and the application of gradients. The discriminating power of RPLC is exemplified by the superior separation of polycyclic aromatic hydrocarbon (PAH) isomers on bonded octadecylsilane (C18) stationary phases, for which subtle differences in molecular shape controls their separation. Over 25 years ago, Ogan and Katz illustrated the improved separation of certain PAH isomers on higher-density C18 stationary phases [1-3]. During the same time period, the U.S. Environmental Protection Agency method 610 [4] required a specific commercial C18 column for the improved RPLC separation of 16 priority pollutant PAHs. It was unclear, however, which properties of the stationary-phase were driving the enhanced selectivity for the PAH isomers as many details regarding the stationary-phase and its preparation were not available. During early investigations of the mechanism of retention for these chromatographic materials [1,3,5,6], it became apparent that the specific chemical modifications of the silica surface in addition to the shape of the PAH isomer were important parameters that influenced selectivity.
