Chromatographic shape selectivity with carbon dioxide-acetonitrile mobile phases. Effect of mobile phase composition and density

Trends in chromatographic shape selectivity with mobile phases consisting of mixtures of carbon dioxide and acetonitrile are investigated. Selectivity is evaluated as a function of mobile phase composition, temperature, and column bonding chemistry. SRM (standard reference material) 869a is used as a probe of shape selectivity, while the selectivity between triphenylene and o-terphenyl is used to investigate planarity selectivity. Four molecular mass 228 polyaromatic hydrocarbon isomers are used to investigate shape selectivity based on differences in length-to-breadth ratio. Shape selectivity trends as a function of temperature and column type are found to be similar to what is seen in reversed-phase liquid chromatography, while the trend seen as the amount of acetonitrile in the mobile phase increases is found to be different than in reversed-phase liquid chromatography. In addition, the effect of mobile phase density, i.e., solvent strength, on shape selectivity is investigated by examining shape selectivity as a function of density with neat carbon dioxide as the mobile phase.