Unexpected retention and efficiency behaviors in supercritical fluid chromatography: A thermodynamic interpretation

Experimental conditions leading to unexpected shift in retention, band compression, and to band enlargement of small molecules in supercritical fluid chromatography are reported. The stationary phase is a 3.0mm×150mm column packed with 1.8μm fully porous high strength silica (HSS) StableBond (SB) C₁₈ particles. The mobile phase is pure carbon dioxide preheated at 107°C and the column back pressure is set at 100bar. The column was thermally insulated in a vacuum chamber at a pressure of 10^-5Torr in order to maintain the integrity of the peak symmetry. The sample solution was prepared by dissolving seven n-alkylbenzenes (from benzene to dodecylbenzene) in pure acetonitrile. The injected sample volume (1μL) was three orders of magnitude smaller than the column volume. Remarkably, the retention time of octylbenzene is found 15% smaller than that expected for this series of homologous compounds. Most strikingly, the plate counts change from about 20000 for the three least retained analytes (benzene, ethylbenzene, and butylbenzene) to 60000 for hexylbenzene and to only 5000 for the three most retained compounds (octylbenzene, decylbenzene, and dodecylbenzene). These unexpectedly high (reduced plate height of 1.3) and low (reduced plate height of 15) column efficiencies observed for closely related compounds are consistent with the overlap between the spatial concentration zone of the sample solvent (acetonitrile, Langmuir isotherm, k≃2) and those of the analytes (competitive linear isotherms, 0