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

- Unexpected retention and performance behaviors may be observed in SFC.
- Retention times may be 20% smaller than those expected for homologous compounds.
- Reduced plate heights may vary from 1.3 to 15 for closely related compounds.
- Data are explained by the overlapping between the sample solvent and the sample zones.
- The retention shift and efficiency anomalies are predicted by chromatography modeling.

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 3.0 mm × 150 mm column packed with 1.8 μm 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 100 bar. The column was thermally insulated in a vacuum chamber at a pressure of 10−5 Torr 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 (1 μL) 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 20 000 for the three least retained analytes (benzene, ethylbenzene, and butylbenzene) to 60 000 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 < k < 10). The present observations are fully supported by chromatogram simulations which assume that the Henry's constants of the infinitely diluted analytes are strongly dependent on the concentration of the sample solvent in the mobile phase.