Fundamental studies in packed column supercritical fluid chromatography: Column efficiency, thermodynamics and retention mechanism

The modifier effects on column efficiency were investigated in packed column supercritical fluid chromatography (SFC) with methanol, acetonitrile, ethanol and 1-propanol used as modifiers. C-18, phenyl and cyano columns were used with both nonpolar and polar solutes. For highly retained nonpolar solutes, addition of modifier significantly increased apparent column efficiency, especially for the C-18 column. For polar solutes, the presence of modifier dramatically improved retention and efficiency with an apparent efficiency dependence on modifier type and amount. Temperature and pressure effects on efficiency were also studied.; In thermodynamic studies, an approach other than the common van't Hoff equation under constant density was proposed which can be applied to SFC with modifiers in the mobile phase. Based on the proposed approach, thermodynamic studies were performed in packed column SFC for both pure CO2 and modified CO2 as the mobile phases. Methanol and acetonitrile were used as the modifiers. The results show a switch from an enthalpic driven to entropic driven process with increase of temperature at constant pressure for most solutes with both pure CO2 and modified CO2, and the switching temperature decreases with increase of homologous solute size. The adsorption of both CO2 and modifier on the stationary phase was also studied.; Retention mechanism in packed column SFC was studied as a combination of adsorption and partitioning. A series of different bonding density ODS columns were employed along with bare silica column with or without modifier in the mobile phase. Partitioning plays a more important role in retention than adsorption for both pure CO2 and 5% methanol CO2 mobile phase. Partition coefficients of nonpolar solutes decreased with increase of bonding density of column. Larger homologous solutes have more contribution from partitioning than smaller solutes. The retention mechanism of polar solute is mainly a competitive adsorption of both solute and modifier on silanol sites.