HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY USING MOBILE PHASES WITH ENHANCED FLUIDITY

The low viscosities of supercritical fluids provide SFC with increased speed of analysis as compared to HPLC. This study demonstrates that similar advantages can be provided to HPLC when low-viscosity fluids are dissolved in common HPLC mobile phases. For example, the time of analysis for the separation of a test mixture decreased when large proportions of carbon dioxide were added to the methanol mobile phase. Another advantage expected when using the low-viscosity mixed mobile phase was increased efficiency. However, because the stationary phase used in this study was porous glassy carbon adsorbent, the efficiency did not vary with viscosity. This is unlike what is found for most LC solid or bonded stationary phases. With PGC, the nonequilibrium contribution to chromatographic band dispersion apparently is controlled by the slow mass-transfer kinetics of the adsorption-desorption process. The addition of carbon dioxide had differing effects on the retention of solutes on the PGC. For example, increasing the proportions of carbon dioxide up to 100% caused the time of analysis of naphthalene to decrease continuously. However, for phenolic compounds or beta-naphthol, a minimum time of analysis as a function of solvent composition was found. Two different mechanisms of solute interaction with PGC are proposed to account for these differences. These mechanisms are supported by data on the solvent strength of the mobile phase and the thermodynamics of solute transfer to the stationary phase.