COMPARISON OF ENHANCED FLUIDITY AND ELEVATED-TEMPERATURE MOBILE PHASES IN REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

Enhanced-fluidity liquid mobile phases (methanol/H2O/ CO2) were studied as possible mobile phases in reversed-phase HPLC. Diffusion coefficients were experimentally determined for a methanol/water mixture and enhanced-fluidity mixtures containing methanol/H2O/CO2. The effects of temperature increase on the diffusion coefficients for the methanol/water mixture and the enhanced-fluidity mixtures were compared. Markedly larger increases in the solute diffusion coefficients were observed when the temperature of the enhanced-fluidity mobile phases were increased compared to that found for the methanol/H2O mixture. The chromatographic advantages of enhanced-fluidity mixtures were compared to those of elevated-temperature HPLC and to the combination of temperature elevation with enhanced fluidity mixtures. Improvements in efficiency and analysis time were observed in the order of room temperature methanol/water < elevated-temperature < enhanced-fluidity < combined elevated-temperature/enhanced-fluidity. However, the selectivity of the separation decreases in this same order. An example is shown in which too much selectivity is lost for optimum chromatographic performance when the eluent is an enhanced-fluidity mixture that is elevated in temperature. Also, the same quality chromatographic separation in the same approximate time was demonstrated for an isocratic separation with an enhanced-fluidity mixture compared to a gradient elution separation of a 16-component test mixture of polycyclic aromatic compounds.