FLUID PHASE-EQUILIBRIA IN SUPERCRITICAL FLUID CHROMATOGRAPHY WITH CO-2-BASED MIXED MOBILE PHASES - A REVIEW

In supercritical fluid chromatography (SFC), cosolvents (modifiers) have been added to CO2 in order to deactivate the analytical column and to improve the solvating power of CO2 for extending the utility of SFC to polar and high molecular weight solutes. However, the use of modifiers has proven to be difficult, and irreproducibility in retention and selectivity is often observed. This comprehensive review focuses on the phase behavior of analytically useful CO2-based based mixed mobile phases in SFC. All of the CO2-based mixed mobile phases which have been used in packed or open tubular column SFC are reviewed. All of the available fluid phase equilibria data for such binary fluids are discussed, and the P-T-x region which is applicable to SFC is compiled in both graphs and tables. Single phase regions (pressure and temperature) are listed by mol % modifier. However, only mixtures of CO2 + benzene, + ethanol, + hexane, + methanol, or + toluene have enough P-T-x data points to enable extrapolation to any other conditions used in conventional SFC (0-20 mol% modifier, 40-400 atm, and 0-200-degrees-C). Problems associated with estimating the critical parameters of the pure components or the mixtures are reviewed. Phase transitions in the syringe pump, transfer lines, injector, column, and restrictor are delineated. Equilibration problems associated with the preparation of mixed mobile phases (premixed cylinders, liquid or gas phase saturation, and dual pumps with on-column mixing) and phase transitions during the operation of SFC instrumentation are discussed. Finally, future research needs are outlined.