Liquid Chromatography at the Critical Condition Using Enhanced-Fluidity Liquid Mobile Phases

The use of enhanced-fluidity liquid mixtures of tetrahydrofuran (THF)/CO 2 for liquid chromatography at the critical condition is described. For polystyrene polymers, the molecular weight range that can be analyzed at the critical condition was compared for using THF/CO 2 and other common liquid mixtures. Because enhanced-fluidity liquids are highly compressible, it was possible to reach the critical condition by changing the pressure of the THF/CO 2 mixture. Approaching the critical condition by pressure variation was compared to that possible with temperature changes. Pressure variation was a more accurate and reproducible means of reaching the critical condition than temperature variation. Finally, THF/CO 2 mixtures were used as mobile phases at the critical condition for polystyrene to characterize copolymers of poly(styrene−methyl acrylate).