IMPACT OF PHASE-EQUILIBRIA ON THE BEHAVIOR OF CYLINDER-STORED CO2-MODIFIER MIXTURES USED AS SUPERCRITICAL FLUIDS

Supercritical solvents such as C02 may be combined with modifiers to extend their range of application. Recently, a number of suppliers have offered cylinders of premixed, modified fluids for supercritical applications. Claims of consistent delivery concentrations for these products in analytical applications have been at variance with the experience of a number of workers. Our analysis of this problem confirms the findings of inconsistent performance and traces its origin to a shifting vapor-liquid equilibrium which changes the concentration of modifier in the liquid phase as cylinder contents are withdrawn. Computer simulation of the vapor-liquid equilibria within the cylinder indicates that maintaining delivery concentration consistency is not possible unless an azeotrope is present. Other difficulties inherent in this type of product formulation are also noted. Our discussion should give users a better understanding of criteria to be considered when introducing modifiers into C02 for use in supercritical extraction and chromatographic applications. © 1973, Oxford University Press. All rights reserved.