Supercritical Fluid (Dense Gas) Chromatography/Extraction with Linear Density Programming

The versatility of dense gases or supercritical fluids as solvents is briefly reviewed., It is pointed out that gas density is the key parameter controlling solvent power and it is argued that density programming should replace pressure programming for chro-matographlc and extractive separations. Our previous theory of dense gas solubility and solubility thresholds is then used to explain why relatively high pressures are desirable for dense gas separations. The theory is also used to examine peak spacing for linear density programs to develop a speeial density program for uniform peak spacing. An experimental linear density programming system utilizing CO2 at 40 C and at pressures up to 300 atmospheres is described and factors affecting separation efficiency are evaluated. A separation of four aromatic compounds is demonstrated using programmed density extraction alone. Chromatographic columns used with polystyrene oligomers and polynuclear aromatic compounds are shown to improve the separation. The spacing of oligomers is shown to be in qualitative accord with theory. Finally, factors to be considered in optimum programming are discussed.