decreasing retention time with increasing times of injection

[Bruce Hamilton]

I have no idea at all about the supercritical chrom then curious that how the system ensures only liquid CO2 but not any gasous CO2 present.

The supercritical state exists between the liquid state and gaseous state, around the critical point. A lot of SFE is actually on the subcritical liquid side of the phase diagram. I'm sure there will be a lot of information on the theory on the WWW. The following is just intended to provide a simple introduction.

Around the supercritical region CO2 has the solvation ability of a non-polar solvent, such as hexane, so adding a modifier such as MeOH will change the solvation power.

Usually a liquid CO2 supply cylinder ( eductor tube ) feeds a high pressure pump with another pump adding any co-solvent/modifier. The fluid state is controlled by flow rates through restrictors and/or back pressure regulators, with either temperature constant, and the pressure varied or vice versa. Provided the conditions are stble, gaseous phase will not form.

The fluid viscosity changes dramatically in the supercritical region, consequently trivial changes in preesure and temperature can create very significant changes in chromatography, so equipment has to be very controlled.

As the supercritical fluid changes temperature/pressure, the solubility of materials changes, so separation occurs. Dropping the temperature or pressure to create subcritical conditions will cause most solutes to precipitate from the fluid.

Increasing the temperature and pressure away from the critical region further into the gaseous region can also lower the solvation power of the fluid, but the cost of equipment also increases, with little benefit..

Bruce Hamilton

[ym3142]

Bruce, Thank you.