Chromatography Forum

Hi All

we are running an SFC system with good success in our lab. I am suspecting that in reality we are doing subcritical chromatography or that our system becomes subcritical (liquid) during gradients.

Our standard method is at 30°C, which is below the Tc of pure CO2.

From a publication (J. Chem. Eng. Data 2000, 45, 932-935) it appears that the ciritcal temperature rises as alcohols are added. For 15% methanol it is approximately 50°C.

Am I getting something wrong?

What are the effects of crossing the Tc during a gradient method at 40°C and 5-40% MeOH?

I guess it wasn't a trivial question since there are no replies.

I guess it wasn't a trivial question since there are no replies.

Ok, here's a reply to render it non-trivial

Actually, saying it's of "academic interest" is probably more accurate than "trivial". SFC is not a wildly popular technique in the first place. You make a good point, but there are probably not many people who are into SFC enough to care.

The effects may not be known and that could be an area of academic interest. Provided there are no published papers in the area.

Are you neglecting the pressure aspect?
What is subctitical at a given temp. can become supercritical when enough pressure is applied...

Thank you for the input!

regarding the comment by DR:

If you look at the phase diagram of CO2, increasing the pressure on liquid CO2 below its supercritical temperature of 31°C would just eventually turn it solid but never supercritical. The critical temperature does not depend on the pressure.

Maybe I will conduct some research work then...when the days in the lab have 25 hours that is.