Jim,
The fact that liquid propane can exist at a range of temperatures is actually
a MAJOR advantage. While you must ensure that the temperature of the liquid
propane is just above its own freezing point, the very high boiling point of
propane ensures that there is liquid-to-solid contact with your sample for
good heat transfer. When the cryogen boils, there is a gas-to-solid contact,
slower cooling, and a greater chance of ice crystal formation.
....
Yes, I agree with the theory, but I also I believe that if you quickly
plunge your crystal into liquid nitrogen, that the high velocity through
the liquid presents fresh liquid nitrogen as you go and the gas-to-solid
contact is neglible. I think speed and having no cold-gas-layer above the
liquid cryogen are the main points of the Warkentin et al. paper cited
previously. I have observed many slow people freezing crystals instead of
flash-cooling them.
But for some crystals flash-cooling is better at temperatures higher than
in the 77 K to 100 K regime for unknown reasons. This can be one reason
why flash-cooling in the gas stream occassionally is better. It is also
why liquid propane worked for Raji E. and Karolin L.: they were using a
temperature above the freezing point of propane.
If you are having problems with flash-cooling, go ahead and try propane
(my preference is CF4 over propane though). And a trick we learned from
the University of Cambridge is to fill a balloon with the gas, put the
balloon opening over a 15 ml Falcon tube and then condense/freeze the gas
in the tube for pouring into vials later. This prevents wasting gas by
bubbling through a coil held in liquid nitrogen. This is just one of
techniques described in the PDF I mentioned.
It is also not a bad idea to practice flash-cooling on lysozyme or
thaumatin crystals and get good at it before working with your own
precious crystals.
Jim
