> 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
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