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Re: liquid propane

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Diao, Jiasheng wrote:
>    I'd like to buy some liquid propane (not liquid nitrogen) to freeze
> protein crystals. If you know the phone number and address of the company
> which sells liquid propane, would you please send me the phone number and
> address of the company?
One piece of advice- don't buy the LP-gas cylinders sold for plumber's soldering 
torches- the gas won't freeze at liquid nitrogen temperature in a reasonable
amount of time. Get pure propane or ethane from a gas distributor. 
The gas is liquefied by filling a balloon and inverting it over a cryo-vial
or other container which is cooled in liquid nitrogen- I could probably find
a reference but I consider this method somewhat obsolete since the advent
of cryotongs (http://www-structure.llnl.gov/Xray/cryo-notes/Cryonotes.html).
Haken Hope has shown that for small objects like thermocouple junctions (and
presumably crystals) cooling is not significantly faster in liquid propane
than in liquid nitrogen. Perhaps this is because there is not enough latent
heat to form a layer of insulating gas around the object.
Given that, the only advantage of the "popsicle method" was that it allowed 
mounting crystals when you didn't have the capability of directing the phi 
axis downward- say on Rigaku/R-axis where the Phi axis points upward, LN2 
would poor out before you remove the vial. Frozen propane leaves a popsicle 
that slowly melts after you pull off the vial, leaving you plenty of  time 
to position the cold stream and roughly center the crystal. 
The cryo-tongs allow you to mount LN2-frozen crystals at any angle, and recover
them for future data collection if desired. They are far easier to use than
frozen hydrocarbons. There is a large number of designs for different types of
pins, but the system sold by Hampton Research is excellent.

Having said all this, I have to admit that the best datasets we collected for 
my favorite protein were from crystals frozen in liquid ethane before we started
using cryotongs. But there are a lot of other variables.

Edward A. Berry, MailStop 3-250
Lawrence Berkeley National Laboratory
1 Cyclotron Road, Berkeley, CA 94720
Phone: +1-510-486-4335
Fax: +1-510-486-6059
Jfax +1-530-323-9836 (you send fax, I receive email)
e-mail: EABerry@lbl.gov