Theoretically? Millions to billions of years. Practically? They will
last until the first time you forget to fill the dewar. Or until the
Sun explodes and in the rush to leave Earth your descendants forget to
pack it. Whichever comes first.
Seriously, as long as the crystals have actually stayed below ~130 K
they are as stable as glass beads. This is because they actually ARE
glass beads at this temperature. The diffusion rate, however, goes up
by a factor of a million at ~140K and that seems to be why there are so
many rumors floating around out there that crystals "change" under lN2.
Perhaps perpetuated by people who want space in the storage dewar.
For example, there is a rumor that gasses like xenon will slowly "leach
out" of cryo-cooled samples. I heard that one about 10 years ago, and
since then I have been periodically checking the occupancy of the Xe
sites in a particular cryo-cooled lysozyme crystal. So far, I can see
no change. But if I ever make a mistake opening the tongs and warm it
to >140K, even for a millisecond, I'm sure it will be ruined.
As for ice that accumulates at 100K, this is always easy to remove with
a jet of liquid nitrogen. We use this commercial product:
http://brymill.com/brymill-cryosurgical-devices/brymill-cry-ac-cry-ac-3.html
and there is a new one from MiTeGen
http://www.mitegen.com/mic_catalog.php?c=iceoff
which is cheaper as it is not a medical device.
You just have to be careful not to knock the crystal off with it, and
not to blow the cryo stream out of the way. Can't just point and
shoot. You need to "sweep" the liquid stream over the crystal. It is
surprisingly effective.
You can also check out these studies on dewar life:
http://dx.doi.org/10.1107/S0021889804025403
and on the thermal spike you can encounter while mounting/unmounting
with cryovials:
http://dx.doi.org/10.1007/s10969-007-9029-0
-James Holton
MAD Scientist
On 5/22/2013 5:38 AM, Careina Edgooms wrote:
Hi
Does anybody know how long one could store a crystal in liquid
nitrogen for before it will no longer diffract well? I'm talking in
the order of weeks to months...
careina
