Hi Mark
we also had a problem like this but in one case it was due to an
instability in the goniometer head system so that the device still
moved even after being "homed". Another few cases were due to an
unstable magnetic base that was wobbling about due to the screw, on
the gonimeter head, not holding the magnetic base in place correctly.
Hope that helps!
Gina
On Jul 21, 2008, at 4:25 PM, Rizkallah, PJ (Pierre) wrote:
Hi Mark,
Cryocooled loops stored in tubes inside dewars do flex a little
when the loops are put on the goniometer. This is natural, as in
the dewar, the whole pin was in liquid nitrogen, but on the
goniometer, it is only the loop that is in the cryo stream. Having
said that, the degree of movement is small enough not to worry
about, but will stabilise within a couple of minutes. Different
makes of loops move to different extents, due to the different
amount of metal in them. I agree that the Mitegen loops move least,
as they are made of more plastic than others, which doesn't change
dimensions upon temperature cycling as much as metal pins.
When it comes to loops disappearing out of the field of view, then
that is almost certainly due to a pin not glued to the base. It
works for a little while, but when you start rotating it, gravity
sets in, and it jumps randomly. The solution is quite simple. Just
a tiny drop of super glue between pin and base, wait long enough
for it to set, then rotate.
As for empty loops, what Eddy Snell described is very likely if
your loop is taken to the cryo stream first, with the stream
obstructed. Eddy gave an accurate description of the problem. It
might be better to put the loops in a tube in a small open dewar to
avoid exactly this problem, then take the tube to the goniometer.
Many different ways of achieving the same result. Isn't research
wonderful!
Pierre
**********************************************************************
*********
Pierre Rizkallah, Daresbury Laboratory, Warrington, Cheshire WA4
4AD, U.K.
Phone: (+)44 1925 603808 Fax: (+)44 1925 603124
e-mail: [EMAIL PROTECTED] html: http://www.srs.ac.uk/px/pjr/
-----Original Message-----
From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On Behalf
Of Mark J. van Raaij
Sent: 21 July 2008 18:47
To: CCP4BB@JISCMAIL.AC.UK
Subject: [ccp4bb] Spooky, moving crystals
Dear all,
in a recent synchrotron trip we had a problem with our crystals moving
after mounting them onto the goniometer, in some cases they moved out
of the beam and even out of the zoomed camera picture - it seemed the
pins, upon equilibrating to room temperature, extended. It happened
with pre-mounted litho-loops only, not with pre-mounted mitegen loops
on the same trip, so one possible cause is different metal allows used
in the pins, somehow the mitegen ones being more suitable.
We used two-component glue to stick the pins into the metal bases
(Spine), so that might be another possible culprit. Perhaps we did not
allow sufficient time for the glue to react before freezing into
liquid N2 and it continued its reaction upon thawing, somehow pushing
the pin a bit out of the base. In this case the difference between
litholoops and mitegen loops may have been the thickness of the pins,
the latter somehow allowing expansion of the glue along the sides, the
former not.
In any case, I am wondering if any of you has seen this before, so we
know how to avoid it in the future.
In some cases, it took 10-20 min. for the crystal to stop moving,
which, with the current data collection speed and robotic mounting, is
significant. Fortunately, it did not affect our trip too much, as we
has sufficient time in the end.
Greetings,
Mark
Mark J. van Raaij
Dpto de Bioquímica, Facultad de Farmacia
Universidad de Santiago
15782 Santiago de Compostela
Spain
http://web.usc.es/~vanraaij/
