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: [log in to unmask] html: http://www.srs.ac.uk/px/pjr/
>
> -----Original Message-----
> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf
> Of Mark J. van Raaij
> Sent: 21 July 2008 18:47
> To: [log in to unmask]
> 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/
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