We had exactly the same - and there were two culprits: 1. frozen gas trapped between the jacket of the pre-mounted Hampton loop and the loop itself 2. ice on the base of the pin. When the ice slowly melted (on contact with the goniometer head at room temp.) the base shifted by infinitesmally small amount - enough to drive the displacement at the end appreciably. Depending on the amount of ice you may experience angular, lateral, or vertical movement.
Drying the pins before use eliminated most of the issues. Artem > 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/ >
