[3dem] Gatan Cryo-Holder

[DocDave50 at aol.com]

Hi Heater,
 
At Yale, we have the 626 which is very stable in terms of temperature,  
vacuum, etc.  In our runs, we constantly were able to achieve well less  than 
0.5A/sec drift throughout an entire session.  
 
One issue that we did experience and, in talking with others  found this to 
be true, is that there is a slight tilt in the holder off of a=0.  I 
noticed this because with a set to 0, the dewar did not hang parallel with the  
microscope center line.  I measured the tilt angle using an  unused thin holey 
carbon grid at room temperature.   The  angular deviation was something 
approximately 5.5o when the holder is set to  a=0. This was determined by 
setting focus at one  side of the grid orthogonal to the tilt axis and then 
refocusing on the other  side the grid using Da.  The same measurement was  made 
along the tilt axis and yielded little to no significant Da.  This was done 
a number of times because it was  clear that the dewar was not hanging 
parallel with the center line of  the microscope and it is possible that either a 
grid was partially bent or the  clip ring not set correctly.  In many 
applications, this slight angular  deviation may not represent a problem.  On the 
other hand, I do find  it to be of concern since a=0 should be better  
controlled.
 
Another issue that we had with our holders was the clip ring  assembly.  In 
the older 60o Gatan Cryo-holders, the clip ring had a slight  screw thread 
to insure that it was tightly onto the grid at -192C.  In the  70o holder, 
on the other hand, there is a tension clip in the tip of the holder  which 
gets loose over time.  While at room temperature you can hear and  feel the 
clip ring insert, at -192C, it was very difficult to ensure that the  clip 
ring was in place such that it wouldn't be lost in the microscope.   Other than 
the obvious issue of having to retrieve a dropped clip ring from  
compustage assembly, a loose clip ring will contribute to significant drift  rates.  
As a testimony to this issue, during one repair to our  compustage on the 
Tecnai 20, 6 clip rings were found inside the Cryo Box.   Unfortunately, lost 
clip rings are something that user's seem to fail to report  to the facility 
manager until there are no clip rings remaining in  inventory.
 
Just a thought.
 
David W. Chester, Ph.D.
currently unemployed due to funding losses
 
Visiting Research Faculty
Department of Cellular and Molecular Physiology
Yale University School of Medicine
333 Cedar Street
New Haven, CT.   06520
_docdave50 at aol.com_ (mailto:docdave50 at aol.com) 
(203) 213-4167
 
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