[3dem] Contamination rates

[Bill Tivol]

On Nov 12, 2008, at 2:56 PM, Ruben Diaz-Avalos wrote:

> I am quite interested in knowing if people have carried out  
> measurements of contamination rates for different microscopes, and  
> what are the results of their measurements, to find out how the  
> different instruments fare in this test. At the NYSBC we have both  
> JEOL and FEI instruments, and we measure significantly different  
> contamination rates between the two brands, therefore I would like  
> to know if we are observing a common trend.
>
> By contamination rate in this case, I mean the rate of accumulation  
> of water on the grid's surface per hour (in nanometers per hour). A  
> low contamination rate obviously is very desirable, since only then  
> a sample can be used for a long session of data collection.


Dear Ruben,
	When our Tecnai scopes were installed, the contamination rates were  
measured as a part of the acceptance tests.  These rates were slower  
than the specified rates for both instruments, but I don't have access  
to the actual numbers at present.  I have not seen significant build- 
up of contamination on the T12 during an entire day of observations-- 
about 8 to 10 hours--and I have been able to collect data for a few  
days in a row on the Polara at LN2 temperature, but there is  
observable contamination from N2 at LHe temperature within several  
hours.  The N2 contamination can be removed by withdrawing the  
specimen into the insertion rod and leaving it at LN2 temperature for  
a few minutes.  If the specimen is left in the column over the weekend  
at LN2 temperature, there will be noticeable contamination, but  
specimens left in the multi-specimen holder during that interval or  
longer and maintained at LN2 temperature look as good as fresh  
specimens when they are introduced into the column.  I don't remember  
any of the users of the Polara finding that their specimens have  
become contaminated while sitting in the multi-specimen holder, so I  
cold not say what the maximum time until noticeable contamination  
might be.  I also have not seen any observable contamination on the  
UEM (a TF20), where I am now working, but I have not had any sessions  
longer than about 8 hours.  I was not working at the UEM when it was  
installed, so I don't know what the contamination rate was found to  
be, although I am pretty sure that it was measured during  
installation.  I have no experience with JEOL instruments, so I cannot  
make any comparisons.
Yours,
Bill Tivol, PhD
EM Scientist
Ultrafast EM Facility
Noyes Laboratory, MC 127-72
California Institute of Technology
Pasadena CA 91125
(626) 395-8833
tivol at caltech.edu