[3dem] RE: TEMs and sample preferences

[Shi, Dan (NIH/NCI) [E]]

Dear Liz,
A 120 kv microscope has limited length of free path(depth penetration without energy loss) and the beam coherence of a LaB6 filament decays pretty quick after a couple of hundreds hrs.  Most people use it for thin and low contrast specimens at medium resolution.

Since cryo-EM normally means low-dose, it should have lowest contamination rate comparing to plastic thin section and negatively stained samples, and the most of contamination generated from the frozen-hydrated specimen would be water. With my experiences, if a plastic thin section specimen was coated carbon on both sides (or one side with C support film then coat C on other side), the contamination would be dramatically dropped since no plastic/resin molecule debris would fly from the section into the column and the charging would be minimized too. For data collection from negatively stained samples, I used low-dose mode to decrease radiation damage which may also limit contamination.
Good luck,
Kind regards,

Dan Shi, PhD
LCB, NCI/CCR
Bldg 50/4306
South Dr, Bethesda, MD 20892
USA

From: Wright, Elizabeth R. [mailto:erwrigh at emory.edu]
Sent: Thursday, November 18, 2010 10:31 PM
To: 3dem at ncmir.ucsd.edu
Subject: [3dem] TEMs and sample preferences

Dear 3DEM Colleagues,

We are in the final stages of setting up our two new microscopes and I have a little question about samples. The 200 kV microscope will be dedicated to the imaging of cryo-specimens. However, we also have a new 120 kV microscope. What are the current preferences about specimens to image with an instrument? The three basic specimen types we image in our EM core lab are cryo, standard sections, and negatively stained samples. Does anyone have information on the types and amounts of contamination that are associated with each? Along with maintenance costs (perhaps)? We also have another TEM we use for basic training and routine screening.

Any and all input is most appreciated.

Sincerely,

Liz
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