You discard your isopentane every 3 months? Is this stuff you've been 
using for freezing, and you keep using the same isopentane? And it 
stays cold the whole time?
I hope not.
The cold isopentane is getting enriched in oxygen as it's being used, 
and this heightens the fire hazard.
Why not freeze into (all right, everyone who's read my rants before 
can ignore me now) slush nitrogen? Colder, faster freezing, better 
freezing, and no flammable/explosive liquids or gases involved.

Phil

>Dear Steve, I presume you are freezing muscle biopsies, or possibly
>research material.
>This isopentane/liquid nitrogen method is more an art than a science.
>Altitude of your lab and humidity of your ambient air, are the 2 major
>limiting factors.  The most important thing in the freezing is not
>measuring the temperature but rather visual assessment of the state of
>the isopentane.  We put the isopentane in a 250 ml stainless steel
>beaker and lower it slowly into the liquid nitrogen.  We wait to freeze,
>until the isopentane has formed ice all around the walls of the beaker
>and all the way up the wall of the beaker, and completely across the
>bottom of the beaker.  It will be syrupy.  We use 25 seconds of freezing
>(I know the books say 5-10 seconds).
>Our lab is at sea level.  It is also often humid here.  We discard the
>isopentane every 3 months when there is no major humidity and more often
>(every 6 weeks) when we have a run of humid weather, or when we have a
>problem with freezing artifact, or when I think my assistant has been
>sloppy with its storage.
>We keep the beaker very clean i.e. we scour with steel wool.  We make
>sure there is no residual water in the beaker (I keep 2 beakers so one
>is freshly clean and dry at all times).
>I have not used the thermometer since I was a young attending a long
>time ago!!!
>Best
>Karen
>
>Karen M. Weidenheim, M.D.
>Professor of Pathology, Clinical Neurology and Clinical Neurosurgery
>Albert Einstein College of Medicine
>Chief, Division of Neuropathology
>Montefiore Medical Center
>111 East 210th Street
>Bronx, NY   10467
>(718) 920-4446
>FAX (718) 653-3409
>Beeper (917) 556 3696
>CONFIDENTIALITY NOTICE:  This email, including any attachments, is for
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>
>>>>  Steven Coakley  7/14/2005 12:07:15 PM >>>
>We're doing allot of liquid nitrogen/isopentane snap freezing and I'd
>like to purchase a good low temp thermometer.  Any suggestions or advise
>out there?
>
>Steve
>
>
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-- 
Philip Oshel
Supervisor, BBPIC microscopy facility
Department of Animal Sciences
University of Wisconsin
1675 Observatory Drive
Madison,  WI  53706
voice: (608) 263-4162
fax: (608) 262-5157 (dept. fax)
http://www.ansci.wisc.edu/microscopy.htm

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