Once the paraformaldehyde solution is prepared, it should be kep in refrigeration (at about 5ºC).
  René J.

"Johnson, Teri"  wrote:
  I had a question today from a researcher asking about the stability of a
formalin solution made from Paraformaldehyde. She indicated that on one
of her samples, freshly prepared fixative worked well, while one day old
fixative (same batch) did not fix properly. She assures me all
pre-fixative steps were the same. The samples were fixed for 2 hours. I
have no details about her experiments or what she was testing (I suspect
mRNA targets). 

I realize that without the addition of methanol, the solution is not as
stable would would re-polymerize with time. I'm wondering if the
fixative started re-polymerizing and that made the difference, but one
day? Is it possible for one day to make the difference between a
positive and negative result? 

How stable/instable is methylene glycol?

I'm also thinking that by adding methanol (or using purchased 100%
formalin (37% formaldehyde)) would be useful for mRNA targets, since
aldehydes tend to fix nucleic acids poorly.

Why do all my most difficult questions arise on Friday afternoon?

Thanks for any insight you can give me.


Teri Johnson, HT(ASCP)QIHC
Managing Director Histology Facility
Stowers Institute for Medical Research
1000 E. 50th St.
Kansas City, MO 64110

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