Dear Atoska,
	You can determine the usability of the fixative by a simple test. 
		1.  check the pH.  If holding close to 7, then it is
probably OK
		2.  check with Schiff's Reagent, used in PAS.  If you get
the same result with old as new fixative, then it is probably OK
		3.  do a visual check.  If there are any flakes of white
'snow' suspended in the fluid or dusting the bottom of the bottle/flask,
then heat to 60oC in a hood with stirring.  If the 'snow' disappears, the
fixative is probably OK.

	Paraformaldehyde is a polymer of formaldehyde.  When you prepare
what you call "4% PFA", you are taking 4g of paraformaldehyde suspended in
100ml of water, raising the temperature to around 60oC with stirring, and
adding a little bit of NaOH to raise the pH so that the paraformaldehyde
will depolymerize into formaldehyde (monomer).

	What you heard was that the 4% PFA would revert back to
paraformaldehyde.
  
	Let's expose the foolishness of errant nomenclature one more time.
If you had 1,000,000 molecules of formaldehyde in 100ml of water, and you
let them polymerize into 10,000 molecules of paraformaldehyde (by
evaporation of water), you would have reduced the number of molecules by a
factor of 100.  You would also have reduced the number of aldehyde molecules
by a factor of 100.

	If you had started with the 10,000 molecules of paraformaldehyde,
had raised the temperature to 60oC and added a little NaOH, you would have
generated 1,000,000 molecules of formaldehyde.  Which would be the stronger
aldehyde fixative?

	Question:  If I wrote an article in which I stated that I fixed
skeletal muscle in 4% paraformaldehyde, what would a graduate chemistry
major do to prepare a fixative as I had described it if s/he had at hand a
bottle of 10% aqueous formaldehyde and a jar of paraformaldehyde?

	I apologize for the blunt prose, but you are the victim of one of
the great hoaxes of 20th century biology.  It is like a little secret from
which all the in's gain superiority over the 'outs'.   A silly little game
that we should grow out of before we become old and grumpy.

Hope this helps,

Fred Monson

Frederick C. Monson, PhD   
Center for Advanced Scientific Imaging
Schmucker II Science Center
West Chester University
South Church Street and Rosedale
West Chester, Pennsylvania, USA, 19383
Phone:  610-738-0437
FAX:  610-738-0437
fmonson@wcupa.edu
CASI URL:  http://darwin.wcupa.edu/casi/
WCUPA URL:  http://www.wcupa.edu/
Visitors URL:  http://www.wcupa.edu/_visitors/
 

> ----------
> From: 	Atoska S. Gentry
> Sent: 	Monday, May 20, 2002 2:52 PM
> To: 	histonet
> Subject: 	4% PFA/PBS
> 
> 
> Dear All, will someone please provide me information on the shelf life of 
> 4%PFA/PBS?  I have a few liters of this unused fixative left over from 
> various experiments.  And I'm wondering if it can be used for routine 
> fixation or if I should just treat it for waste disposal?  A couple of 
> years ago I heard by word of mouth that after a certain period of time the
> 
> PFA reverts back to  regular formaldehyde.  Does anyone have a reference 
> you can recommend whereby I can get my questions answered?  On the other 
> hand if you know for a fact that it can be used as  either formaldehyde or
> 
> 10% formalin please will you share this info with me?  If it can be used 
> does it need to be stored at 4C?  Thanks, Atoska
> 
> 
> 
> 
> 
> 
> Atoska S. Gentry B.S., HT(ASCP)
> Research Assistant III
> Scott-Ritchey Research Center
> College of Veterinary Medicine
> Auburn University, AL  36849
> Phone# (334)844-5579  Fax# (334)844-5850
> 
> 
>