You need to go back to the books on formal-stuff.

   1. Formaldehyde is a gas.

   2. Formalin is the gas dissolved in water: 37-40%. (It's very soluble.)

   3. Paraformaldehyde is a solid polymer of formaldehyde. It is
      depolymerized by heating to 60C, usually in a buffer at pH 7.4.
      It is a source of pure formaldehyde; formalin contains about
      10% methanol and traces of formic acid (which increase in
      amount with the passage of time, on a scale of years).

   4. 4% formaldehyde = 10% formalin (approximately).

-----------------------------------------------------------

On Fri, 13 Oct 2000, BrosnanWatters, Gayle wrote:

> Thanks John.  I am really dumb, however, and now I have another question.  I
> had been told that the perfusate could not contain formaldehyde, but had to
> be formalin (my friend at Syracuse University who wrote a paper on the Tago
> back in the 90s told me that).  So my fixative has formalin and
> glutaraldehyde - so is formalin okay?  I have done the Tago my self back in
> 1991, so I know how to do it, but we used fresh tissue at Syracuse.  Fresh
> does work, but I thought I'd prefer fixed.  By the way, Kutcher's paper was
> well-received - he figured out that the most important thing was to have the
> pH correct.  
> 	Thanks for helping the dumb psychologist!
> 	gayle
> 
> > ----------
> > From: 	J. A. Kiernan
> > Sent: 	Friday, October 13, 2000 7:09 AM
> > To: 	BrosnanWatters, Gayle
> > Cc: 	'histonet'
> > Subject: 	Re: Tago technique for acetylcholinesterase
> > 
> > On Thu, 12 Oct 2000, Gayle Brosnan-Watters wrote:
> > 
> > > Can anyone tell me if it is okay to perfuse mice with a perfusate
> > containing
> > > glutaraldehyde when I am going to do the Tago technique for
> > cholinesterase
> > > staining?  
> > 
> > Yes.
> > 
> > The original paper (Tago, Kimura & Maeda 1986 J Histochem Cytochem
> > 34: 1431-1438) says fix for 1 to 4 days in 1 to 4% formaldehyde, 
> > with or without 0.5 to 2% glutaraldehyde. Alternatively, fix in 4%
> > formaldehyde and 0.2% picric acid in 0.1M phosphate buffer, pH 7.4.
> > 
> > The principle of this method is that you use a 100X diluted medium
> > of the Karnovsky-Roots type. The reaction product (copper ferrocyanide)
> > has a peroxidase-like activity, and is amplified by a DAB-H2O2
> > incubation. It is sometimes necessary to inhibit endogenous peroxidase
> > activity, especially in unfixed cryostat sections, by putting them
> > in 0.1% H2O2 for 30 minutes before starting the AChE incubation.
> >  
> > 
> >  John A. Kiernan,
> >  Department of Anatomy & Cell Biology,
> >  The University of Western Ontario,
> >  LONDON,  Canada  N6A 5C1
-----------------------------------------------------

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1