Folks,

I have used prepared paraformaldehyde solutions from Electron 
Microscopy Sciences very happily for years, so if you don't want to 
bother making it, I suggest:

Paraformaldehyde Solution -16% (Formaldehyde Solution, 16%)
CAS #30525-89-4
Prepared from Paraformaldehyde crystal, 16% aqueous solution, 
methanol free. Microfiltered and stored in pre-scored ampoules.

http://www.emsdiasum.com/ems/chemicals/nma.html#15700

I have no interest the company. The following is from the Purdue 
Cytometry list:

From: Zbigniew Darzynkiewicz
Formaldehyde (CHOH) is a gas. A solution of approx. 37 % (by weight)
formaldehyde (gas) in water, generally with 10-15 % methanol added to
prevent polymerization is called formalin, also called "full strength
formalin", "100 % formalin" or Formalin 40 (to denote that there is
about 40 g formalin in 100 ml of solution). This is the widely
available solution which after appropriate dilution with water or PBS
is commonly used as a crosslinking fixative.
Paraformaldehyde (polyoxymethylene) is a polymerized formaldehyde and
as such cannot be used as a fixative.
The confusion regarding the names stems from the fact that in some
fixative solutions the presence of methanol is undesired. It was
common, then, in the past to prepare oneself the formaldehyde
solution from paraformaldehyde, by boiling it. Erroneously, the name
"paraformaldehyde" was stuck to denote formaldehyde obtained by this
way. It really defined a solution of formaldehyde without methanol.
Nowadays many vendors (e.g. Polysciences) sell a solution of
formaldehyde (37 - 40 %) without methanol. I presume that they add
some other stabilizer (in lieu of methanol) which prevents
formaldehyde polymerization. They do not specify the stabilizer,
although the solution being sold in ampules has no contact with
oxygen (the solution in ampules, I was told by the vendor, is packed
in under nitrogen rather than in air).
To avoid a confusion I would suggest that describing the fixative one
should specify whether it is formaldehyde with methanol (e.g. 1 %
formaldehyde and 0.2 % methanol) or formaldehyde without methanol. In
most applications the presence of methanol is acceptable. The name
"paraformaldehyde" to define a fixative is, of course, erroneous,
since by its chemical nature it is a polymer with poor crosslinking
properties.


>Dear Confocalists,
>
>does anybody know of a method and a supplier for preparing Formaldehyde
>solutions (4%) other than weighing out (and inhaling) Paraformaldehyde
>powder? We tried Paraformaldehyde tablets from Merck, but after
>dissolving them the solution still remained turbid, so we didn't want to
>use it. Formaldehyde in solution (37%, also from Merck) is stabilized
>with 10% Methanol, and it seems that cells which are fixed with this
>(diluted to 3.7% Formaldehyde / 1% Methanol) show a lower fluorescence
>intensity after protein detection than cells fixed with 4%
>Paraformaldehyde solution.
>Could anybody comment on the effect of the 1% Methanol? Are Formaldehyde
>solutions otherwise chemically identical to dissolved Paraformaldehyde,
>or are there any differences?
>
>Thank you for your efforts,
>
>Joachim
>
>
>--
>------------------------------------------------------------------
>Joachim Walter, Dipl. Phys.
>Institut für Anthropologie und Humangenetik der LMU München
>AG Cremer
>Richard-Wagner-Straße 10/I
>D-80333 München               Tel. +49 - 89-2180-6713
>Germany                       Fax. +49 - 89-2180-6719
>------------------------------------------------------------------


Mario M. Moronne, Ph.D.
NanoMed Technologies
Berkeley, CA
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