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. Paraforaldehyde (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. Erronously, the name "paraformaldehyde" was stuck to denote formaldehyde obtained by this way. It really defined a solution of formaldehyde without methanol. Nowdays 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 ampulles, 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, erroneus, since by its chemical nature it is a polymer with poor crosslinking properties. Zbigniew Darzynkiewicz
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