Polymethanal (paraformaldehyde)
Polymethanal is a common fixative in biology. It is used to reduce degradation in cells, tissue and entire organisms before further experiments, e.g. antibody staining.
Recipes for polymethanal buffers
Some people make 16 or 24% stocks to be diluted at a later point. But the solid polymethanal / PFA is very hard to dissolve at these high concentrations. Stocks or working solutions can be preserved by freezing for later use. Otherwise, methanal reacts with itself to form methanol and methanoate (acidification).
PFA 4%
for 50ml
2.0 g PFA +5 ml H2O +drop of NaOH (0.5-1.0M) +45 ml PBS (correct pH to 7.4 with HCl)
PFA 1%
for 10ml
0.1 g paraformaldehyde powder in small glass tube +0.5 ml distilled water +drop of 0.5-1.0M sodium hydroxide heat to ~80°C for 2-3 mins shake in water bath until PFA dissolved (beaker of very hot water) +9.5ml PBS correct pH with HCl if necessary
Too many names
There's plenty of confusion regarding this chemical because too many synonyms exist. Paraformaldyhde is probably still the most common term among biologist, but it ignores attempts to introduce meaningful and systematic names in chemistry. Polymethanal is clearer: poly - polymer, meth - single carbon, -al - aldehyde group.
Here's a list of equivalent terms: polymethanal, polyoxymethylene, polyformaldehyde, paraformaldehyde, paraform, polytrioxane,.. (regards from Babel) Common abbreviations are PFA, pMeO.
CAS number = 30525-89-4
Comparison with methanal
Polymethanal is often preferred over methanal which is also used in fixation of live material. This is because polymethanal is a solid which makes it easier to transport and a little safer to use. The chemistry of fixation is similar due to the partial depolymerisation of polymethanal in the buffer making process (basic hydrolysis).
Chemistry of fixation
The aldehyde group, especially of methanal, is very reactive. It readily combines with amino groups to amides or with alcohol groups to esters. These reactions are mostly irreversible. Unreacted molecules have to be removed prior to subsequent experiments since they easily damage proteins like antibodies.
Polymethanal safety data sheets
Links
Wikipedia
