On Tue, 23 Jan 2001, Jane Radford wrote:

> Can Periodate lysine paraformaldehyde fixative be frozen?

You should not freeze any solution containing formaldehyde. 
Low temperatures (even without freezing) accelerate
polymerization and a white precipitate of paraformaldehyde
forms in the bottle. This lowers the concentration of
dissolved formaldehyde (which may not matter much) and
gets insoluble paraformaldehyde onto and sometimes also
within the specimen. This remains there and can make for
difficult sectioning.

Two of the ingredients of PLP (lysine and formaldehyde)
react chemically when mixed, so the working fixative
solution should be made immediately before use. You
can make concentrated stock solutions (lysine, sodium
periodate, phosphate-buffered formaldehyde) and mix these
when needed. There is no advantage in keeping the stock
solutions cold (except possibly the lysine, which might
get infected).

Note that the name PLP was poorly chosen by the inventors
of this mixture, because the solution does not contain
any paraformaldehyde. For a good paper on how it works,
see DC Hixson et 4 al 1981 J Histochem Cytochem 29:561-566.

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