A while ago, there was a discussion re how long a formalin fixative for
EM could be stored before being considered unsuitable. By chance I came
across an article which addresses this point :

Mount  SL., Schwarz JE and Taatjes DJ.  Prolonged storage of fixative
for electron microscopy: effects on tissue preservation for diagnostic
specimens.
Ultrastructural Pathology  21:   195-200.  1997. The abstract states:

"Prolonged storage of Karnovsky's fixative, at both 4 and -20 degrees C,
is possible in the electron microscope setting. Ultrastructural detail
was not compromised in specimens processed in fixative that had been
stored for 6 months. Evidence of smooth muscle and neuroendocrine
differentiation was present in the form of actin filaments/dense bodies
and neurosecretory granules, respectively. No difference in preservation
was detected between specimens fixed in freshly prepared Karnovsky's
fixative  and fixative that had been stored at either 4 or -20 degrees C
for up to 6 months. Thuis community hospitals can be provided with
Karnovsky's fixative on a semiannual basis for surgical pathology
specimens that may require electron microscopy for diagnosis."

I guess those of us who prefer to make the fixative up fresh are being
somewhat conservative.


Barry