help with PFA fixation

[sthomaska at yahoo.com]

dear sir,

while I am well fixed tissue (rat-pup brain) for
vibratome sectioning, 4% PFA fixation for
immunohistochemistry is yielding poorly fixed tissue
even with over night fixation. 

Could I
       a. use 4% PFA in PBS instead of 4% PFA in   
           phosphate bufer.       
       b. increase the concentration of PFA as the
          powder is quite old.

Is their some other alternative.

Dr. Sunil Thomas,
National institute of mental health and neurosciences,
India. 



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