Re: Carnoy's fixative

From:"J. A. Kiernan"

On Wed, 26 Sep 2001 wrote:
> Where can I purchase Carnoy's fixative in MA?

Why buy? 

Carnoy is made in the lab from its 3 cheap
ingredients. The only other items needed
are a glass measuring cylinder and a glass
jar to put it in. Some types of plastic, such 
as polyethylene, are OK instead of glass. 
Avoid polystyrene, cellulose esters and other 
plastics that are attacked by organic solvents.

Carnoy is:

 Ethyl alcohol  60 ml
 Chloroform     30 ml
 Acetic acid    10 ml

It is one of the best fixatives for animal tissues
that will be embedded in paraffin. Microanatomical
preservation is much better than that obtained
with aqueous formaldehyde solutions. The chemical
composition of Carnoy changes with time, and stored
Carnoy has the fruity smell of ethyl acetate. This 
may not matter. With aged alcoholic Bouin-type 
fixatives the presence of ethyl acetate and other
reaction products improves structural preservation
of some tissues (see papers by RE Gregory, 1980 in 
Stain Technol. 55: 143-149 and 151-160). 

A commercially marketed Carnoy that's more than
a few days old will not be the same as a fresh 
lab-made fixative. This may not matter, but has
there been an investigation of aged Carnoy that's
comparable to Gregory's 2 papers on aged alcoholic
Bouin? If it costs less to buy pre-mixed Carnoy
than it does to make your own, try it out on 
some unimportant specimens. Rat or mouse kidney
and brain are good test-tissues for fixatives
(see Histochem. J. 17: 1131-1146, 1985 for 
illustrated criteria of good vs bad fixation).

The chloroform in Carnoy may not be necessary.
Clarke's fixative, which dates back to the
1850s, is a 3:1 mixture of alcohol and glacial
acetic acid. It has performed as well as or 
better than Carnoy in published comparisons
of fixatives.

All the statements in the preceding paragraphs
relate to appearances in paraffin sections.
For frozen sections or resin embedding, you
need a cross-linking aldehyde fixative such as
formaldehyde, glutaraldehyde or acrolein, but
that's another collection of stories.

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

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