Re: Xylene substitutes (rather long reply)

From:"J. A. Kiernan"

> Valerie Hannen wrote:
> Does anyone know of any "great" xylene substitutes?
> We need one that is good for all aspects of processing.
> Processing, staining, coverslipping. It can't have a terrible
> citrus smell (tried one like that and it was horrible). Must
> be ok to use with IHC. 

The properties you list are those of xylene itself!

I don't think there's another inexpensive liquid
that mixes in all proportions, quickly, with alcohol,
melted wax and all the resinous mounting media.
There are other solvents that qualify (benzyl benzoate
and cedarwood oil come to mind), but they are too
expensive to use in large volumes. It may be possible
to avoid xylene by using 2 alternative solvents, one
for the transition between alcohol and wax, and another
for coverslipping - but that's not what you're asking for.

>  Our safety officer wants us to try
> "safer" clearing agents.

Xylene is probably as safe as anything else if you
are fairly careful about not setting it on fire
(alcohol is a much bigger risk - flash point 17C 
compared with 29C for xylene) or inhaling the
vapour either in high concentration (impossible
at room temperature) or for long periods at low
concentration (a real risk, but only in a small, 
badly ventilated lab, which should not be there
in an institution big enough to have a safety

The symptoms of acute and chronic exposure to
xylene (and of oral ingestion too) are very
well documented. The Merck Index (12th edn 1996)
has a substantial paragraph describing these,
with a reference to longer account in a book.
Every lab should have a Merck Index. Not having
that $50 volume is tantamount to criminal
negligence, in my humble opinion. 

You might consider asking your safety officer
whether xylene is any less safe than any other
solvents, given that it presents almost no
hazard at all if used in a lab that has a
healthy rate of replacement of its air. If
someone does experience symptoms, it is easy
to compare them with the established symptoms
of too much xylene. If xylene vapour is making
people feel ill, then it is the safety officers
duty to insist on adequate ventilation in the
building. The local code requirements for offices
or kitchens will probably be good enough for
a histotechnology lab.

Most of the less expensive alternatives to xylene
do not have the same miscibility with alcohol,
wax and (especially) resinous mountants, and nearly 
all are sold under trade names without any obvious 
disclosure of the chemical(s) of which they are 
composed. If your safety officer suggests a
proprietary solvent, ask him/her what it has in
it, what its flash-point is, and what the expected 
symptoms of overexposure are, or where this
information could easily be obtained.

In case you think I have a prejudice against
safety officers, please note that I have been one
myself, in university departments in Britain and
Canada. This office was (and still is) an unpaid
and time consuming chore, involving confrontations 
with ignorant and officious people who have somehow 
wormed their way into senior administrative 

Anyone whose full time paid job is being a safety
officer (in a place where all has been and still
is pretty safe) is someone to be VERY CAREFUL with.
You must always go along with their recommendations,
but leave a few infractions to be detected at the
next inspection. 

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

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