The dye sold by Sigma is not methyl green. It is
ethyl green, which is better. They call it methyl
green on the label, but with the correct CI number
(the one for ethyl green). Ethyl green can be found
also under its right name in the Sigma catalog, with 
a cross-reference to the methyl green entry (which has 
ethyl green as a synonym, and the correct molecular
formula for ethyl green). I don't know why they
go on calling their ethyl green methyl green. The
latter is a dye nobody would want these days because 
of its inevitable contamination with crystal violet. 
The CV originates during manufacture of MG and is 
also produced by spontaneous degradation in the stored
dye powder. Ethyl green is made in a different way
and it is also much more stable than methyl green.

If anyone sells real methyl green, it must be from 
very old stock because that dye has not been
manufactured for many (?30-40) years. See Floyd Green: 
Sigma-Aldrich Handbook of Stains, Dyes and 
Indicators (1991) for explanation of why ethyl
green in not contaminated and does not change
slowly into crystal violet. The differences between
the two dyes are explained also in both the 9th 
and the 10th editions of Conn's Biological Stains.

The staining method for DNA & RNA has, in reality,
been ethyl green-pyronine for many years. Recognition
of this fact in staining manuals and catalogs is 
long overdue. A standardized EGP method (Hoyer et al
1986 Histochem. J. 18:90-94) is technically simpler 
than MGP procedures that date from the days of real
methyl green.
-- 
-------------------------------
John A. Kiernan
Department of Anatomy and Cell Biology
The University of Western Ontario
London,   Canada   N6A 5C1
   kiernan[AT]uwo.ca
   http://publish.uwo.ca/~jkiernan/
   http://instruct.uwo.ca/anatomy/530/index.htm
_______________________________
Gayle Callis wrote:
> 
> Following the thread here.
> 
> One can purchase methyl violet free Methyl reen from Sigma.  This avoids
> the messy procedure to remove methyl violet using toxic chloroform and a
> separating funnel inside a fume hood. We changed to Sigmas MG instead of
> taking a chance other companies methyl green dye lots were not MV free.
> 
> Gayle Callis
> MT,HT,HTL(ASCP)
> Research Histopathology Supervisor
> Veterinary Molecular Biology
> Montana State University - Bozeman
> PO Box 173610
> Bozeman MT 59717-3610
> 406 994-6367 (lab with voice mail)
> 406 994-4303 (FAX)
> 
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> Histonet mailing list
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