Tom,
Caveat Emptor: My experiences with urea are based entirely on
protein folding experiments, not gels. In general, urea goes bad because
of formation of cyanate and ammonia with time. The equilibrium between the
three is temperature-dependent and is larger than one might guess (up to 20
mM cyanate for 8 M urea). See, for example, Advances in Protein Chemistry,
1985, p. 306.
Bio Rad has references to urea cleanup with their mixed-bed resins.
In the past, I have used AG501-X8 to remove the cyanate and ammonia. One
can either stir the resin with the solution and filter, or pour the urea
through a column of the resin. My hunch is that the stirring method is
better for critical application because the rate of reaction is with the
mixed bed exchanger takes several hours to go to completion, according to
Bio Rad's brouchure.
I can't say empirically how long this will stay clean because I
haven't had much success with the colorimetric tests for cyanate (can you
post what test you are using?). However, some years ago, Boehringer
Mannheim published a technical note which claimed that the resin-treated
urea was good for at least a week at 4 ƒC. Storing the solution frozen in
small batches is another option.
Hope this helps.
Chris Halkides
Christopher Halkides
Dept. of Chemistry, UNCW
601 S. College Road
Wilmington, NC 28403-3297
(910) 962-7427
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