trace metals in minimal media

[J. P. Germanas]

   We're trying to produce labeled proteins by expression in
E. coli in defined media, and have had inconsistent results.
We've thought of adding a solution of trace elements (e.g.
Fe, Co, Mo salts) to the media. Does anyone have any 
experience with E. coli using the trace element solution,
and have a reference they could share for the recipe
for such a solution?

                           Thanks much

-- 
J. P. Germanas