Postembedding EMICC problem

[whitnall at usuhsb.usuhs.mil]

I am having the following problem with postembedding labeling
of LR White sections. After the peroxidase/DAB reaction,
I make the reaction product electron-dense with a brief
incubation in sodium gold chloride. If I examine the grids
immediately after rinsing and drying, they're OK. If I
let them sit (in atmosphere, at room temp) for a few days,
they are covered with a precipitate that looks exactly
like colloidal gold. Round, black spheres from a few nm
to ~15 nm in diameter. This precipitate is only over tissue,
not empty plastic. This just happened last weekend, when
the humidity was quite high. The tissue is unosmicated.
No other heavy metals were used, i.e., there was no
contrast enhancement with lead or uranyl salts.
Any suggestions?   

--------------------------------------------
Mark H. Whitnall, Ph.D.
Dept. of Physiology
Armed Forces Radiobiology Research Institute
8901 Wisconsin Avenue
Bethesda, MD 20889-5603

tel (301) 295-0277
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