[ecnerwalc3321]

If a compound (tetraethylammonium bromide) has hardened over time but you can still chip away at it, is it possible to dry this compound in a vacuum oven and use it?  Or has the chemical composition changed so it's a different compound?

[Arkcon]

It really depends on how stable the compound is to heating, and some solids are volatile enough to be lost in a vacuum.  You'll have to look up each compounds and see how the general practice goes for drying a particular compound.  For this example, you can get a quick Google result:  http://books.google.com/books?id=PTXyS7Yj6zUC&pg=PA182&lpg=PA182&dq=tetraethylammonium+bromide+drying&source=bl&ots=ZwP-5EsxkC&sig=MRRYwFGnOFQv_ApAWFACRZ-pLU0&hl=en  They suggest recrystallizing before vacuum heating, which seems like a very good way to remove water and potential contaminants.