I probably shouldn't have used the word key. We are not intending to put together a database per se (we might later but we certainly wouldn't use CAS as the primary key). We really just want assert that 'Post ID X is about a material that is the same as CAS/InChi/PubChem SID/ChemSpider ID Y' and the same for whatever identifiers we can find. Its not a primary database key in any sense, just the most practical search term that I'm after.