Hi, I am not sure if this is the correct section for this question but it seems to involve both physics and chemistry so I will start here. In a recent conversation a friend stated that no substance within the earth's atmosphere can be 100 percent pure. It needs to be in space or some other kind of vacuum. Is this true? I don't know enough about physics or chemistry to refute the claim but it didn't sound correct. The context was in a discussion about deet bug spray. The aerosol spray can stated the contents were 99.9% deet. Then my friend stated the above as the reason why it could not have been 100% deet. Thanks for any help.