You strain out the impurities so that only raw succubus-ness remains. It takes squeezing the succubus concentrate out of several succubi to have enough for one pure succubus.
Although if you mean "pure" as in "not corrupted by lust", then they have to be virgins at some point, right? There's a lot of variation on just how demonic demons are, with some just being humans with wings and a tail.