[Doormatthew]

That seems like a much better option than Create, especially since most biochemical substances are effective in small amounts and the point of Create is creating large amounts of your chosen substance, rather than enough to coat a single quill.

I'm not sure how to create the limitation that the character must understand the substance they are creating. I suppose you could require a roll against the appropriate skill (Poisons, for example), but considering that requiring an attribute roll is only worth -5% to -10%, and having a high attribute is much more difficult than a high skill, this probably wouldn't be worth very much.

As an alternative, GURPS Powers has rules for giving your advantages Minor Benefits and Drawbacks. Rather than purchasing a specific limitation, you could just find some equally minor benefit as a trade-off.