The cost to use an Elder Sign to seal a gate is 1 Sanity and 1 Stamina. If you are at 1 on either one of these (or both), can you essentially sacrifice yourself in order to use it? Will the sealing occur before you become insane/wounded/devoured?
This is explicitly allowed in the rule book, p18
To use an elder sign, do the following:
The player removes 1 Sanity and 1 Stamina from his investigator sheet. This may knock the investigator unconscious or drive him insane, but the elder sign still takes effect.
Pat has come up with an authoritative answer, but even if the rulebook had left things ambiguous, that scenario sounds so atmospheric that, if it were my house and it came up, I would definitely want to allow it.
Arkham Horror is a game that thrives on its atmosphere: if something so cinematic was disallowed by the rules, I'd feel a bit cheated by them!