Or just fill up his chamber with carbon monoxide and he just goes to sleep forever. Quick, cheap and painless.
People have been offing themselves by this method for a long, long time now.
That's what I thought until watching a show recently which claimed that one of the symptoms can be painful abdominal cramps.
I would think the nitrogen might work well. I heard about a man who mistakenly plugged his air mask into a nitrogen source before entering a deposition chamber. He passed out and was removed from the chamber.
No cause was determined for his passing out so, when he sufficiently recovered, he entered the chamber again and passed out a second time. Only then was the mistake with his air line discovered.
I didn't hear that he suffered any pain.