Nitrogen Asphyxiation - causes rapid and painless onset of unconsciousness. It is so rapid and unnoticeable that there is a specific warning for high nitrogen environments in the safety sheets
Strap the condemned down,
Put on a brainwave monitor
put on a medical breathing mask
hook up to a 99% nitrogen gas supply
flow nitrogen at 1.1 atmospheres
Wait till the brainwave monitor has shown zero activity for 5 min