I like that.
I think you are a bit optimistic with the time though. 30 to 60 seconds? I can hold my breath for 90+ seconds ...or at least I could when I was in my 20s.
Correct but when you hold your breath after a deep breath the pressure (partial pressure of oxygen) is greater as the pressure in your lungs is greater than normal because you have a large volume of air in your lungs and are holding it. Thus you can remain conscious for a greater time than normal.
60 to 90 seconds of pure nitrogen will do the trick for unconsciousness but it must be continued for several minutes after to ensure death.
You can accomplish a change of the gas mix from normal atmosphere to pure nitrogen without a mask or the knowledge of the subject. The only clue is a light headed feeling as you pass out from insufficient oxygen. Totally odorless and tasteless. The standard cyanide gas chamber has a distinct odor and takes 3 to 13 minutes. The subject is aware of the effects and there is a toxic gas to clear before the body can be removed. The “holding your breath” to avoid the poison was usually an exercise in panic. That doesn’t occur with nitrogen.