I was doing my last bit of research tonight before buying a CO2 tank and building my euthanasia chamber tomorrow when I stumbled upon a disturbing article. You can read it for yourself here
http://www.upc-online.org/nr/33005co2.htm
but the cliff notes are that CO2 suffocation is very painful and considered inhumane. The upshot is that suffocation via nitrogen or argon is NOT painful and found to be much more humane mode of euthanasia. Since nitrogen is lighter than O2 it would be pretty hard to build an efficient chamber but argon gas is HEAVIER than O2 just like CO2 so would work in the same way as a CO2 Chamber. .8 Ar .2 CO2 is readily available as a welding mix. I am going to try this.............any thoughts?