Quote Originally Posted by ambaker View Post
Nitrogen is relatively inert. It does combine with oxygen, NOx for dragsters etc.
N2 and NOx are very very different beasties. N radicals have huge reaction energies because N2 is at such a low-energy state. In other words, the very-inert nature of N2 (difficulty in making it react without bringing it up to very high temperatures and/or using catalysts) is the very thing that makes N compounds (NOx and azides for example) so powerfully reactive.

Quote Originally Posted by ambaker View Post
Argon is not only completely inert, it was the cover gas of choice for liquid sodium breeder reactors, it is heavier than air.
Argon is an excellent choice, but harder to find (in small, affordable quantities) than butane for most people. It's hard to beat a $3 can that will last 5 years of topping off bottles...

Quote Originally Posted by ambaker View Post
Both should work well for oxygen exclusion in photo chemical preservation. Though, theoretically Aragon a bit better due to being completely inert, and heavier than air.
Butane is quite a bit heavier than Argon. Still, I expect that at STP in a little bottle, the gases will be pretty well-mixed anyway just thermally. Note that N2, O2, Ar and CO2 in our atmosphere are all really uniformly mixed despite some big differences in nominal density.

Either way, a nice trick is to put in a couple atmospheres worth of the inert gas then let it down to near 1 atmosphere. That will reduce the partial-pressure of oxygen even further.