My innner chemist just got interested.
Looking at
https://www.ready.noaa.gov/documents/TutorialX/files/Chem_henry.pdf which shows how much each gas dissolves into water, here are the relative solubilities of some common gasses that aren't acidic nor basic:
Gas Solubility (molar/atm) reactive?
Oxygen 0.0001 YES (so don't use this)
Hydrogen 0.0008 YES
Nitrogen 0.0003 NO
Argon 0.001 NO
Butane 0.001 Maybe (I really don't know here because I don't know the reduction potentials of the developers, but think it's higher than 0.8v)
In using gasses to purge oxygen out of the headpace of a solution, the main consideration is how much dissolves. If you add a very soluble gas over the solution, it will dissolve, creating a vacuum in the headspace. That vacuum will pull air into the bottle, and your developer-preservation plan is foiled. So, avoiding those gasses that will react with your developer (O
2 and H
2), nitrogen is the next best of the common gasses. Nitrogen also has one MASSIVE advantage over all the other gasses: it's already saturated in the solution! It got there when the water came out of your aerating faucet (provided you didn't boil the water before making your developer).
In the labs I worked in we always used nitrogen to displace oxygen unless we were using a reaction where nitrogen was one of the reactants (like nitrogenase studies).
So, who sells canned nitrogen gas?
P.S. This always works best when you sparge the solution with the headspace preservative, ie. blow the gas deep into the solution. Little red straws for the win.