This is a valid Idea. I work in a research lab (I'm a molecular biologist), and Nitrogen gas is used to store sensitive chemicals that get oxidized by air oxygen.
The Nitrogen gas is N2, the same in the air (most of the air is N2). It does not react with anything, is pretty inert, Is not N03 as someone asked earlier on.
The way you do it, with a chemical on a glass bottle with a screw cap, you hold the jar slightly open with one hand, while with the other hand you blow N2 into the bottle/jar, for a few seconds, this replaces the air with N2. Then while blowing N2 gas into the opening you screw on the bottle cap. And you are all set, the gas in the jar/bottle is almost if not completely N2. N2 is dirty cheap by the way.
Co2 is also pretty inert and won't react with anything, Oxygen in the air is the bad thing. You could probably drop a little piece of dry ice into the jar with weed (make sure it doesn't touch the weed so it doesn't freeze it. Then wait some time for it to evaporate, Co2 is denser than regular air, it will expand and push out the air out of the jar (there has to be zero breeze for this), then screw the cover on and you are all set, you've gotten rid of oxygen in your jar. I might try this.