I'll have to put some balloon gas into my system with the mass spec. Could be O2, but might just be plain air, probably for cheapness as much as for any other reason. I know the welding grade stuff I get is pretty pure, having checked that. All of it is very dry, because the way they make it is to liquefy air, then distill off the gases one by one -- and water is poison for welding, one of the major reasons you use the special gases (hydrogen embrittlement). I do know that "real" helium gives more lift than the party store cheap tanks do.
I've done some fooling around with cathodes and materials for emission. The tri-alkali mix is indeed messed up by air, fairly badly, you can usually only get a couple exposures before emission falls so much the thing is useless. From the books, you start with a mix of carbonates, and during activation lose CO2 and wind up with oxides and some reduced metal. Air by itself doesn't ruin this, but any water does, by making hydroxides which won't re-activate again. So the balloon gas might be fine if it's dry and dry N2 (cheapest at the welding store) should also be fine.
After reading some more, I discovered that Y2O3 (yttria) works a lot better in these applications. It doesn't seem to mind air exposure at all. You only get perhaps half the emission of the other mix, but it is stable and "forever". We put it on via electrophoresis, thin, and it reduces the required cathode temperature (tungsten filament from a small 12w halogen) for a given emission from more or less white hot to orange (hotter than most vacuum tubes, but still reasonable). For example, in this case we were going for 10 ma emission off a 6v, 2a filament. To get that we needed more or less full voltage without a coating, but only 3.4v with one in a little triode/ion gage I made. We got half a pound of this to fool with, a multi lifetime supply as only a few milligrams makes the slurry that will coat many filaments/cathodes.
I hear the really serious electron microscopes made by Crewe at U of Chicago (the first ones to resolve atoms, just before scanning tunneling microscopes came along and stole all the thunder) used a single crystal tip field emission source of tungsten, cold. Not much current, but a really good point source, effectively about one atom in size. I have messed with making these too, and it's not really hard (selective electrolytic etching) if you can deal with the low current you get, and keep back-accelerated ions from heating and sputtering the tip off. It sometimes takes a few tries to get a good one, you have to get lucky with the crystal orientation in your rod stock, but welding rods seem to be a good source of mostly correct orientation.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.