The gas made from DIY CO2
should be 100% CO2
, while the gas in pressurized CO2
should be very nearly 100% as well, depending on the grade (although I think it's really a matter of 99% or 99.9%). The problem comes when air from another source gets into the reactor, such as if some gets into a canister during a WC
and the canister spits it into the reactor. It's not so much a matter biproduct.
That being said, there is a noticeable caveat with DIY CO2
. When you first start your batch, the entire CO2
fermenter (and any gas separator you use) will be filled with atmosphere, which is 80% nitrogen, 20% oxygen (and much smaller percents other things), which is all insoluble. CO2
is denser than either of those, so your reactor will initially release those two gases until the entire space has been replaced by CO2
, after which point it will mostly be pure CO2
. Those gases will take a while to dissolve and may linger in your reactor for some time. This can be reduced after the first batch by disturbing the gas separator as little as possible and filling the fermenting bottle as much as possible with your mash, leaving relatively little air in the top (but leave some, else you'll make other problems.