I carbonate my kegs using the natural carbonation from fermentation, release the initial gas which contains the impurities you don't need, the back pressure will carbonate the beer with what is left for the fermentation. I use a hand pump on some of my beers and natural carbonation of a keg equals the mouthfeel and taste of that. Even though bottled gas is classed as pure it is actually a by product, there is an article I will source and post it up, and as we are producing CO2 why waste it and then buy it!