You have a chamber with constant amount of gas (say either helium or argon). You create the plasma with an arc and turn it off. How much of the gas used to create the plasma returns back to helium or argon as opposed to dissipating into the chamber or creating new elements?
I think it would be a good idea if you could enlarge on your question, explaining how it arose and what experience you have with such plasmas.
The first point is that the gas is already in the chamber, so it cannot "dissipate into the chamber."
The short answer to your second question is that an arc does not create new elements. New elements can be created by radioactive decay, by irradiation with neutrons or with particles accelerated to high energy, or by thermonuclear fusion. This last can occur in a Hydrogen bomb, where there are very high temperatures and densities. It can also occur in a laboratory plasma, but only when extreme methods are used to heat and compress the plasma. Helium and argon do not undergo fusion.
If you started with helium or argon in the chamber, it will still be there after you turn the arc off, if the chamber is sealed. If the chamber is attached to a vacuum pump, it will be pumped out. That's about it. Hope this helps.