Argon was a gas.
The Tenth Doctor described fresh air as "Nitrogen, oxygen, a pinch of argon, a soupçon of carbon dioxide and a squeeze of H₂O" after filtering the ammonia from Chelsea 426 to remove Rutan spores that infected humans. (PROSE: The Taking of Chelsea 426) The Eighth Doctor could detect trace amounts of argon in Earth's atmosphere. (PROSE: Father Time)
A control panel on Orbit Station 3 had three dials to control the mixture of gasses. Dial A was for oxygen, dial B was for nitrogen, and dial C was for a mix of argon, carbon dioxide, and other gases. (PROSE: The Planet of Oblivion)