A plasma can be created using many gases. Argon is usually preferred because of its characteristics. As all noble gases, it is mono-atomic, chemically inert and its ionization energy is high (15.76 eV). The use of Argon has many advantages:
Its only limitation is its low thermal conductivity that may also be an advantage, simplifying the design of instruments. Some instruments using Helium or air have been designed in the past. The high cost of Helium and the drawbacks of air with a complex spectrum emitted, the formation of stable compounds with elements and its lower ionization energy have limited the use or the performance of such instruments.
All relevant information for elemental analysis is in the 120-800 nm wavelength range. Some instruments can only access the 160-800 nm range, limiting the range of applications that can be addressed by ICP-OES. The 120-160 nm range is particularly helpful for halogen analysis or for alternative lines in some special applications.