What is Plasma?

Plasma is an electrically neutral gas made up of positive ions and free electrons. Plasmas have sufficiently high energy to atomize, ionize, and excite virtually all elements in the periodic table, which are intentionally introduced into it for the purpose of elemental chemical analysis.
There are many types of plasma (direct current, microwave induced, etc.), the inductively coupled plasma (ICP) has demonstrated the most useful properties as an ion source for analytical mass spectrometry.

Direct current plasma (DCP) is obtained when a direct current field is established across electrodes,
ICP is obtained when a high-frequency (hf) field is applied through a coil,
Microwave-induced plasma (MIP) is obtained when a microwave field is applied to a cavity.

DCP was the first described and commercialized plasma. However, the ICP is currently the most commonly used plasma because of some unique properties. Originally, the ICP was designed for the production of crystals. The first analytical applications of the ICP were published in 1964 and 1965.

Gases such as argon, helium, nitrogen, and air have been used to sustain plasma useful for analytical purposes; however, the inert gases give some advantages, because of their desirable ionization properties and their availability in relatively pure form.

Impurities in the plasma support gas can result in spectral interferences, leading to inaccurate quantitative measurements.
Inert gases, specifically argon, also have the advantageous property of minimal chemical reactivity with various analyte species, which can also result in undesirable analytical results.