Further Observations on the Spatial Emission Properties of a Microwave-Induced Plasma in Argon

Recently in this journal we presented a study of the spatial emission properties of a microwave-induced plasma (MIP). That particular study used a TM010 Beenakker cavity to generate a plasma in argon with analyte elements introduced into the plasma in a vapor form from an electrothermal (ET) atomizer. We observed that analyte emission from metallic species was localized to a narrow region at the inlet side of the plasma (i.e., at low heights in the plasma) and that no significant emission was observed higher in the plasma. These effects, together with the observation of metal deposition on the discharge tube walls and decreased analyte emission at higher microwave powers, led us to suggest that analyte atoms were being removed from the plasma before reaching the most favorable excitation region at the center of the discharge. We postulated that the loss of analyte atoms from the plasma might result after initial ionization of analyte atoms (soon after their entry into the plasma) by subsequent drift of these ions in the inhomogeneous microwave field surrounding the discharge. This present report introduces some further experimental evidence that supports the above hypothesis of analyte removal from MIP sources.