Approaches for clarifying excitation mechanisms in spectrochemical excitation sources

It is shown that the state of an ICP plasma can be adequately described by a measured electron density distribution. Even though the plasma is not in full local thermal equilibrium (LTE), the electron temperature can be deduced from the density with the LTE relation with more accuracy than direct measurements permit. From energy and mass balances considerations it is argued that the deviations from LTE are sufficiently small; the ground state of the argon atom is underpopulated with respect to LTE. Spatially resolved measurements of excited state densities of argon neutrals and analyte neutrals and -ions are in good agreement with this " close enough to LTE "-concept. Only the levels of excited analyte ions which are resonant with the argon ion ground level show a significant overpopulation with respect to LTE. This is shown to be caused by the charge transfer process.