Abweichungen vom lokalen thermischen Gleichgewicht im axialen Kanal eines induktiv gekoppelten Argon-Plasmas

Summary

Measurement of the excited state level population of a thermometric species like FeI injected into the axial channel of an inductively coupled plasma (ICP) is a powerful method to describe the plasma, even when the level population distribution in case of deviations from local thermal equilibrium (LTE) does not define a uniform plasma temperature.

The injection of the thermometric species as ferrocene vapour allows to study the specific influence of operating conditions and normal sample components such as water on the plasma conditions by adding independently the different components.

It is shown that deviations from LTE-state decrease with increasing generator power and decreasing carrier gas-flow rate and reach an axial minimum in the normal analytical zone (NAZ). Although differences of more than 1000 K between excitation temperature of low lying FeI-levels and levels close to the ionization-limit have been observed, deviation of the levels population density from an LTE-distribution is smaller than a factor of 5 under reasonable analytical conditions. Under such conditions the LTE-concept seems to be a useful first approximation to describe the thermodynamics of the ICP and to predict trends.