Evaluation of the oscillator frequency of a free running RF generator as a diagnostic tool for inductively coupled plasma-optical emission spectrometry

The frequency response of a free running RF generator was evaluated for its ability to provide plasma diagnostic information. A small probe coil in the plasma torch housing and a frequency counter were used for frequency measurement. A linear dependence between the RF generator frequency and the flow of a foreign gas (CO2, H2, He, N2—introduced into the central channel of the ICP) was encountered. Moreover, the inductively coupled plasma (ICP) was found to extinguish at a constant, characteristic frequency when the flow of foreign gas was too high, allowing a simple prediction of the maximal tolerable foreign gas flow. Spectroscopic plasma diagnostic techniques (Mg II/Mg I ratio, excitation temperature and rotational temperature) were also investigated but provided little indication of instable plasma conditions due to the addition of foreign gases.