Characterization of a very low power argon CCP

Abstract

The very low power Ar rf capacitively coupled plasma (CCP) (10.9 MHz, 5–75 W) at atmospheric pressure could be sustained at low gas flow-rates (0.1–2 l min⁻¹) on a sharp conducting electrode. The Ar plasma is characterized from four points of view: stability, plasma parameters, influence of the plasma on the waveform of the rf oscillations and analytical capabilities. The stability diagram was obtained by observation of the behaviour of the plasma as a function of Ar flow-rate and plasma power. Six different regions were identified, the most important of them being the region defined as stable plasma. This state offers the optimum conditions for its characterisation and for its use as a spectral source. The characteristic temperatures (rotational, excitation and ionization) were determined using as thermometric species the OH radical, Ar, Pb and Fe. The work proposes a new method to calculate the electric parameters (resistance, capacitance and absorbed power) of CCP. The method is based on the measurement of the frequency of generated oscillations, the current flowing through the plasma and the voltage on the plasma-sustaining electrode and on the electric model of the plasma. The presence of the plasma inside the resonant circuit of the radiofrequency oscillator determines the change in rf oscillations waveform. By Fourier analysis of the oscillations waveform in the absence and in the presence of the plasma, the high order harmonics absorbed by the plasma were identified. The plasma was tested as a spectral source for the analysis of pneumatically nebulized liquid samples. The detection limits for 16 elements are presented.