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A DC current, which was driven by the self-bias voltage, could be conducted in a radio-frequency-powered glow discharge plasma by connecting a low-pass filter circuit and a load resistor with the discharge tube. This current enhanced the intensity of emission spectra from the plasma largely. The intensities of iron atomic lines increased 35 -50 times, whereas the sputtering rate was not changed by the current introduction. Boltzmann plots for iron atomic (Fe I) and ionic lines (Fe II) were investigated when the bias current was conducted, so that the excitation process relating to the intensity increase could be clarified. While the excitation temperature of the Fe I lines was slightly changed (3000 -3600 K), that of the Fe II lines was drastically reduced from 7600 to 4300 K, which was close to the temperature of the Fe I lines at higher bias currents. Therefore, the plasma was changed towards an LTE condition so that both the Fe I and the Fe II lines could be excited through a common major process. The bias-current enhanced the density of electrons enabling low-lying excited energy levels (3 -5 eV) of iron atom/ion to be much more populated, and they became the major colliding partners for the excitation of these iron species.

An advanced detection method based on a modulation technique is described in radio-frequency-powered glow discharge plasma optical emission spectrometry (r.f. GD-OES). A frequency-sensitive separation using a fast Fourier transform (FFT) analyser, where a pulsated bias-current was introduced into an r.f. GD plasma, was available for improving the limit of determination for the atomic emission analysis. The FFT analyser has an ability to disperse signal components by frequency, and it is thus employed to select the component of a particular frequency. A dc bias current introduced into the GD plasma can enhance the emission intensities of analyte species greatly, and furthermore, it can be easily pulsated to modulate the emission intensities from the plasma. The modulated emission signal was selectively detected with the FFT analyser, with removing any noise components from the overall signal. The duty ratio of the pulsed bias current largely affected the amplitude of the FFT frequency components, because the pulse waveform comprised sine-function components having frequencies integral-times as much as the fundamental frequency, whose contribution coefficients depended on the duty ratio. This detection method was applied to the determination of vanadium and molybdenum in low-alloyed steel samples. The detection limits were obtained to be 6.2 × 10 -3 mass% V and 2.0 × 10 -3 mass% Mo in low-alloyed steel samples.