New techniques for controlling the excitation source in glow discharge optical emission spectrometry

This paper describes three advanced detection techniques in glow discharge optical emission spectrometry: (a) an applied voltage modulation method for a d.c.-powered glow discharge emission source, (b) a bias-current introduction method and (c) an amplitude modulation method for an r.f.-powered glow discharge emission source. In the applied voltage modulation method, an alternating component superimposed on a d.c. bias voltage is applied to the excitation source. Only the emission component corresponding to the alternating frequency can be selectively detected at very low noise levels with a lock-in amplifier, thus contributing to an improvement in the signal-to-noise ratio. In the bias-current introduction method, a d.c. current driven by the self-bias voltage is conducted through the plasma body by connecting a low-pass filter circuit and a load resistor with the glow discharge lamp. The electrons introduced into the plasma can cause various excitation processes more actively and, as a result, the emission intensities increase, which yields a better detection limit in the quantitative analysis. In the amplitude modulation of an r.f. voltage, the emission detection with a lock-in amplifier enables the measurement to be performed with a better signal-to-noise ratio, whereas the sputtering rate and the sampling amount are reduced. This method can be successfully applied to determine depth elemental composition of nm-order thin films.