Application of a bias-current modulation technique to radio-frequency glow discharge optical emission spectrometry

A 13.56-MHz Grimm-style glow discharge plasma in which pulsated bias currents are introduced was investigated concerning the analytical applications. The d.c. bias currents driven by a self-bias potential are conducted through the r.f. plasma body by using a low-pass filter circuit and a variable resistor. Excitation by a modified r.f. plasma can yield larger emission intensities. By modulating the bias current associated with phase-sensitive detection, the emission intensities can be measured at very low noise levels, enabling the detection limit to be improved more. A detection limit of 3x10(-4) mass % was obtained for Mn determination in Fe-based binary alloy samples, whereas 1.4x10(-2) mass % Mn was obtained in the conventional excitation of an 80-W r.f. discharge plasma.