Application of a bias-current modulation technique to radiofrequency 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 3 x 10^-4 mass % was obtained for Mn determination in Fe-based binary alloy samples, whereas 1.4 x 10^-2 mass % Mn was obtained in the conventional excitation of an 80-W r.f. discharge plasma.