A 13.56-MHz Grimm-type glow discharge plasma on which external d.c. voltages are superimposed is investigated for the application to optical emission spectrometry. With a voltage modulation technique associated with phase-sensitive detection, the emission intensities can be measured at very low noise levels, enabling the detection power to be improved. The experimental parameters: d.c. voltage, modulation frequency, and Ar pressure, are investigated for obtaining the optimum conditions. It is possible to obtain a detection limit (Cu I 327.40 nm) of 8 × 10-4 wt.% for Cu in Fe-based alloys, while 6.6 × 10-3 wt.% in the case of conventional detection.
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