An applied voltage modulation technique was applied to the high-sensitivity determination of phosphorous and sulfur in steel samples in glow discharge optical emission spectrometry. Emission signals from the glow discharge plasma were modulated by a cyclic variation of the discharge voltage so that only the desired signals could be detected without any noises with a DSP lock-in amplifier. The relative lock-in signals from sputtered species were enhanced compared to those of gas species. The PI 177.49-nm, and the SI 180.73-nm and SI 182.72-nm lines, were selected as the analytical lines. The limit of determination for the PI 177.49-nm line was estimated to be 1.O×10-3 mass% P.
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