Effect of the driving frequency on the emission characteristics of a radio-frequency glow discharge excitation source boosted by the introduction of a d.c. bias current

In a radio-frequency-powered glow discharge lamp, a d.c. bias current which is driven by a self-bias voltage can lead to an enhancement of the emission intensities excited by the plasma. The driving frequency of the r.f. plasma is an important parameter to determine the self-bias voltage; lower r.f. frequencies induce greater self-bias voltages. The effects of the bias current introduction on the emission characteristics were compared between a 13.56-MHz plasma and a 6.78-MHz plasma. As a result, the 6.78-MHz plasma offered a better analytical performance, probably due to higher self-bias voltages, if the introduced Ar pressure was optimized. This method was applied to a Mo determination in Fe-matrix alloy samples. At bias currents of 40 - 50 mA, the emission intensities of the Mo I 379.82-nm line were about 10-times larger than those obtained with the conventional plasma when the 6.78-MHz plasma was produced at an r.f. power of 60 W. The detection limit obtained for this calibration was 2.0 × 10^-4 mass % Mo at an 80-W r.f. power and at a d.c. bias current of 68 mA.