Microwave-induced Plasma Boosted Microsecond-pulse Glow Discharge Optical Emission Spectrometry

Abstract

A microsecond-pulse (µs-pulse) glow discharge (GD) source boosted by a microwave-induced plasma (MIP) has been developed and studied for optical emission spectrometry (OES). The excitation processes of the tandem GD source were investigated. The analytical characteristics of the GD-OES source in the presence and absence of the MIP were compared, including the operating parameters, signal-to-background ratios (S/B) and relative standard deviation (RSD). The results show that under a relatively low discharge pressure (<180 Pa), the µs-pulse GD can couple fairly well with the MIP and emit intense analytical lines. When the GD source is operated under a pressure higher than 200 Pa, two emission peaks appear, independent in time, for a given resonance atomic line, because sample atoms are independently structurally excited, first by the µs-pulse GD and then by the MIP. The time interval between the two peaks for Zn I 213.8 nm is longer than that for Cu I 324.7 nm, which is believed to be due to the faster diffusing velocity of copper atoms. When the µs-pulse GD lamp is operated under a gas pressure higher than 220 Pa, the ion population is so high that Cu II ionic line at 224.7 nm ‘becomes’ two peaks because of a possible self-absorption. The results show that the supplementary use of an MIP can eliminate the self-absorption of ionic and atomic lines. When the µs-pulse GD source is coupled with the MIP, S/Bs are improved by a factor of more than one order of magnitude for several analytical lines. A short-term RSD of 0.2% is achieved for the ‘µs-pulse GD+MIP’ configuration compared with that of 1.0% for ‘µs-pulse GD only’ mode. The experimental results show that the MIP boosted µs-pulse GD is a promising technique for solid sample and surface analysis.