The role of oxygen in analytical glow discharges: GD-OES and GD-ToF-MS studies

The influence of up to 0.8% volume fraction of oxygen on the argon plasma in a direct current analytical glow discharge was studied for iron, titanium, copper and gold samples using both time-of-flight mass spectrometry and high resolution optical UV-Vis Fourier transform spectrometry. A sudden decrease in sputtering rates, optical emission intensities and ion signals is observed at a current and material dependent threshold fraction of oxygen, and is attributed in part to the formation of an oxide layer on the surface. Beyond the threshold oxygen fraction, a further large continuing decrease in ion signals is observed as the oxygen fraction is increased. This smooth decrease of the observed ion signals is attributed to quenching of ions and argon metastable atoms by oxygen in the flowing afterglow in the flow tube used to promote ion transport to the mass spectrometer sampling orifice. At the threshold oxygen fraction, signals from the mass spectrometer decrease by orders of magnitude more than the optical emission observed from analyte and gas ions. This disproportionate decrease is also likely to be due to reactions in the flow tube. The negative mass spectra were also measured for iron, titanium and copper samples and a significant increase in ion signals is observed at material dependent critical oxygen concentration. Results of studies using calamine (a sample with oxide layer) in pure argon are reported and compared with argon/oxygen mixtures.