Characterization of a helium discharge for hollow anode furnace atomization non-thermal excitation spectrometry

Abstract

A discharge in He has been characterized for hollow anode furnace atomization non-thermal excitation spectrometry (HA-FANES). The small surface area of the cathode and high current densities make it possible to stabilize He discharges up to 1200 hPa using a d.c. power supply operating in the constant current mode. Current—voltage characteristics were determined for pressures up to 800 hPa. Characterization at higher pressures was not possible owing to limitations of the power supply. The release of thermionic electrons from the cathode surface at temperatures in excess of 1750 K caused a dramatic decrease in the discharge potential during the atomization cycle. Increased ohmic heating of the cathode with increased currents caused a time shift in the release of thermionic electrons. Excitation temperatures, obtained from Boltzmann plots of five He I lines, decreased slightly as a function of increasing pressure and increased as a function of increasing current. Dramatic decreases in the discharge potential were observed for high concentrations (250 mg ml–1) of NaCl. The potential decreases were too short-lived to permit the accurate measurement of excitation temperatures. Integrated analytical signals increased linearly with increasing pressure.