CHARACTERIZATION OF A HELIUM DISCHARGE FOR HOLLOW ANODE FURNACE ATOMIZATION NONTHERMAL EXCITATION SPECTROMETRY

A discharge in He has been characterized for hollow anode furnace atom ization non-thermal excitation spectrometry (HA-FANES). The small surf ace 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 oper ating in the constant current mode. Current-voltage characteristics we re determined for pressures up to 800 hPa. Characterization at higher pressures was not possible owing to limitations of the power supply. T he release of thermionic electrons from the cathode surface at tempera tures 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 Boltzman n plots of five He I lines, decreased slightly as a function of increa sing pressure and increased as a function of increasing current. Drama tic decreases in the discharge potential were observed for high concen trations (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 press ure.