PULSE OPTIMIZATION CRITERIA FOR THE MICROCAVITY HOLLOW-CATHODE DISCHARGE EMISSION SOURCE

Criteria are presented that are important for optimizing the emission intensity in a microcavity hollow cathode (MCHC) discharge source operated in the pulsed mode. The d.c. component must be sufficiently large to ensure a continuous discharge inside the HC. It is shown that the pulse parameters that result in maximum light output for the spectral lines of the cathode material do not result in maximum light output from sodium as an analytical sample which has been mounted inside the HC. This lack of correlation may be due to either the physical and chemical properties of the species or to the manner in which the species enters the HC discharge plasma. Voltage-current characteristics are presented for the MCHC to illustrate the role of the d.c. component during pulsed mode operation. The d.c. current of the discharge must be >5 mA in order to maintain a discharge in the HC. At currents <5 mA for this system the discharge is restricted to the planar surface of the cathode and will not participate in the excitation of analytes thrown into the hollow space during the pulse.