Investigation into the Operating Characteristics of a “Microarc” Atmospheric-Pressure Glow Discharge

Abstract

Details of the processes occurring during sample atomization from a “microarc” discharge have been studied photometrically, by use of high-speed color cinematography and through current-voltage waveforms. The microarc studied here is an atmospheric-pressure inert-gas glow discharge supported between 0.25 mm diameter tungsten wires; quiescent argon-1% H2 provides reactive-sputtering conditions and improved behavior in the presence of oxygen impurities. Excitation temperatures of ca. 5000°K are measured for the argon glow. Samples of Na, Al, and Sr illustrate the influence of volatile, refractory, insulating, and electron-emitting sample properties on the temporal-spatial-electrical behavior of the discharge. The step-by-step events occurring in the discharge are described qualitatively and a variety of processes are invoked to explain sample volatilization, including sputtering, chemical reactions, and purely thermal effects. In the first stages of the discharge, instabilities are related to the placement and insulating character of deposits. With heating, electron emission becomes important in directing the discharge to or away from the sample; abnormal glow wandering and glow-to-arc transitions can ensue. Improved stability is achieved by uniformly depositing multi-element samples along the electrode, which localizes the initial discharge and promotes ablative cooling of the sample and electrode.

A new, versatile, direct-current helium atmospheric-pressure glow discharge

“…Rogoff 33 studied gas-heating effects in hydrogen APGDs and their relationship with the transition to an arc. Hieftje et al 17,34 developed an atmospheric-pressure glow discharge generated in Ar that was termed a ''microarc'', and studied its application as a sample-introduction device. Mezei et al 35,36 considered the deviation from similarity laws and the dependence of the positive-column cross section for dc helium and nitrogen glow discharges at several pressures, including atmospheric pressure.…”

Microsample introduction into the microwave-induced nitrogen discharge at atmospheric pressure using a microarc atomizer

“…At least three factors appeared to affect the precision of the measurement: the electrode spacing and relative geometry, the position of the sample droplet on the cathode, and the surface characteristics of the sample electrode. These factors have been excellently characterized by Bystroff et al, who operated the microarc under different conditions than presented here (23). As a result, the qualitative discussion below will dwell on the effects the flowing nitrogen atmosphere has on these parameters.…”

“…A factor that limited reproducibility and signal stability in initial microarc-MINDAP trials was uncertainty in the duration of the desolvation process. If desolvation were incomplete, initiating the arc resulted in intense broad-band emission from the MINDAP, caused presumably by tungsten oxide liberated from the sample electrode (23). On the other hand, waiting a sufficient period of time to ensure complete desolvation decreased the throughput of sample analysis.…”

Computer-controlled source for atomic emission spectrometry

“…EXPERIMENTAL SECTION Microarc and Electrode Design. A computer-controlled microarc, based on the design of Hieftje et al (1,3,4), was used in this work. A block diagram of the experimental setup is presented in Figure 1 and the equipment used is listed in Table I.…”