Hybrid arc/glow microdischarges at atmospheric pressure and their use in portable systems for liquid and gas sensing

This paper reports on dc pulse-powered microdischarges in air at atmospheric pressure and their potential utility in chemical sensing. For electrode gaps of 50-100 [micro]m, microdischarges take the form of a glow discharge, an arc discharge, or a hybrid of the two. Arc microdischarges have high optical intensity but suffer from high background emission. Glow microdischarges have low background emission, but the prominent emission is confined in the UV-blue region of the spectrum. The arc--glow hybrid has characteristics that are intermediate between the two and can be tuned by circuitry to suit the chemical sensing application. A handheld system for chemical analysis using synchronized emission spectroscopy of these pulsed microdischarges is demonstrated. The system employs an exchangeable sensor chip (different for gas and liquid samples), a control circuit, and a commercially available portable spectrum analyzer coupled to a handheld computer. A pump and inert carrier gases are not utilized. The system can generate one or a series of single-shot microdischarges per chemical analysis. The gas discharge microchip, which utilizes electroplated copper electrodes on a glass substrate, has an electrode separation of 75 [micro]m and an active area of 300 x 300 [micro][m.sup.2]. The handheld system has been used to detect 17 ppm of acetone vapor in air. The liquid discharge microchip also has an electrode gap of 75 [micro]m and an active area of 1 x 1 [mm.sup.2]. It uses a porous cathode fabricated by micromolding and sintering glass frit slurry in a microchannel. When a microdischarge is initiated between the metal anode and the wet cathode, the liquid is sputtered into the microdischarge and emits characteristic line spectra. In this configuration, the system can detect 2 ppm of aqueous Cr without preconcentration. Index Terms--Chemical sensing systems, gas detectors, gas discharge devices, microarc, microdischarge, microplasma, water chemistry.