On the Temporal Evolution of Micro-discharge Spectra and Detection of Organic Vapors in Air

This paper reports the use of time resolved pulse discharge spectroscopy for detecting organic vapors in ambient air. The impact of both sample duration and delay is investigated. A pulsed micro-discharge, consuming only 400 µJ of energy, is struck by applying a high voltage pulse between two closely spaced electrodes patterned on a glass substrate. As a result, the ambient gas in the glow region of the micro-discharge breaks down into molecular and atomic fragments. The time resolved spectrum of the light emitted by these fragments is obtained with a spectrometer, which is triggered a programmed time delay after the discharge pulse. It is observed that better detection is achieved by limiting the sample duration to 2.1 ms and with a delay of <1 ms after initiation of the micro-discharge. This is verified for isopropyl alcohol (100 ppm) and acetic acid (250 ppm) vapor samples. 1. Introduction In the quest for miniaturized vapor and gas sensors, a variety of transduction methods have been explored. Chemi-resistors, chemi-capacitors, chemi-mechanical sensors, and metal oxide sensors measure changes caused by analytes to thin films [1]. However, film properties can drift and change with contaminants, package stress, and ambient temperature. Some applications require greater speed and specificity than these approaches offer. Miniaturized sensors that use plasma spectroscopy of target species can complement other sensors in many respects, but typically require low operating pressures (and hence a pump) and do not favor an air ambient [2]. Pulsed arcs (micro-discharges) can, however, operate at atmosphere in air, and have been used for spectroscopically detecting inorganic impurities in water [3]. Resulting emissions have also been used for fluorescent detection of bio-chemicals [4]. This paper describes the first use of pulsed discharge spectroscopy (using a micromachined device) for detecting organic vapors in an air ambient at atmospheric pressure. It also describes how the temporal evolution of spectra affects the signal 2. Microsystem Design The sensor (Fig. 1) utilizes two closely spaced Ti electrodes patterned on a glass substrate. The gas flows through the channel formed between the substrate and two glass spacers bonded to it. The gas molecules are fractionated as they pass through the micro-discharge region [5]. The emitted spectrum, which is characteristic of the fragments, is observed using a portable off-chip spectrometer (Ocean Optics USB2000). The micro-discharge is powered by a capacitor bank under computer control, which provides a current pulse of about 2 µs duration (Fig. …