Spectral detection of metal contaminants in water using an on-chip microglow discharge

This paper reports on the detection of trace contaminants in water by spectroscopy of micro glow discharges that operate in air or at moderate vacuum using liquid electrodes. A liquid electrode spectral emission chip (LEd-SpEC) has been developed to perform this function. The device is fabricated by a four mask process, and provides a reservoir and channels in a glass substrate, along with electrodes that bias the water sample. Liquid from the cathode is sputtered into the discharge, for spectroscopic detection of impurities. Using a commercial spectrometer, Na concentrations <10 ppm, and Pb concentrations of 5 ppm, and Al and Cr concentrations of 10 ppm have been measured. The ratio of Na spectral intensity to that of ambient N₂ is shown to be a suitable measure of Na impurity concentration over several orders of magnitude. The addition of HNO₃ to lower the pH of the liquid solution increases this ratio by almost an order of magnitude. By selectively doping the solution, the device can also be used as a customizable optical source for UV and visible wavelengths.