Microfluidic electrodischarge devices with integrated dispersion optics for spectral analysis of water impurities

This paper reports a microfluidic device that integrates electrical and optical features required for field-portable water-chemistry testing by discharge spectroscopy. The device utilizes a dc-powered spark between a metal anode and a liquid cathode as the spectral source. Impurities are sputtered from the water sample into the microdischarge and characteristic atomic transitions due to them are detected optically. A blazed grating is used as the dispersion element. The device is fabricated from stacked glass layers, and is assembled and used with a charge-coupled device (CCD) sensing element to distinguish atomic spectra. Two structural variations and optical arrangements are reported. Detection of Cr and other chemicals in water samples has been successfully demonstrated with both devices. The angular resolution in terms of angular change per unit variation in wavelength (partial derivative theta/partial derivative lambda) is experimentally determined to be approximately 0.10 rad/mu m, as opposed to the idealized theoretical estimate of 0.22 rad/mu m. This is because the microdischarge is uncollimated and not a point source. However, this is sufficient angular resolution to allow critical spectra of metal impurities to be distinguished.