The United States National Academy of Engineering (NEA) lists access to clean water supplies as one the fourteen greatest challenges facing humanity in the twenty-first century. Organic and inorganic contamination of drinking water supplies can result from human efforts and the erosion natural deposits. Metal ion contamination from industrial wastewater is a significant threat and is regulated by government environmental agencies. Point sources refer to the site of chemical emissions into water sources and are often monitored by sampling on-site and analysis in off-site laboratories. Development of field-portable water quality analysis devices is critical for quickly determining the potability of water sources. Plasma and mass spectrometers are traditionally used for water analysis and miniaturized versions of these tools have been reported for on-site monitoring. However, many of these miniaturized devices require cumbersome ancillary hardware such as vacuum equipment, tanks for flow gases, and RF power supplies and are often produced using relatively complex and expensive fabrication techniques.
At Louisiana Tech University, we have developed and patented (U.S. Patent No. 7746451) a low-cost, compact DC plasma spectroscopic sensor for on-site water quality monitoring. Spectroscopic analysis is performed in air, at atmospheric pressure, eliminating the need for cumbersome vacuum equipment and gas tanks in field-portable applications. This essentially disposable sensor is batch fabricated on an inexpensive borosilicate glass substrate with thin-film Cr microheater/electrodes patterned using simple, standard photolithographic processes.
Water impurities are detected in a sample by observing the atomic spectral emissions of contaminants sputtered into an on-chip plasma discharge. A water sample reservoir is sandblasted into a borosilicate glass substrate, housing 10 μL samples (Fig. 1). Electrical current is supplied to a 200 nm Cr microheater/cathode patterned to the bottom of the reservoir to partially evaporate the water sample, forming a sample particle coating over the microheater/cathode and anode leads (Fig. 2). Evaporating a portion of the water from the sample increases the relative concentration of impurities in the sample, providing a means of increasing the detection capabilities of the sensor. A DC bias is applied to the Cr cathode and anode leads to produce an on-chip plasma discharge in air at atmospheric pressure. Water contaminants in the sample coating are sputtered into the plasma discharge, doping its spectral emissions. Spectral data is collected with a fiber optic cable connected to an off-chip spectrometer (Ocean Optics HR2000 UV-VIS) with an integration time of 300 ms. Spectral data is analyzed with Ocean Optics’s SpectraSuite software installed on a laptop computer.
Cu and Fe impurities are detected at 10 ppm in a 2.5% HNO3 solution (Fig. 3-4) and Ca and Mg contaminants are detected at 100 ppm (Fig. 5-6). The on-chip microheater yields temperature changes as high as 96 °C when supplied with 100 mA (Fig. 7).
