An array of quartz crystal microbalances (QCMs) has been developed to measure and identify trace quantities of volatile organic compounds (VOCs) in water. In one set of experiments, a total of nine polymer-coated QCMs were tested with varying concentrations of twelve VOCs while frequency and damping voltage were measured. QCMs are rugged, low power, easily miniaturized, and capable of direct chemical sensing in liquids. Moreover, QCMs can be adapted for many different uses by developing coatings that respond to different target molecules, adding to their versatility.
The ability to provide real-time monitoring of chemical contaminants in water samples can be used for a variety of applications:
Background - QCMs are piezoelectric thickness-shear-mode resonators where the resonant frequency has long been known to vary linearly with the mass of rigid layers on the surface when the device is in contact with air. Developments in QCM sensor technology have progressed in the area of gas phase analysis since the first report in 1964. Since then, reports of other detection schemes for different gas phase analytes have appeared in the literature. More recently, these devices were also determined to be sensitive to property changes in liquids that they contact. Many new highly selective coatings amenable for piezoelectric transducers in liquid media have been developed in addition to more traditional polymer coatings. Coatings such as cyclodextrins, cavitands, and calixarenes have shown potential for making sensors selective for certain compounds or classes of compounds.
An alternative and more versatile approach is to use an array of devices with different coatings that have only partial selectivity and respond in some way to all compounds. The pattern of responses from this sensor array can be analyzed using chemometrics or pattern recognition techniques to identify the chemical being detected and determine its concentration. The array we have developed has AT-cut quartz crystals mounted in flow cells as shown in Figure 1.
|Figure 1.Schematic of liquid test cell for a single QCM sensor.|
This stainless steel flow cell houses a QCM between an o-ring on the liquid side and a polycarbonate spacer on the opposite side where electrical contacts are made. Six QCM flow cells arranged in series comprise the array, as shown in Figure 2.
|Figure 2. Schematic of six QCM sensor array prototype system.|
Water samples with trace quantities of VOCs are pumped through the flow cell array to monitor the level of contamination. The first cell in line contains an uncoated QCM which provides a reference to note changes in density or viscosity of the solution. The remaining cells in the array house polymer-coated QCMs.
To evaluate the utility of QCM sensors to detect a range of VOC contaminants, the following compounds were tested:
Figure 3 shows results for a typical experimental run using deionized (DI) water and a water sample contaminated with successively higher concentrations of chloroform. Rapid and reversible responses were noted.
|Figure 3. Response of polymer-coated quartz crystal microbalance to part per million (ppm) levels of chloroform in water.|
In addition, response patterns were analyzed using the Sandia-developed pattern recognition technique referred to as Visually Empirical Region of Influence (VERI). Better than 98% correct identification of chemical contaminants could be obtained from five coated QCM sensors assuming sensitivity drift is low.
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