The SnO2 electrode has received wide-spread application for the headspace detection of volatile organic compounds such as ethanol. The sensor is actually an Al2O3 ceramic tube which is coated with a rough (high surface area) SnO2 layer and heated. Semiconducting oxide gas sensors of this type are based on the conductivity changes caused by the absorption and desorption of the gas molecules on the surface of the heated sensing structure. The response mechanism [1] is shown in Figure 1. Most previous research has focused on the design of miniature tin oxide sensors [2], often for food freshness sensing [3].
Inexpensive tin oxide electrodes are commercially available. The Vernier company sells a ethanol sensor based on this same electrode for chemical education experiments. Consumer electronic companies such as Polulu and the Robot Shop sell a variety of sensors including the tin oxide one, promoted for use in a home-built breath analyzer. However, this sensor has not been applied to other volatile organic compounds. We have previously shown that headspace detection of a wide range of alcohols, ketones, amines, and other volatile polar organics can be detected in the mM range using this tin oxide sensor when placed over a small vial containing the sample. Myrosinase enzyme activity has also been followed. Care must be taken because the sensor cannot be immersed or touch a liquid.
A higher throughput method for the determination of volatile compounds in aqueous samples would be desirable. In our study, a headspace flow injection analysis (HS-FIA) instrument was built and applied for quantitation of ethanol and esters in water. The instrument is composed of a standard HPLC pump and injection valve with a 200 microliter loop with the outlet tubing equipped with a fitting for droplet generation. The tin oxide electrode is mounted just above the forming drop. Upon exceeding the size of the stable droplet, the excess solution flows down a concentric plastic tube and then a new droplet forms. Despite the droplet formation process, the baseline is steady.
Author(s): Zhichang Yin, Chemistry Major
Advisor(s): Neil Danielson, Department of Chemistry and Biochemistry
Ruichao Xie, Department of Chemistry and Biochemistry
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