Guang Chen

Date of Award

2002

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Electrical Engineering (MSEE)

Department

Electrical and Computer Engineering

Advisor

David Frankel

Second Committee Member

John F. Vetelino

Third Committee Member

Mauricio Pereira da Cunha

Abstract

The widespread use of pesticides on commercial food crops can result in short and long term health problems for both f m workers and consumers as well as serious impacts on the environment. Consequently, there is an increasing need to develop low cost portable detection systems that can be used to screen for pesticide residues on food products. Since phosmet (C1 IH12N04PS2) is one of the most commonly used organophosphorous pesticides within the state of Maine, it is selected as the target pesticide to detect in this thesis. Two sensing techniques were investigated. One is the quartz crystal microbalance (QCM), coated with a polyepichlorohydrin (PECH) film and expose to phosmet in a liquid solution. The second technique uses a chemiresistive sensor, with tungsten trioxide (W03) used as the sensing film. The data obtained fiom the QCM experiments shows that the PECH coated QCM sensor displays good reproducibility to injections of phosmet solutions concentrations range up to 25 ppm phosmet in de-ionized water. The minimum detectable concentration of phosmet is lppm and the sensitivity of the PECH coated QCM is 15p2 Hdppm. The PECH coated QCM was also shown to detect phosmet in organic blueberry juice. In order to determine the response of the W03 film to phosmct, a positive pressure gas delivery system was built. The resistance of the pure W03 film exposed to phosmet vapor in compressed air was measured fiom room temperature (25OC) to 400°C, while the temperature of phosmet solid was kept at '500~. The resistance of various gold doped W03 film exposed to phosmet vapor in compressed air was also measured while the film temperature was kept at 350°C. The data obtained fiom SMO experiments shows that gold doped W03 films maintained at 350 OC are very sensitive to phosmet gas exposures in a carrier gas of air at 50% humidity. The concentration of phosmet gas within the gas delivery system is not known, but is estimated to be lower than lppb. The gold doped W03 film is poisoned by sulfur, a break down component of heated phosmet. The gas chromatography/mass spectrometry (GCIMS) was used to determine the thermally induced break down products of phosmet and also to identifjr possible interferent molecules fiom crushed blueberries. The data shows that Phosphordithioic acid, ion 172, which is a main break down component of phosmet, can be used as an indicator to detect phosmet in treated blueberry samples. The advantage of the polymer coated QCM sensor is that it can be used in both a gas and liquid environment, while the SMO sensor only can be used in a gas environment. Since the concentration of phosmet in liquid phase samples will be higher than when using gas phase detection, polymer coated QCM sensors have much more potential in phosmet detection.

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