Survey of Odor Discrimination and Indoor Air Monitoring by High-Sensitive Sensor based on Quartz Crystal Resonator

Response characteristics of quartz crystal resonator (QCR) coated with organic sensory film prepared by radio-frequency sputtering of organic materials, either a synthetic polymer or a biomaterial was investigated from the view point of application to a sensor array or electric nose for monitoring indoor air quality. As a model of indoor air, odors vaporized from six kinds of essential oils of woody note (cedar wood, rose wood, sandal wood, eucalyptus, star anise, and pine) at small intensity as a sensing threshold for human olfactory system were monitored with QCR sensors. All film coated QCRs showed Langmuir-type adsorption/absorption characteristics to each odor that should be a mixture of several gaseous substances. By means of principal component analysis (PCA), data plots of six odors were well separated in the scoreplot obtained through the PCA with an accuracy of 91 % according to k-nearest neighbor method. The compactly assembled QCR sensor module (6.5 cm × 9.5 cm ×5 cm) was set in a smoking area, and continuous monitoring under room air was demonstrated. Simultaneously, monitoring with a carbon dioxide sensor and a relative humidity/temperature sensor were performed. The frequency vs. time response curve of each film coated QCRs was composed of a long-span baseline drift and a quick response recorded as a pulse-shape curve within several minutes, which revealed QCR sensor's ability of monitoring under normal room air. The baseline shift seemed synchronized with the change in relative humidity. Some pulse-shape responses appeared when no other sensor's response was observed, which indicated the film coated QCRs only could detect some changes of the indoor air in the smoking area.