Classification and characterization of atmospheric VOCs based on sorption/desorption behaviors of plasma polymer films

The effects of adsorbed/absorbed (sorbed) water on the sensor response of a quartz crystal resonator (QCR) coated with plasma polymer films (PPFs) were investigated. Conventionally, the performance of QCR sensors for volatile organic compounds (VOC) has generally been measured under ideal conditions using dry synthetic air as a dilution or carrier gas. On the assumption of use for real indoor-air monitoring, a gas-flow setup is developed for assessing the sensor capability of PPF-coated QCRs in terms of a VOC plume generated from cellulose beads incorporating a VOC under normal atmospheric conditions. Sensor response patterns are classified into three types, and the water sorbed on the PPFs before chemical exposure is considered to play a significant role. We have presented an explanatory model describing sorption/desorption behavior based on the hydrophilic/hydrophobic interactions between VOCs, sorbed water, and PPFs. PPF weight loss is induced by desorption of the sorbed water, owing to the reduction of affinity to the PPF. Principal component analysis (PCA) has revealed that the VOCs in atmospheric air can generally be classified in terms of their polarity and carbon chain length, by using the strength and width of the sensor response curves towards the pulse-like exposure of the VOC plume.