N. Barua, I. Williams, T. Hutter
The University of Texas at Austin,
United States
Keywords: sensors, preconcentrator, gas adsorption, benzene, BTEX, silica
Summary:
The detection and monitoring of volatile organic compounds (VOCs) especially Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX) in ambient air are crucial due to their significant impact on public health and environmental safety. Among these, benzene is particularly important as it is classified as a carcinogen and poses serious health risks including respiratory tract irritation, lung cancer, and leukemia. Current demands for portable low-cost sensors to measure BTEX are mainly driven by the need to monitor indoor air quality, and industrial occupational safety. Existing portable technologies for VOC detection include photoionization detectors (PID), metal oxide, non-dispersive infrared (NDIR), and electrochemical sensors. Those devices are either non-selective, can measure only one gas, or lack adequate sensitivity. PIDs, the most popular VOC detectors, can measure concentrations down to the low parts-per-billion level but cannot differentiate between VOCs. To address the selectivity limitations of PIDs, we have integrated PID with a preconcentration system using mesoporous silica as the preconcentration material. Preconcentration enhances sensitivity by concentrating target analytes over time before rapid desorption, effectively increasing the sample concentration presented to the detector. Our previous work demonstrated the efficacy of this approach, achieving preconcentration factors up to 20 for VOCs and showing improved sensitive detection. In this study, we focus on modulating the desorption rate and temperature to achieve temporal separation of the BTEX components based on their volatility and interaction with mesoporous silica. We investigated the effects of desorption rate on the desorption characteristics of benzene and toluene. We tested benzene and toluene (0.63 – 8 ppm) at four different desorption rates: 5 °, 1, 0.5 °, and 0.3 °C/sec and found that at a specific desorption rate, the desorption temperature for each VOC is fixed regardless of the VOC concentration. At 1°C/sec desorption rate, the desorption temperature for benzene and toluene is 40±0.59 °C, and 60±0.31 °C, respectively. We also found out that at a lower desorption rate, the signal widens, implying that the desorption of the VOC occurs for a long range of time compared to when the desorption rate is higher. This unique peak broadening feature is essential to understanding the desorption kinetics and the interaction between the VOC and preconcentrating materials. The integration of preconcentrator with PID has the potential to selectively detect benzene in the presence of other VOCs, which is crucial for compliance with increasingly stringent regulatory standards for occupational safety.