Abstract
Monitoring of volatile organic compounds (VOCs) is of increasing importance in many application fields such as environmental monitoring, indoor air quality, industrial safety, fire detection, and health applications. The challenges in all of these applications are the wide variety and low concentrations of target molecules combined with the complex matrix containing many inorganic and organic interferents. This paper will give an overview over the application fields and address the requirements, pitfalls, and possible solutions for using low-cost sensor systems for VOC monitoring. The focus lies on highly sensitive metal oxide semiconductor gas sensors, which show very high sensitivity, but normally lack selectivity required for targeting relevant VOC monitoring applications. In addition to providing an overview of methods to increase the selectivity, especially virtual multisensors achieved with dynamic operation, and boost the sensitivity further via novel pro-concentrator concepts, we will also address the requirement for high-performance gas test systems, advanced solutions for operating and read-out electronic, and, finally, a cost-efficient factory and on-site calibration. The various methods will be primarily discussed in the context of requirements for monitoring of indoor air quality, but can equally be applied for environmental monitoring and other fields.
Highlights
Measurements of volatile organic compounds (VOCs) are becoming ever more important due to stringent environmental regulations and increasing health concerns
We have developed a new approach based on open pre-concentrators, i.e., absorbing material based on metal organic frameworks (MOF) deposited on a micro-hotplate similar to the gas sensor substrates
Bringing together the different aspects outlined above—highly sensitive sensor elements; optimized dynamic operation (TCO, Electrical Impedance Spectroscopy (EIS), Gate Bias Cycling (GBC)) of the sensors; high-performance electronics for dynamic operation combined with advanced signal processing to achieve high sensitivity, selectivity and stability; low-cost pre-concentration to boost sensitivity and selectivity further; gas test bench for ppb and sub-ppb VOC concentrations, efficient factory, and on-site calibration—is a pre-requisite for the systematic development of low-cost sensor systems for VOC detection in various applications and for their validation in field tests
Summary
Measurements of volatile organic compounds (VOCs) are becoming ever more important due to stringent environmental regulations and increasing health concerns. The specific challenge for VOC measurements are the low target concentrations: the respective guideline threshold values for some critical substances (in indoor air) are 0.1 mg/m3 (81 ppb) for formaldehyde and 0.01 mg/m3 (1.9 ppb) for naphthalene according to the World Health Organization (WHO) [4], and 5 μg/m3 (1.6 ppb) for benzene according to EU guidelines [6]. In other fields such as industrial monitoring and workplace safety, higher values apply, but these are currently trending down sharply, e.g., in Germany [7]. Sensor-based monitoring, on the other hand, would allow both an improved temporal resolution and a wider detection spectrum, as sensors will respond to practically all VOCs including VVOCs and SVOCs
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