Increasing pollution due to vehicular traffic and industrial activities is a major problem nowadays, especially in larger cities or industrial areas. Capillary air quality monitoring is mandatory to ensure healthy living conditions, but available air quality monitoring systems are bulky and expensive.
Simple low-cost systems based on solid state gas sensors were recently proposed. However, lack of selectivity of MOX gas sensors is one of the major issues in applications where single compounds within complex mixtures must be detected and quantified. Some of the compounds typically found in air quality samples, like e.g. water vapour, though being harmless to population, are present at high concentrations and can vary rapidly with time, producing significant shifts in the baseline of MOX sensors. Other compounds, like e.g. benzene, are toxic or even carcinogenic at ppb concentrations.
A miniaturized gas-chromatographic-like system was developed, combining Silicon micromachining technology and innovative chemical pre-concentration and separation techniques together with MOX gas sensors used as detectors, aiming at the detection of sub-ppb levels aromatic volatile organic compounds (VOCs) in environmental monitoring applications.
The systemic approach includes the study of innovative pre-concentration materials, micromachining of GC components and devices, the fabrication and characterization of complete system prototypes as well as the development of application-specific control electronics.
A palm-sized prototype was fabricated, integrating quinoxaline-bridged cavitands as pre-concentration materials, a micromachined packed gas chromatographic column and a MOX sensor array as detector in a simple fluidic system comprising some commercial components (mini-valves and pumps). The use of MOX sensors and the innovative pre-concentration materials allow using on-board generated filtered air as GC carrier gas, therefore no external bottles are required. Miniaturized application-specific control and read-out electronics were also designed and fabricated using small footprint stacked SMD PCBs.
The system was characterized both in laboratory conditions and partially in real outdoor air. The plots below show chromatograms of mixtures of benzene, toluene, ethylbenzene and m-xylene (BTEX) at different concentrations are reported, and the non-linear calibration curve of the system, defined as the peak area versus the VOC concentrations.
A palm-sized miniaturized GC system prototype was designed, fabricated and characterized. The mini-GC does not require any external gas bottles, since the carrier gas is generated directly on-board. Detection limits lower than 0.1 ppb were proved for benzene in laboratory conditions, when pre-concentrating the sample for 50 minutes.
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